A2B′B″O6 perovskites: A review

Investigation of the phase transition to the Ruddlesden-Popper phase in La- or Nb-doped Sr2Fe1.5Mo0.5O6-δ double perovskites and the impact of lanthanum or niobium doping

Sr2Fe1.5Mo0.5O6-δ (SFM) is a well-known representative of the double perovskite family, recognized for its remarkable properties, such as good conductivity in air and hydrogen. However, this material can undergo a phase transition under reductive atmospheres, which might be a challenge for its practical use. Herein, we focus on the impact of lanthanum or niobium dopants, which would not only stabilize the structure during the reduction but also have a beneficial impact on the properties of the material, e.g., electrical conductivity. As a result, lanthanum doping (LSFM - La0.3Sr1.7Fe1.5Mo0.5O6-δ) was found to be the most stable and the lowest amount of a new Ruddlesden-Popper phase was formed during the reduction. Moreover, the aliovalent La-doping resulted in increased electrical conductivities in both air and hydrogen compared to those of pristine SFM. Niobium doping resulted in a behavior similar to that of SFM with only slight stabilization, but the exsolution process in this material was found to be more intense. In situ studies during oxidation allowed us to retrieve the original structure at 700 °C. Ex situ XAS analyses enabled us to focus on the electronic state, which in most cases was restored almost to the original state after the re-oxidation process. This showed that not only the crystallographic structure but also the local atomic structure were re-established. The use of wavelet transform on the Fe K-edge allowed us to differentiate contributions from the Fe-Fe and Fe-Mo bonds in LSFM.

Mössbauer Spectroscopic Study of the Ordering Degree in Sr2-xBaxFeNbO6 Double Perovskites Prepared via the Sol-Gel Method

Sr2-xBaxFeNbO6 double perovskite materials were synthesized for the first time by using the sol-gel method, and their ordering degree was analyzed with Mössbauer spectroscopy. By analyzing the changes in the Fe ion quadrupole splitting before and after Ba2+ doping, Mössbauer spectroscopy was used to quantify the ordering degree of the double perovskite materials. The double perovskite samples prepared by the novel sol-gel method exhibited a high degree of ordering. VSM measurements confirmed the Mössbauer results, revealing a significant correlation between the square of the slope of the magnetization curve and the ordering degree obtained from Mössbauer spectroscopy, further validating the feasibility of using Mössbauer spectroscopy for quantitative analysis of the ordering degree in double perovskite samples. This study not only overcomes the previous challenge of synthesizing Sr2FeNbO6 via the sol-gel method but also offers a new perspective for investigating the ordering degree of double perovskite materials through Mössbauer spectroscopy.

Recent Advances on the Gas-Sensing Properties and Mechanism of Perovskite Oxide Materials - A Review

Perovskite oxide-based materials (ABO3) have gained much attention as promising candidates for advanced gas-sensing applications due to their versatile structures, tunable properties, and excellent stability. This review discusses recent developments in the synthesis, structural optimization, and functionalization of perovskites to enhance their gas-sensing performance. Strategies such as doping, creating oxygen vacancies, tuning morphology, and forming heterojunctions have significantly improved their sensitivity, selectivity, response, and recovery times. Specific advances include the incorporation of nanostructures, porous morphologies, and catalytic elements, which have optimized the adsorption and desorption processes for various target gases, including volatile organic compounds, NO2, and CO2. Mechanistic insights into the role of oxygen vacancies, surface defects, and charge carrier dynamics are also addressed. These developments position perovskite materials as important components in next-generation gas sensors for environmental monitoring and industrial applications.

Engineering magnetism in hybrid organic-inorganic metal halide perovskites

The chemical and structural flexibility of hybrid organic-inorganic metal halide perovskites (HOIPs) provides an ideal platform for engineering not only their well-studied optical properties, but also their magnetic ones. In this review we present HOIPs from a new perspective, turning the attention to their magnetic properties and their potential as a new class of on-demand low-dimensional magnetic materials. Focusing on HOIPs containing transition metals, we comprehensively present the progress that has been made in preparing, understanding and exploring magnetic HOIPs. First, we briefly introduce HOIPs in terms of composition and crystal structure and examine the synthesis protocols commonly used to prepare those showing magnetic properties. Then, we present their rich magnetic behavior and phenomenology; discuss their origin and guidelines for tuning them by changing the perovskite phase, chemical composition and dimensionality; and showcase their potential application in magneto-optoelectronics and spintronics. Finally, we describe the current challenges in the field, such as their integration into devices, as well as the emerging possibilities of moving from magnetic doping to pure transition metal-based HOIPs, which will motivate further studies in the future.

Research Progress and Prospect of Perovskite and Anti-Perovskite Solid Electrolytes for Sodium Solid-State Batteries

Sodium solid-state batteries (SSSBs) are poised to revolutionize energy storage by capitalizing on sodium's exceptional crustal abundance (2.36% vs 0.0017% for lithium) and cost-effectiveness, addressing critical sustainability challenges of lithium-dependent technologies. Solid electrolytes (SEs) with high ionic conductivity and stability have gained significant attention. The compositional and structural flexibility of perovskites and anti-perovskites make them competitive, and the combination of advanced computer simulations and synthesis techniques can achieve stable synthesis of the materials. Importantly, the high ionic conductivity and high stability of perovskite and anti-perovskite SEs at room temperature endow them with enormous potential for the construction of SSSBs. In this review, the research progress of perovskite and anti-perovskite SEs for SSSBs is summarized, different optimization strategies for improving the ionic conductivity of SEs are compared, and an in-depth discussion on the chemical and electrochemical stability of SEs is provided. Specifically, key technical indicators reflecting their structural tolerance and future application potential have been summarized and discussed for the first time. Among these, anti-perovskites, due to their diversity and the presence of more ion transport channels, have the potential to become commercial SEs. Finally, the future challenges and development directions of perovskite and anti-perovskite SEs for SSSBs have been prospected.

Magnetic, Phonon, and Optical Properties of Pure and Doped Ba2FeReO6 and Sr2CrReO6-Bulk Materials and Nanoparticles

On the basis of a microscopic model and employing Green's function technique, the effects of temperature, size, and ion doping on the magnetization and phonon energy of the A1g mode in double perovskites Ba2FeReO6 and Sr2CrReO6-both in bulk and nanoscale samples-are investigated for the first time. The Curie temperature TC and magnetization M decrease as nanoparticle size is reduced. Doping with rare-earth ions such as Sm, Nd, or La at the Ba or Sr sites further reduces M. This behavior originates from the compressive strain induced by the smaller ionic radii of the dopant ions compared to the host ions. As a result, the antiferromagnetic superexchange interaction between Fe or Cr and Re ions is enhanced, along with an increase in the magnetic moment of the Re ion. The dependence of the band gap energy of Sr2CrReO6 on temperature, size, and doping is also studied. Near the magnetic-phase-transition temperature TC, anomalies in phonon energy and damping indicate strong spin-phonon coupling. The theoretical calculations show good qualitative agreement with experimental data.

Characterization of the Spin-Frustration in Doubly Ordered Perovskite NaYbZnWO6 Obtained by High-Pressure Synthesis

We present the high-pressure synthesis of a novel doubly ordered perovskite NaYbZnWO6 composed of the rare earth magnetic Yb3+ ions and its comprehensive magnetic characterization. The structure consists of alternating layers of Yb3+ and Na+ ions along the c-axis, with Yb3+ ions forming slightly distorted two-dimensional (2D) square lattices of kite-shape (Yb3+)4 units. Low-temperature magnetic susceptibility measurements indicate that the ground state of Yb3+ can be described by an effective Jeff = 1/2 Kramers doublet. Further, specific heat analysis reveals an internal magnetic field of the order of 1.48 K; however, magnetization data do not exhibit magnetic ordering down to 0.4 K. The spin exchanges of NaYbZnWO6 evaluated by density functional theory (DFT) calculations unveil spin frustration in the compound. These findings suggest that NaYbZnWO6 is a promising candidate for realizing a magnetically disordered quantum state.

Synergistic Integration of Halide Perovskite and Rare-Earth Ions toward Photonics

Halide perovskites (HPs), emerging as a noteworthy class of semiconductors, hold great promise for an array of optoelectronic applications, including anti-counterfeiting, light-emitting diodes (LEDs), solar cells (SCs), and photodetectors, primarily due to their large absorption cross section, high fluorescence efficiency, tunable emission spectrum within the visible region, and high tolerance for lattice defects, as well as their adaptability for solution-based fabrication processes. Unlike luminescent HPs with band-edge emission, trivalent rare-earth (RE) ions typically emit low-energy light through intra-4f optical transitions, characterized by narrow emission spectra and long emission lifetimes. When fused, the cooperative interactions between HPs and REs endow the resulting binary composites not only with optoelectronic properties inherited from their parent materials but also introduce new attributes unattainable by either component alone. This review begins with the fundamental optoelectronic characteristics of HPs and REs, followed by a particular focus on the impact of REs on the electronic structures of HPs and the associated energy transfer processes. The advanced synthesis methods utilized to prepare HPs, RE-doped compounds, and their binary composites are overviewed. Furthermore, potential applications are summarized across diverse domains, including high-fidelity anticounterfeiting, bioimaging, LEDs, photovoltaics, photodetection, and photocatalysis, and conclude with remaining challenges and future research prospects.

Tunable NIR Emission of Cr3+-Activated Double-Perovskite Tantalates and Improvement of Luminescence Properties via Codoping Yb3

Broadband near-infrared (NIR) luminescent materials exhibit great potential in many fields including nondestructive examination, biological imaging, and night vision. Herein, the tunable NIR emission can be realized based on Sr2MTaO6:Cr3+ (M = Ga, Sc, In) tantalate phosphors with a double-perovskite structure, with the emission peak varying from 782 to 880 nm via cation modulation. Specifically, after being stimulated by a 460 nm blue light, the Sr2GaTaO6/Cr3+ phosphor demonstrates a broadband NIR emission with the full width at half-maximum (fwhm) over 180 nm, originating from two octahedral sites of Ga3+ and Ta5+ in Sr2GaTaO6 that can be occupied by Cr3+ ions. By using the Yb3+ codoping strategy, the efficient energy transfer process (Cr3+ → Yb3+) enables remarkable expansion of the fwhm to 304 nm and significant improvement in thermal stability due to the suppressive thermal quenching behavior of Cr3+ ions. Ultimately, by mixing the Sr2GaTaO6/Cr3+, Yb3+ phosphor with an epoxy adhesive and coating on a 460 nm blue LED chip, an NIR pc-LED device was successfully fabricated, and its prospective usage in nondestructive testing and night vision fields was evaluated. This work contributes to the exploration of blue light effectively stimulating broad-band NIR-emitting phosphors.

Insights into Chemical and Structural Order at Planar Defects in Pb2MgWO6 Using Multislice Electron Ptychography

Switchable order parameters in ferroic materials are essential for functional electronic devices, yet disruptions of the ordering can take the form of planar boundaries or defects that exhibit distinct properties from the bulk, such as electrical (polar) or magnetic (spin) response. Characterizing the structure of these boundaries is challenging due to their confined size and three-dimensional (3D) nature. Here, a chemical antiphase boundary in the highly ordered double perovskite Pb2MgWO6 is investigated using multislice electron ptychography. The boundary is revealed to be inclined along the electron beam direction with a finite width of chemical intermixing. Additionally, regions at and near the boundary exhibit antiferroelectric-like displacements, contrasting with the predominantly paraelectric matrix. Spatial statistics and density functional theory (DFT) calculations further indicate that despite their higher energy, chemical antiphase boundaries (APBs) form due to kinetic constraints during growth, with extended antiferroelectric-like distortions induced by the chemically frustrated environment in the proximity of the boundary. The three-dimensional imaging reveals the interplay between local chemistry and the polar environment, elucidating the role of antiphase boundaries and their associated confined structural distortions and offering opportunities for engineering ferroic thin films.

Synthesis, Characterization, and Electrocatalytic Properties of PrMn0.5M0.5O3 (M = Cr, Fe, Co, Ni) Perovskites

In this paper, the synthesis, characterization, and investigation of electrocatalytic properties of perovskites of general formula PrMn0.5M0.5O3 (M = Cr, Fe, Co, Ni) are presented. The synthesis was conducted by the solution combustion method using glycine as a fuel. The perovskite with the formula PrMn0.5Fe0.5O3 was also synthesized by the sol-gel combustion method with citric acid as fuel. The obtained perovskites were investigated by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), infrared spectroscopy, and cyclic voltammetry. The XRPD patterns showed that the compounds are pure and isostructural within the series. The unit cell parameters of the compounds were determined within the Pnma space group, and several crystallochemical parameters were calculated and discussed. The recorded SEM images of the perovskites revealed a porous morphology, while the EDX analysis confirmed the 2:1:1 atomic percentage ratio of Pr:Mn:M. Within this investigation, the electrocatalytic properties of the obtained perovskites towards oxidation of OH- ions and H2O2 oxidation in phosphate buffer were studied by cyclic voltammetry, using a paraffin-impregnated graphite electrode (PIGE) modified with microcrystals of the investigated perovskites. PrMn0.5Fe0.5O3 showed high electrocatalytic activity for OH- oxidation, while both PrMn0.5Fe0.5O3 and PrMn0.5Co0.5O3 exhibited significant efficiency for H2O2 oxidation, with a distinct oxidation peak with a peak potential of 0.6 V.

Variable Valence Ce-Based Cs2CeAgBr6 Perovskite Nanocrystals for Highly Selective Photoconversion of CO2 to CH4

Lead halide perovskites demonstrate outstanding luminescent characteristics. However, the inclusion of lead components restricts their extensive utilization. Halide perovskite materials, formulated as A2M(III)M(I)X6 or A2M(IV)X6, possess the potential to serve as stable and eco-friendly substitutes for optoelectronic applications. Nevertheless, their wide bandgap (>3 eV) hinders the practical implementation across various domains. Here, the variable valence Ce-based Cs₂CeAgBr₆ perovskite nanocrystals (NCs) are first synthesized with a bandgap of 2.65 eV. Intriguingly, the coexistence of trivalent and tetravalent Ce can cause localized spin of the f-layer electrons of Ce, leading to Ce3+/4+ (the Ce valence state ranges between III and IV) defects. By manipulating trivalent and tetravalent Ce source proportions, a dual Ce-based perovskite achieves a minimal Ce3+/4+ defect content of 1.4%. The as-prepared Cs₂CeAgBr₆ NCs exhibit exceptional efficiency in CO2 reduction driven by sunlight, with a CH4 selectivity greater than 70% and a super high charge transfer rate of 802.5 µmol·g-1 h-1, far surpassing previously reported findings. Additionally, theoretical calculations have elucidated the photocatalytic mechanism involved in CO₂ reduction. The outcomes of this investigation are expected to stimulate design and fabrication of novel lead-free perovskite nanocrystals.

Insights into the electronic, magnetic structure, and photocatalytic activity of Y2CuMnO6 double perovskite

This research aims to develop Y2CuMnO6 double perovskite, using a citrate auto combustion method, to be used as a photocatalyst for the degradation of organic dyes and antibiotics. XRD and Raman characterization revealed the synthesis of pure-phase Y2CuMnO6 double perovskite. The X-ray photoelectron spectroscopy results show the presence of +4 and +2 oxidation states of Mn and Cu ions. Our electronic structure analysis, Mott-Schottky, and UV-vis-NIR analysis suggest strong UV and visible region absorption. Our density functional theory analysis reveals that Y2CuMnO6 exhibits characteristics of a ferromagnetic semiconductor with low effective mass. The Jahn-Teller active Cu2+ ion induces local distortions, contributing to the stabilization of the low-symmetry monoclinic structure (P21/n). The ferromagnetic superexchange mechanism is attributed to the overlap between the empty eg band of Mn4+ and the partially filled eg band orbital of Cu2+. The Y2CuMnO6 double perovskite resulted in degradation efficiencies of 99%, 96%, and 95% of rhodamine B, methylene orange dyes, and tetracycline antibiotics, respectively. This study reveals that the Y2CuMnO6 double perovskite achieved enhanced photocatalytic activity compared to commercial P25 TiO2. It demonstrated the remarkable photocatalytic properties of the Y2CuMnO6 catalyst indicating its significant potential for diverse environmental applications.

Thermally Controlled A-site Cation Ordering and Coupled Polarity in Double Perovskite NaLaZr2O6

A-site cation ordering in double perovskites is crucially important for their physical properties. In this study, polycrystalline samples of Zr-based double perovskite NaLaZr2O6 were synthesized via high-temperature solid-state reactions, and the influence of the heating temperature and cooling rate on their crystal structures was investigated using synchrotron X-ray diffractometry and optical second harmonic generation. The samples prepared at 1200 °C, followed by slow cooling to room temperature, crystallize in a polar P21am structure, exhibiting partial A-site cation ordering, with Na- and La-rich A-site layers alternately stacked along the c axis. Remarkably, this structure transforms to a previously reported nonpolar Pnam phase with a disordered A-site cation arrangement when the slow-cooled samples are reheated at 1300 °C or higher and then rapidly quenched to room temperature. Theoretical analyses reveal that in the P21am phase, the a-a-c+ octahedral rotations trigger antiparallel displacements of the A-site cations in the Na- and La-rich A-site layers to induce spontaneous polarization. This is in contrast to the case of the Pnam phase, in which the rotation-induced antiparallel displacements of the equivalent A-site cations are antipolar. This work represents a rare example of AA'B2O6 perovskites exhibiting layered A-site ordering and polarity and also demonstrates a novel mechanism for reversible thermal switching from polar to nonpolar states in perovskites.

Insights into the electronic structure, optical properties, and photocatalytic potential of Gd2CoCrO6 perovskite: a comprehensive theoretical and experimental investigation

In this study, we present a comprehensive theoretical and experimental investigation into the electronic structure, optical properties, and photocatalytic potential of Gd2CoCrO6 (GCCO) double perovskite. Using first-principles calculations with the generalized-gradient-approximation plus Hubbard U (GGA + U) method, we explored the effects of Coulomb interactions on the electronic properties. Our calculations revealed that GCCO exhibits a half-metallic nature, displaying metallic behavior for up-spin and semiconducting behavior for down-spin states. The optimized U eff value of 4.2 eV accurately reproduces the direct bandgap of 2.25 eV, which aligns closely with experimental results obtained through UV-visible absorption spectroscopy and photoluminescence analysis. Additionally, time-resolved photoluminescence (TRPL) measurements indicate a mean charge carrier lifetime of 2.37 ns, suggesting effective charge separation. Mott-Schottky analysis and valence band X-ray photoelectron spectroscopy (XPS) confirm the n-type semiconducting nature of GCCO with favorable band edge positions for redox reactions. The combination of theoretical insights and experimental characterization indicates that GCCO holds significant promise as a photocatalyst for applications in renewable energy production and environmental remediation, particularly in solar-driven water splitting and pollutant degradation. Our study provides crucial insights into the electronic structure and optical properties of double perovskites like GCCO, highlighting their suitability for photocatalytic applications. Furthermore, the research paves the way for future work in the compositional engineering and defect modulation of double perovskites to optimize their photocatalytic efficiency.

Dynamics of magnetic inhomogeneity in La2CoMnO6 films probed by Raman spectroscopy

We report the dynamic effects of magnetic inhomogeneity on the temperature evolution of the Raman modes in polycrystalline La2CoMnO6 (LCMO) films. The LCMO films were obtained via chemical solution deposition and annealed at different temperatures, 700, 800 and 900 °C. Temperature-dependent Raman spectroscopic studies uncover anomalous phonon energy behaviors, associated with strong spin-phonon couplings revealed even at ambient conditions. This effect, which is observed to occur well above ferromagnetic ordering temperature is ascribed to short-range Mn4+/Co2+ ferromagnetic clusters. Moreover, our study has shown that spin-phonon coupling strength is governed by competing antiferromagnetic (AFM) and ferromagnetic (FM) interactions. These results significantly enhance the understanding of the complex spin-phonon coupling mechanism to provide insights into magnetic inhomogeneity in systems with two or more magnetic sublattices. These findings suggest the presence of similar effects in other double perovskites within the RE2CoMnO6 (RE = rare earths) family, which exhibit analogous magnetic sublattice and order-disorder defects.

Advanced Computational Insights Into Cs₂NaScX₆ (X = Cl, Br) ₆ Double Perovskites: Structural Stability, Elastic Properties, and Optical Characteristics for Next-Generation Photovoltaics

We investigate the comprehensive analysis's structural, electronic, optical, and elastic properties of Cs₂NaScX₆ (X = Cl, Br) double perovskites using density functional theory (DFT) implemented by the WIEN2k code. The results show that both compounds are in cubic phases. The calculated tolerance factors show both are stable compounds. The computed optimized lattice parameters are Cs₂NaScX₆ (X = Cl, Br) are 10.72 Å and 12.01 Å, respectively. Employing a modified Becke-Johnson (mBJ) potential electronic nature shows that both compounds are in semiconductor nature, that is, 3.138 eV and 3.977 eV. The calculated elastic constant and perimeters show the Cs₂NaScX₆ (X = Cl, Br) are mechanical stables and also ductile and anisotropic nature. The optical properties described the range of photon energies from 0 to 10 eV, revealing pronounced absorption within the visible spectrum, highlighting their considerable promise for transformative innovations in photovoltaic technology. These double perovskites exhibit superior absorption characteristics compared to their Cs₂NaScX₆ (X = Cl, Br) analogues, thus laying the groundwork for significant advancements in solar energy conversion and photovoltaic applications.

Recent Advances in Ruddlesden-Popper Phase-Layered Perovskite Sr2TiO4 Photocatalysts

Sr2TiO4, a prominent member of the Ruddlesden-Popper (RP) perovskite family, has garnered significant interest in photocatalysis, primarily owing to its distinctive two-dimensional (2D) layered structure. In this review, we provide an insightful and concise summary of the intrinsic properties of Sr2TiO4, focusing on the electronic, optical, and structural characteristics that render it a promising candidate for photocatalytic applications. Moreover, we delve into the innovative strategies that have been developed to optimize the structural attributes of Sr2TiO4. These strategies aim to maximize light absorption, improve charge separation, and accelerate the photocatalytic reaction rates. By highlighting these unique approaches, we strive to contribute to a more profound understanding of the material's potential and stimulate further advancements in developing Sr2TiO4-based photocatalytic systems. The review not only synthesizes the existing knowledge but also offers a perspective in future directions for research and application. As the field of photocatalysis continues to evolve, Sr2TiO4 stands poised to play a pivotal role in the quest for more efficient and sustainable solar energy conversion technology.

B-Site-Ordered and Disordered Structures in A-Site-Ordered Quadruple Perovskites RMn3Ni2Mn2O12 with R = Nd, Sm, Gd, and Dy

ABO3 perovskite materials with small cations at the A site, especially with ordered cation arrangements, have attracted a lot of interest because they show unusual physical properties and deviations from general perovskite tendencies. In this work, A-site-ordered quadruple perovskites, RMn3Ni2Mn2O12 with R = Nd, Sm, Gd, and Dy, were synthesized by a high-pressure, high-temperature method at about 6 GPa. Annealing at about 1500 K produced samples with additional (partial) B-site ordering of Ni2+ and Mn4+ cations, crystallizing in space group Pn-3. Annealing at about 1700 K produced samples with disordering of Ni2+ and Mn4+ cations, crystallizing in space group Im-3. However, magnetic properties were nearly identical for the Pn-3 and Im-3 modifications in comparison with ferromagnetic double perovskites R2NiMnO6, where the degree of Ni2+ and Mn4+ ordering has significant effects on magnetic properties. In RMn3Ni2Mn2O12, one magnetic transition was found at 26 K (for R = Nd), 23 K (for R = Sm), and 22 K (for R = Gd), and two transitions were found at 10 K and 36 K for R = Dy. Curie-Weiss temperatures were close to zero in all compounds, suggesting that antiferromagnetic and ferromagnetic interactions are of the same magnitude.

Efficient Exploratory Synthesis of Quaternary Cesium Chlorides Guided by In Silico Predictions

Exploratory synthesis of solids is essential for the advancement of materials science but is also highly time- and resource-intensive. Here, we demonstrate an efficient strategy to explore solid-state synthesis of quaternary cesium chlorides in the search space of CsnAIBCl6 (n = 2 or 3, A = Li, Na or K, and B = d or p-block metal), where the target compositions are selected from a pool of candidates based on computationally predicted stabilities and availability of viable precursor powders. Synthesizability of the targets is assessed by observing the evolution of starting phases upon heating under in situ synchrotron X-ray diffraction. Laboratory synthesis is attempted for promising targets, and resulting materials are characterized by powder X-ray and neutron diffraction and subsequent Rietveld refinement. We focus on how computational predictions can be bridged to experimental characterizations in exploratory synthesis and report on successful and failed synthesis attempts for compounds of type Cs2AIBIIICl6, revealing underexplored variants including new polymorphs of Cs2LiCrCl6 and Cs2LiRuCl6, and a new compound Cs2LiIrCl6.

Nitride Tuning of Magnetic Frustration in the Double Perovskite Ba2MnWO6

The new double perovskite oxynitride Ba2MnWO4.42N1.58 has been obtained by topochemical ammonolysis at 700 °C of B-site ordered Ba2MnWO6 using a high NH3 flow rate of 600 cm3/min. The relatively low synthesis temperature hinders the cation mobility, allowing the order of Mn and W cations of the precursor oxide to be unperturbed in the oxynitride. Synchrotron X-ray diffraction, electron diffraction, and neutron diffraction indicate that Ba2MnWO4.42N1.58 crystallizes in the Fm3̅m space group with a larger parameter of 8.2434(5) Å compared to Ba2MnWO6 [8.20337(1) Å]. Magnetic susceptibility measurements show that Ba2MnWO4.42N1.58 orders antiferromagnetically below T N = 3.8 K, and the observed Curie-Weiss temperature θCW = -70(3) K indicates a frustrated Mn lattice with frustration parameter f = |θCW|/T N ≈ 18, which is significantly larger than for the oxide (f ≈ 7.2). The substitution of oxide by the nitride anion induces the oxidation of Mn2+ to Mn3+/4+ and a subsequent decrease of the paramagnetic effective moment from 6.28 to 5.16 μB/f.u. The observation in the oxynitride of a frustration parameter which is larger than twice that of the oxide precursor is rationalized as caused by the relative enhancement of the nearest neighbors' magnetic interactions (J 1) Mn-(O/N)-(O/N)-Mn at 90°, compared to next-nearest neighbors' interactions at 180° (J 2) Mn-(O/N)-W-(O/N)-Mn, due to the smaller electronegativity of nitrogen compared to oxygen that increases the covalency of bonding. These results expand the chemical and structural diversity of complex transition metal oxynitrides and provide a new route to tailor spin frustration in transition metal double perovskites.

Observation of Enhanced Long-Range Ferromagnetic Order in B-Site Ordered Double Perovskite Oxide Cd2CrSbO6

A B-site ordered double perovskite oxide Cd2CrSbO6 was synthesized under high-pressure and high-temperature conditions. The compound crystallizes to a monoclinic structure with a space group of P21/n. The charge configuration is confirmed to be that of Cd2+/Cr3+/Sb5+. The magnetic Cr3+ ions form a tetrahedral structural frustrated lattice, while a long-range ferromagnetic phase transition is found to occur at TC = 16.5 K arising from the superexchange interaction via the Cr-O-Cd-O-Cr pathway. Electrical transport measurements indicate that Cd2CrSbO6 is an insulator that can be described by the Mott 3D variable range hopping mechanism. First-principles calculations reproduce well the ferromagnetic and insulating ground state of Cd2CrSbO6 with an energy band gap of 1.55 eV. The intrinsic ferromagnetic insulating nature qualifies Cd2CrSbO6 as a promising candidate for possible spintronics applications.

Kinetic energy driven two-sublattice double-exchange: a general mechanism of magnetic exchange in transition metal compounds

One of the most important phenomena in magnetism is the exchange interaction between magnetic centres. In this topical review, we focus on the exchange mechanism in transition-metal compounds and establish kinetic-energy-driven two-sublattice double-exchange as a general mechanism of exchange, in addition to well-known mechanisms like superexchange and double exchange. This mechanism, which was first proposed (Sarmaet al2000Phys. Rev. Lett.852549), in the context of Sr2FeMoO6, a double-perovskite compound, later found to describe a large number of 3d and 4d or 5d transition metal-based double perovskites. The magnetism in multi-sublattice magnetic systems like double-double and quadrupolar perovskites involving 3d and 4d or 5d transition-metal ions have also been found to be governed by this as a primary mechanism of exchange. For example, the numerical solution of a two-sublatice double exchange with additional superexchange couplings for the FeRe-based double double and quadrupolar perovskites are found to reproduce the experimentally observed magnetic ground state as well as the high transition temperature of above 500 K. The applicability of this general mechanism extends beyond the perovskite crystal structures, and oxides, as demonstrated for the pyrochlore oxide, Tl2Mn2O7and the square-net chalcogenides KMnX2(X = S, Se, Te). The counter-intuitive doping dependence and pressure effect of magnetic transition temperature in Tl2Mn2O7is explained, while KMnX2(X = S, Se, Te) compounds are established as half-metallic Chern metals guided by two sublattice double exchange. While the kinetic energy-driven two-site double-exchange mechanism was originally proposed to explain ferromagnetism, a filling-dependent transition can lead to a rare situation of the antiferromagnetic metallic ground state, as found in La-doped Sr2FeMoO6, and proposed for computer predicted double perovskites Sr(Ca)2FeRhO6. This opens up a vast canvas to explore.

Multiferroic and Phonon Properties of the Double Perovskite Pr2FeAlO6

With the help of a microscopic model and Green's function technique, we studied the multiferroic and phonon properties of the recently reported new multiferroic Pr2FeAlO6 (PFAO) compound, which belongs to the double perovskite A2BB'O6 family. The magnetization decreases with the increase in temperature and disappears at the ferromagnetic Curie temperature TCFM. The polarization increases with the application of an external magnetic field, indicating strong magnetoelectric coupling and confirming the multiferroic behavior of PFAO. In the curves of dependence of the phonon energy and their damping with respect to temperature, a kink is observed at TCFM. This is due to the strong anharmonic spin-phonon interactions, which play a crucial role below TCFM and are frequently observed in other double perovskite compounds. Above TCFM, only anharmonic phonon-phonon coupling remains. The phonon mode is controlled by an external magnetic field.

Global Optimization of Cation Ordering in Perovskites by Recommendation-Based Basin-Hopping

Cation ordering in multication perovskites is related to many important material properties and performances, but computational determination of the cation ordering remains a major challenge. Here, we propose a new computational approach by introducing a machine learning recommender system into the basin-hopping framework (RBH) for optimizing cation ordering. Taking the electrocatalyst Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF5582) as a showcase example, we found that the efficiency of RBH in identifying low-energy configurations outperforms the methods of cluster expansion and conventional basin-hopping. The RBH results revealed that the BSCF5582 catalyst tended to have a layered ordering of A-site cations and disordered B-site cations both in bulk and on the surfaces. Further, on the A-site-terminated surface, we found the segregation of large Ba atoms. Similarly, on the A-site- terminated surface of the recently developed Cs0.2Sr0.8Co0.4Fe0.6O3 (CSCF2846) catalyst, layered ordering at the A-site and surface enrichment of large Cs atoms were also found. The layered ordering was robust against thermal effects, as found from molecular dynamics simulations at 800 K. This work provides a new approach for thermodynamic global optimization of chemical ordering in multicomponent materials.

Three Perovskite Phases with Different Cation Orders in Sm2MnMn(Mn4-xSbx)O12

Cation order, which can be controlled by synthesis conditions and stoichiometry, plays an important role in properties of perovskite materials. Here we show that aliovalent doping by Sb5+ in Sm2MnMn(Mn4-xSbx)O12 quadruple perovskite solid solutions can control cation orders in both A and B sites. Samples with 0.4≤x≤2 were synthesized by a high-pressure, high-temperature method at 6 GPa and 1770 K. Three regions with different cation orders were found at 0.5≤x≤1.0, x=1.5-1.6, and x=1.8. The 0.5≤x≤1.0 compositions have a B-site-disordered and A-site columnar-ordered structure with space group P42/nmc; the x=1.5 and 1.6 samples have a B-site rock-salt-ordered and A-site columnar-ordered structure with space group P42/n; the x=1.8 sample has a B-site rock-salt-ordered and A-site-disordered structure with space group P21/n. All the samples show one ferrimagnetic transition: TC increases from 35 K to 73 K for 0.5≤x≤1.0, TC=81 K for x=1.5 and 1.6, and TC=53 K for x=1.8.

(Ca0.5Mn0.5)2MnTeO6 - An Anomalously Stable High-Pressure Double Perovskite

High pressure high temperature treatments of the composition CaMnMnTeO6 are found to yield only an A2BB'O6-type double perovskite (Ca0.5Mn0.5)2MnTeO6, rather than a AA'BB'O6 double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO6 and CaMnMnWO6 with similar cation sizes. Double perovskite (Ca0.5Mn0.5)2MnTeO6 adopts a monoclinic structure in space group P21/n with a framework of highly tilted MnO6 and TeO6 octahedra enclosing disordered Ca2+ and Mn2+ cations. Magnetic measurements show that (Ca0.5Mn0.5)2MnTeO6 is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K.

Ba4RuMn2O10: A Noncentrosymmetric Polar Crystal Structure with Disordered Trimers

Phase-pure polycrystalline Ba4RuMn2O10 was prepared and determined to adopt the noncentrosymmetric polar crystal structure (space group Cmc21) based on results of second harmonic generation, convergent beam electron diffraction, and Rietveld refinements using powder neutron diffraction data. The crystal structure features zigzag chains of corner-shared trimers, which contain three distorted face-sharing octahedra. The three metal sites in the trimers are occupied by disordered Ru/Mn with three different ratios: Ru1:Mn1 = 0.202(8):0.798(8), Ru2:Mn2 = 0.27(1):0.73(1), and Ru3:Mn3 = 0.40(1):0.60(1), successfully lowering the symmetry and inducing the polar crystal structure from the centrosymmetric parent compounds Ba4T3O10 (T = Mn, Ru; space group Cmca). The valence state of Ru/Mn is confirmed to be +4 according to X-ray absorption near-edge spectroscopy. Ba4RuMn2O10 is a narrow bandgap (∼0.6 eV) semiconductor exhibiting spin-glass behavior with strong magnetic frustration and antiferromagnetic interactions.

Atomic order of rare earth ions in a complex oxide: a path to magnetotaxial anisotropy

Complex oxides offer rich magnetic and electronic behavior intimately tied to the composition and arrangement of cations within the structure. Rare earth iron garnet films exhibit an anisotropy along the growth direction which has long been theorized to originate from the ordering of different cations on the same crystallographic site. Here, we directly demonstrate the three-dimensional ordering of rare earth ions in pulsed laser deposited (EuxTm1-x)3Fe5O12 garnet thin films using both atomically-resolved elemental mapping to visualize cation ordering and X-ray diffraction to detect the resulting order superlattice reflection. We quantify the resulting ordering-induced 'magnetotaxial' anisotropy as a function of Eu:Tm ratio using transport measurements, showing an overwhelmingly dominant contribution from magnetotaxial anisotropy that reaches 30 kJ m-3 for garnets with x = 0.5. Control of cation ordering on inequivalent sites provides a strategy to control matter on the atomic level and to engineer the magnetic properties of complex oxides.

Synthesis and Properties of the Ba2PrWO6 Double Perovskite

We report details on the synthesis and properties of barium praseodymium tungstate, Ba2PrWO6, a double perovskite that has not been synthesized before. Room-temperature (RT) powder X-ray diffraction identified the most probable space group (SG) as monoclinic I2/m, but it was only slightly distorted from the cubic structure. X-ray photoelectron spectroscopy confirmed that the initial (postsynthesis) material contained praseodymium in both 3+ and 4+ charge states. The former (Pr3+) disappeared after exposure to UV light at RT. Photoluminescence studies of Pr3+ revealed that Ba2PrWO6 is an insulator with a band gap exceeding 4.93 eV. Pressure-dependent Raman spectroscopy excluded the possibility of a phase transition up to 20 GPa; however, measurements between 8 and 873 K signified that there might be a change toward the lower symmetry SG below 200 K. Electron paramagnetic resonance spectra revealed the presence of interstitial oxygen which acts as a deep electron trap.

Comprehensive first principles to investigate optoelectronic and transport phenomenon of lead-free double perovskites Ba2AsBO6 (B[bond, double bond]Nb, Ta) compounds

In the current work we studied the structural, elastics, electrical, optical, thermoelectric, as well as spectroscopic limited maximum efficiency (SLME) of oxide based Ba2AsBO6 (B[bond, double bond]Nb, Ta) materials. All the calculations were performed using first-principles calculation by employing the WIEN2k code. We checked the stability in diverse forms such as optimization, phonon dispersion, mechanical, formation energy, cohesive energy, and thermal stability is computed. The semiconducting nature of these Ba2AsBO6 (B[bond, double bond]Nb, Ta) systems is revealed by calculating the direct band gap values are 1.97 eV and 1.49 eV respectively. Additionally, we determined the optical properties which analyze the utmost absorption and transition of carriers versus photon energy (eV). Moreover, Ba2AsNbO6 has an estimated SLME of 32 %, making it an encouraging alternative for single-junction solar cells. Lastly, we studied the transport properties against temperature, the chemical potential for p-type and n-type charge carriers at various temperatures. At 300 K, the zT values are found to be 0.757 and 0.751 for Ba2AsBO6 (B[bond, double bond]Nb, Ta) compounds respectively. Both materials were examined as having strong absorption patterns and an excellent figure of merit (ZT), indicating that materials are appropriate for daily life applications.

Structure and phase transitions in niobium and tantalum derived nanoscale transition metal perovskites, Ba(Ti,MV)O3, M=Nb,Ta

The prospect of creating ferroelectric or high permittivity nanomaterials provides motivation for investigating complex transition metal oxides of the form Ba(Ti, MV)O3, where M = Nb or Ta. Solid state processing typically produces mixtures of crystalline phases, rarely beyond minimally doped Nb/Ta. Using a modified sol-gel method, we prepared single phase nanocrystals of Ba(Ti, M)O3. Compositional and elemental analysis puts the empirical formulas close to BaTi0.5Nb0.5O3-δ and BaTi0.5Ta0.5O3-δ. For both materials, a reversible temperature dependent phase transition (non-centrosymmetric to symmetric) is observed in the Raman spectrum in the region 533-583 K (260-310 °C); for Ba(Ti, Nb)O3, the onset is at 543 K (270 °C); and for Ba(Ti, Ta)O3, the onset is at 533 K (260 °C), which are comparable with 390-393 K (117-120 °C) for bulk BaTiO3. The crystal structure was resolved by examination of the powder x-ray diffraction and atomic pair distribution function (PDF) analysis of synchrotron total scattering data. It was postulated whether the structure adopted at the nanoscale was single or double perovskite. Double perovskites (A2B'B″O6) are characterized by the type and extent of cation ordering, which gives rise to higher symmetry crystal structures. PDF analysis was used to examine all likely candidate structures and to look for evidence of higher symmetry. The feasible phase space that evolves includes the ordered double perovskite structure Ba2(Ti, MV)O6 (M = Nb, Ta) Fm-3m, a disordered cubic structure, as a suitable high temperature analog, Ba(Ti, MV)O3Pm-3m, and an orthorhombic Ba(Ti, MV)O3Amm2, a room temperature structure that presents an unusually high level of lattice displacement, possibly due to octahedral tilting, and indication of a highly polarized crystal.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

The impact of A-site cations on the crystal structure and magnetism of the new double perovskites ALaCoTeO6 (A = Na and K)

In this work, we report the structural and magnetic characterization of two new B-site rock-salt ordered double perovskites ALaCoTeO6 (A = K+ and Na+) with mixed A-site cations. KLaCoTeO6 crystallizes in the space group P4/nmm with a long-range ordering degree of 84.8% for the A-site K+/La3+ cations, whereas NaLaCoTeO6 adopts an unexpected triclinically distorted I1̄-structure with Na/La3+ disordering, validated by combined Rietveld refinements against high-resolution neutron diffraction data and Cu Kα1 X-ray powder diffraction data. Magnetic susceptibility at low temperatures shows clear antiferromagnetic (AFM) transitions for both compounds. KLaCoTeO6 exhibits the highest AFM transition temperature of 20 K amongst all the Co/Te-ordered 3C-type A2CoTeO6 (A = Pb2+, Sr2+, and Ca2+) and ALaCoTeO6 double perovskites due to its larger Co2+-O-Te6+ bond angle and A-site cationic ordering-induced larger distortion of the Co2+-based face-centered cubic sublattice. Moreover, we found that the average radius of the A-site cations plays a decisive role in the AFM transition temperatures of all these ordered double perovskites, that is, a larger A-site cation always results in a higher AFM transition temperature. This provides a strategy to subtly manipulate the magnetic properties of ordered double perovskites.

High-Pressure Synthesis of Quadruple Perovskite Oxide CaCu3Cr2Re2O12 with a High Ferrimagnetic Curie Temperature

An AA'3B2B'2O12-type quadruple perovskite oxide of CaCu3Cr2Re2O12 was synthesized at 18 GPa and 1373 K. Both an A- and B-site ordered quadruple perovskite crystal structure was observed, with the space group Pn-3. The valence states are verified to be CaCu32+Cr23+Re25+O12 by bond valence sum calculations and synchrotron X-ray absorption spectroscopy. The spin interaction among Cu2+, Cr3+, and Re5+ generates a ferrimagnetic transition with the Curie temperature (TC) at about 360 K. Moreover, electric transport properties and specific heat data suggest the presence of a half-metallic feature for this compound. The present study provides a promising quadruple perovskite oxide with above-room-temperature ferrimagnetism and possible half-metallic properties, which shows potential in the usage of spintronic devices.

Interpretable machine learning-assisted screening of perovskite oxides

Perovskite oxides are extensively utilized in energy storage and conversion. However, they are conventionally screened via time-consuming and cost-intensive experimental approaches and density functional theory. Herein, interpretable machine learning is applied to identify perovskite oxides from virtual perovskite-type combinations by constructing classification and regression models to predict their thermodynamic stability and energy above the convex hull (Eh), respectively, and interpreting the models using SHapley Additive exPlanations. The highest occupied molecular orbital energy and the elastic modulus of the B-site elements of perovskite oxides are the top two features for stability prediction, whereas the Stability Label and features involving the elastic modulus and ionic radius are crucial for Eh regression. A classification model, which displays an accuracy of 0.919, precision of 0.937, F1-score of 0.932, and recall of 0.935, screens 682 143 stable perovskite oxides from 1 126 668 virtual perovskite-type combinations. The Eh values of the predicted stable perovskites are forecasted by a regression model with a coefficient of determination of 0.916, and root mean square error of 24.2 meV atom-1. Good agreement is observed between the regression model predicted and density functional theory-calculated Eh values.

Structure, Spin Correlations, and Magnetism of the S = 1/2 Square-Lattice Antiferromagnet Sr2CuTe1-xWxO6 (0 ≤ x ≤ 1)

Quantum spin liquids are highly entangled magnetic states with exotic properties. The S = 1/2 square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr2CuTeO6 and Sr2CuWO6 are physical realizations of this model but have distinctly different types of magnetic order and interactions due to a d10/d0 effect. Long-range magnetic order is suppressed in the solid solution Sr2CuTe1-xWxO6 in a wide region of x = 0.05-0.6, where the ground state has been proposed to be a disorder-induced spin liquid. Here, we present a comprehensive neutron scattering study of this system. We show using polarized neutron scattering that the spin liquid-like x = 0.2 and x = 0.5 samples have distinctly different local spin correlations, which suggests that they have different ground states. Low-temperature neutron diffraction measurements of the magnetically ordered W-rich samples reveal magnetic phase separation, which suggests that the previously ignored interlayer coupling between the square planes plays a role in the suppression of magnetic order at x ≈ 0.6. These results highlight the complex magnetism of Sr2CuTe1-xWxO6 and hint at a new quantum critical point between 0.2 < x < 0.4.

Microstructural Characterization and Magnetic, Dielectric, and Transport Properties of Hydrothermal La2FeCrO6 Double Perovskites

Double perovskite La2FeCrO6 (LFCO) powders were synthesized via the hydrothermal method, which crystallized in an orthorhombic (Pnma) structure and exhibited a spherical morphology with an average particle size of 900 nm. Fourier transform infrared spectroscopy demonstrated the presence of fingerprints of vibrational modes of [FeO6] and [CrO6] octahedra in the powders. The XPS spectra revealed dual oxide states of Fe (Fe2+/Fe3+) and Cr (Cr3+/Cr4+) elements, and the oxygen element appeared as lattice oxygen and defect oxygen, respectively. The LFCO powders exhibited weak ferromagnetic behavior at 5 K with a Curie temperature of 200 K. Their saturation magnetization and coercive field were measured as 0.31 μB/f.u. and 8.0 kOe, respectively. The Griffiths phase was observed between 200 K and 223 K. A butterfly-like magnetoresistance (MR)-magnetic field (H) curve was observed in the LFCO ceramics at 5 K with an MR (5 K, 6 T) value of -4.07%. The temperature dependence of resistivity of the LFCO ceramics demonstrated their semiconducting nature. Electrical transport data were fitted by different conduction models. The dielectric behaviors of the LFCO ceramics exhibited a strong frequency dispersion, and a dielectric abnormality was observed around 260 K. That was ascribed to the jumping of electrons trapped at shallow levels created by oxygen vacancies. The dielectric loss showed relaxation behavior between 160 K and 260 K, which was attributed to the singly ionized oxygen vacancies.

Effect of A-Cation Radius on the Structure, Luminescence, and Temperature Sensing of Double Perovskites A2MgWO6 Doped with Dy3+ (A = Ca, Sr, Ba)

Herein, we report a case of A2MgWO6 (A = Ca, Sr, Ba) doped with 2%Dy3+, 2%Li+, in which the influences of the cation substitution are exhibited through the crystal structure, the charge transfer band (O2--W6+), the emission spectrum, the color of the luminescence, and the luminescent decay time. The substitution of Ca2+ and Sr2+ ions for larger ions (Ba2+) led to the crystal structure alteration from cubic to monoclinic and tetragonal, respectively. These structure changes also lowered the crystallography symmetry site of Dy3+, tuned the color of the emitted light from the whitish to yellowish region, and caused a blue shift of the CTB. Furthermore, a significant decline in the lifetime of the 4F9/2 → 6H13/2,15/2 transitions was noticed, from 748, 199, to 146 μs corresponding to Ba, Sr, Ca sample owing to the reduction in the local symmetry of Dy3+. Moreover, the thermal sensing properties of 2%Dy3+-doped samples were investigated based on the fluorescence intensity ratio technique in the range of 80-325 K. Under 266 nm excitation wavelength, the maximum relative sensitivity of the investigated samples was remarkably enhanced from 2.26%/K, 3.04%/K, to 4.38%/K corresponding to Ba, Ca, and Sr samples, respectively. In addition to providing a comprehensive understanding of the effects of compositional modifications on the optical properties, the results also present a viable pathway to manipulate the temperature sensing performance.

S-scheme heterojunctions based on novel Sm2CeMnO6 double perovskite oxide and g-C3N4 with excellent photocatalytic dye degradation performances

It has become of utmost importance to preserve marine life and human health by protecting aquatic environments from contaminants. Therefore, using photocatalytic materials in treatment of contaminated water is a promising and innovative technique. Novel double perovskite Sm2CeMnO6 was synthesized through a modified Pechini sol-gel method. Also, urea and melamine were utilized to synthesize graphitic carbon nitride (g-C3N4). Combination of Sm2CeMnO6 and g-C3N4 produced several S-scheme heterojunction materials in diverse components ratios. Average crystallite sizes of Sm2CeMnO6 and Sm2CeMnO6/g-C3N4 (20:80) samples were calculated by Debye-Scherrer and Williamson-Hall methods to be 19.77, 22.72 nm and 42.01, 43.73 nm, respectively. The coexistence of g-C3N4 (002) with a d-spacing of 0.325 nm and Sm2CeMnO6 planes of (222), (111), and (400) with spacing values of 0.314, 0.302, and 0.294 nm, respectively, was depicted in the HR-TEM image of the Sm2CeMnO6/g-C3N4 (20:80). The estimated bandgaps for the g-C3N4, Sm2CeMnO6, and Sm2CeMnO6/g-C3N4 (20:80) were 2.70, 2.60, and 2.65 eV, respectively. Their application was investigated in photocatalytic degradation of methylene blue (MB) dye as typical pollutant. The estimated degradation pathway of MB was also provided through LC-MS analysis. Under the identical conditions, the best photocatalytic performance was found for Sm2CeMnO6/g-C3N4 (20:80) composite. Using response surface methodology (RSM), operational parameters of the photocatalytic degradation were modeled and optimized by the best composite through central composite design approach. Applying optimized parameters led to 96% degradation of MB (8 mg/L) at pH 10 under 120 min visible light irradiation (λ > 365 nm) using 0.15 g of Sm2CeMnO6/g-C3N4 (20:80) composite in 100 mL aqueous solution. Due to low intrinsic charge transfer resistance, modified Eg, and good performance in e‒/h+ pairs production, Sm2CeMnO6/g-C3N4 (20:80) nanocomposite was introduced as a promising S-scheme photocatalyst.

Magnetic Properties of Tetravalent Pu in the Perovskites BaPuO3 and SrPuO3

BaPuO3 and SrPuO3 were synthesized, and their structures were refined in the orthorhombic space group Pbnm, a common distortion from the classic Pm3̅m cubic perovskite. Magnetic-susceptibility measurements, obtained as a function of temperature over the range of 1.8-320 K, exhibit temperature-dependent behavior, with evidence of long-range magnetic order at temperatures higher than their lanthanide and actinide analogues: BaPuO3 below 164(1) K and SrPuO3 below 76(1) K. Effective moments of 1.66(10)μB for BaPuO3 and 1.84(8)μB for SrPuO3 were obtained by fitting their paramagnetic susceptibilities using the Curie-Weiss law. Both are below the free-ion value of 2.68 μB expected for a Pu4+ 5I4 ground level. Ab initio wave function calculations, performed at the relativistic complete active space level including spin-orbit coupling and with an embedded cluster approach that neglects interactions between Pu centers, were used to generate embedded-cluster Pu4+ magnetic susceptibilities. The calculations agree well with experimental data at higher temperatures, providing evidence that a single-ion representation is sufficient to account for the observed paramagnetic behavior without the need to invoke charge transfer, disproportionation, strong covalent bonding, or other more complex electronic behavior.

Investigation on the predictive power of tolerance factor τ for A-site double perovskite oxides

We have used a recently introduced new tolerance factor τ to create a stability map of all possible A-site double perovskite titanates AA'Ti2O6 and niobates AA'Nb2O6. The predictive power of τ is relatively good based on comparisons with available experimental data for A-site double perovskites. We carried out quantum chemical calculations on two hypothetical double perovskite compositions CsScTi2O6 and YRbTi2O6, where τ predicts high probability for their existence. In both cases, we found limits in the predictive power of the new tolerance factor for ion combinations on the A and A' site which are very different in size. A difference in oxidation state may decrease the accuracy, as well. Overall, the A-site double perovskite stability mapping provides a starting point for the discovery of novel A-site double perovskites.

Decoration of Au Nanoparticles over LaFeO3: A High Performance Electrocatalyst for Total Water Splitting

Electrocatalytic water splitting has emerged as a promising approach for clean and sustainable hydrogen production. The LaFeO3 perovskite structure exhibits intriguing properties such as mixed ionic-electronic conductivity, high stability, and abundant active sites for electrocatalysis. However, its OER and HER activities are limited by the sluggish kinetics of these reactions. To overcome this limitation, Au nanoparticles (NPs) are decorated onto the surface of LaFeO3 through a facile synthesis method. The Au NPs on the LaFeO3 surface provide additional active sites for water splitting reactions, promoting the adsorption and activation of water molecules. The presence of Au enhances the charge transfer kinetics via the heterostructure between Au NPs and LaFeO3 and facilitates electron transport during the OER and HER process. The catalyst requires only 318 and 199 mV as overpotential to attain a 50 mA cm-2 current density in 1 M KOH solution. Our results demonstrate that the Au@LaFeO3 catalyst exhibits significantly improved electrocatalytic activity compared to pure LaFeO3 and other catalysts reported in the literature. The enhanced performance is attributed due to the synergistic effects between Au NPs and LaFeO3, including an increased surface area, improved conductivity, and optimized surface energetics. Overall, the Au-decorated LaFeO3 catalyst presents a promising candidate for efficient electrocatalytic water splitting, providing a pathway for sustainable hydrogen production. The insights gained from this study contribute to the development of advanced catalysts for renewable energy technologies and pave the way for future research in the field of electrochemical water splitting.

Phosphors Ba2 LaTaO6 :Mn4+ and its alkali metal charge compensation for plant growth illumination

A series of Mn4+ -doped and Mn4+ ,K+ -co-doped Ba2 LaTaO6 (BLT) double-perovskite phosphors was synthesized using a high-temperature solid-state reaction. The phase purity and luminescence properties were also studied. The optimum doping concentration of Mn4+ and K+ was obtained by investigating the photoluminescence excitation spectra and photoluminescence emission spectra. The comparison of BLT:Mn4+ phosphors with and without K+ ions shows that the photoluminescence intensity of K+ -doped phosphors was greatly enhanced. This is because there was a charge difference when Mn4+ ions were doped with Ta5+ ions in BLT. Mn4+ -K+ ion pairs were formed after doping K+ ions, which hinders the nonradiative energy transfer between Mn4+ ions. Therefore, the luminescence intensity, quantum yield, and thermal stability of phosphors were enhanced. The electroluminescence spectra of BLT:Mn4+ and BLT:Mn4+ ,K+ were measured. The spectra showed that the light emitted from the phosphors corresponded well with chlorophyll a and phytochrome PR . The results show that the BLT:Mn4+ ,K+ phosphors had good luminescence properties and application prospects and are ideal materials for plant-illuminated red phosphors.

Research Progress on Photocatalytic CO2 Reduction Based on Perovskite Oxides

Photocatalytic CO2 reduction to valuable fuels is a promising way to alleviate anthropogenic CO2 emissions and energy crises. Perovskite oxides have attracted widespread attention as photocatalysts for CO2 reduction by virtue of their high catalytic activity, compositional flexibility, bandgap adjustability, and good stability. In this review, the basic theory of photocatalysis and the mechanism of CO2 reduction over perovskite oxide are first introduced. Then, perovskite oxides' structures, properties, and preparations are presented. In detail, the research progress on perovskite oxides for photocatalytic CO2 reduction is discussed from five aspects: as a photocatalyst in its own right, metal cation doping at A and B sites of perovskite oxides, anion doping at O sites of perovskite oxides and oxygen vacancies, loading cocatalyst on perovskite oxides, and constructing heterojunction with other semiconductors. Finally, the development prospects of perovskite oxides for photocatalytic CO2 reduction are put forward. This article should serve as a useful guide for creating perovskite oxide-based photocatalysts that are more effective and reasonable.

Multilevel resistive switching memory in lead-free double perovskite La 2 NiFeO 6 films

Dense and flat La2 NiFeO6 (LNFO) films were fabricated on the indium tin oxide-coated glass (ITO/glass) substrate by sol-gel method. The bipolar resistive switching behavior (BRS) could be maintained in 100 cycles and remained after 30 days, indicating that the LNFO-based RS device owned good memory stability. Surprisingly, the multilevel RS characteristics were firstly observed in the Au/LNFO/ITO/glass device. The high resistance states (HRSs) and low resistance state (LRS) with the maximum ratio of∼  500 could be remained stably in 900 s and 130 cycles, demonstrating the fine retention and endurance ability of this LNFO-based RS device. The BRS behavior of Au/LNFO/ITO/glass devices primarily obeyed the SCLC mechanism controlled by oxygen vacancies (OVs) dispersed in the LNFO layer. Under the external electric field, injected electrons were captured or discharged by OVs during trapping or detrapping process in the LNFO layer. Thus, the resistive state switched between HRS and LRS reversibly. Moreover, the modulation of Schottky-like barrier formed at the Au/LNFO interface was contributed to the resistive states switchover. It was related to the change in OVs located at the dissipative region near the Au/LNFO interface. The multilevel RS ability of LNFO-based devices in this work provides an opportunity for researching deeply on the high density RS memory in lead-free double perovskite oxides-based devices.

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Facile Preparation of SrZr1-xTixO3 and SrTi1-xZrxO3 Fine Particles Assisted by Dehydration of Zr4+ and Ti4+ Gels under Hydrothermal Conditions

In recent decades, perovskite-type compounds (ABB'O3) have been exhaustively studied due to their unique ferroelectric properties. In this work, a systematic study aiming to prepare fine particles in the binary system SrZrO3-SrTiO3 was conducted under hydrothermal conditions in a KOH (5 M) solution at 200 °C for 4 h under a constant stirring speed of 130 rpm. The precursors employed were SrSO4 powder (<38 μm size) and coprecipitated hydrous gels of Zr(OH)4•9.64 H2O (Zr-gel) and Ti(OH)4•4.5H2O (Ti-gel), which were mixed according to the stoichiometry of the SrZr1-xTixO3 in the compositional range of 0.0 > x > 100.0 mol% Ti4+. The XRD results revealed the formation of two crystalline phases rich in Zr4+, an orthorhombic structured SrZr0.93Ti0.07O3 and a cubic structured SrZr0.75Ti0.25O3 within the compositional range of 0.1-0.5 mol of Ti4+. A cubic perovskite structured solid solution, SrTi1-xZrxO3, was preferentially formed within the compositional range of 0.5 > x > 0.1 mol% Ti4+. The SrZrO3 and SrZr0.93Ti0.07O3-rich phases had particle sizes averaging 3 μm with a cubic morphology. However, a remarkable reduction in the particle size occurred on solid solutions prepared with hydrous Ti-gel over contents of 15 mol% Ti4+ in the reaction media, resulting in the formation of nanosized particle agglomerates with a cuboidal shape self-assembled via a 3D hierarchical architecture, and the sizes of these particles varied in the range between 141.0 and 175.5 nm. The limited coarsening of the particles is discussed based on the Zr-gel and Ti-gel dehydration capability differences that occurred under hydrothermal processing.

Magnetic and Magnetocaloric Properties of the A2LnSbO6 Lanthanide Oxides on the Frustrated fcc Lattice

Frustrated lanthanide oxides are promising candidates for cryogen-free magnetic refrigeration due to their suppressed ordering temperatures and high magnetic moments. While much attention has been paid to the garnet and pyrochlore lattices, the magnetocaloric effect in frustrated face-centered cubic (fcc) lattices remains relatively unexplored. We previously showed that the frustrated fcc double perovskite Ba₂GdSbO₆ is a top-performing magnetocaloric material (per mol Gd) because of its small nearest-neighbor interaction between spins. Here we investigate different tuning parameters to maximize the magnetocaloric effect in the family of fcc lanthanide oxides, A₂LnSbO₆ (A = {Ba²⁺, Sr²⁺} and Ln = {Nd³⁺, Tb³⁺, Gd³⁺, Ho³⁺, Dy³⁺, Er³⁺}), including chemical pressure via the A site cation and the magnetic ground state via the lanthanide ion. Bulk magnetic measurements indicate a possible trend between magnetic short-range fluctuations and the field-temperature phase space of the magnetocaloric effect, determined by whether an ion is a Kramers or a non-Kramers ion. We report for the first time on the synthesis and magnetic characterization of the Ca₂LnSbO₆ series with tunable site disorder that can be used to control the deviations from Curie-Weiss behavior. Taken together, these results suggest fcc lanthanide oxides as tunable systems for magnetocaloric design.

Structural, magnetic and optical properties of disordered double perovskite Gd2CoCrO6 nanoparticles

We have synthesized disordered double perovskite Gd2CoCrO6 (GCCO) nanoparticles with an average particle size of 71 ± 3 nm by adopting a citrate sol-gel method to investigate their structural, magnetic, and optical properties. Rietveld refinement of the X-ray diffraction pattern showed that GCCO is crystallized in a monoclinic structure with space group P21/n, which is further confirmed by Raman spectroscopic analysis. The absence of perfect long-range ordering between Co and Cr ions is confirmed by the mixed valence states of Co and Cr. A Néel transition was observed at a higher temperature of TN = 105 K compared to that of an analogous double perovskite Gd2FeCrO6 due to a greater degree of magnetocrystalline anisotropy of Co than Fe. Magnetization reversal (MR) behavior with a compensation temperature of Tcomp = 30 K was also observed. The hysteresis loop obtained at 5 K exhibited the presence of both ferromagnetic (FM) and antiferromagnetic (AFM) domains. Super-exchange and Dzyaloshinskii-Moriya (DM) interactions between various cations via oxygen ligands are responsible for the observed FM or AFM ordering in the system. Furthermore, UV-visible and photoluminescence spectroscopy demonstrated the semiconducting nature of GCCO with a direct optical bandgap of 2.25 eV. The Mulliken electronegativity approach revealed the potential applicability of GCCO nanoparticles in photocatalytic H2 and O2 evolution from water. Due to a favorable bandgap and potentiality as a photocatalyst, GCCO can be a promising new member of double perovskite materials for photocatalytic and related solar energy applications.