Composition-driven structural phase transitions in rare-earth-doped BiFeO3 ceramics: a review

Theoretical Justification of Structural, Magnetoelectronic and Optical Properties in QFeO3 (Q = Bi, P, Sb): A First-Principles Study

One of the primary objectives of scientific research is to create state-of-the-art multiferroic (MF) materials that exhibit interconnected properties, such as piezoelectricity, magnetoelectricity, and magnetostriction, and remain functional under normal ambient temperature conditions. In this study, we employed first-principles calculations to investigate how changing pnictogen elements affect the structural, electronic, magnetic, and optical characteristics of QFeO3 (Q = Bi, P, SB). Electronic band structures reveal that BiFeO3 is a semiconductor compound; however, PFeO3 and SbFeO3 are metallic. The studied compounds are promising for spintronics, as they exhibit excellent magnetic properties. The calculated magnetic moments decreased as we replaced Bi with SB and P in BiFeO3. A red shift in the values of ε2(ω) was evident from the presented spectra as we substituted Bi with Sb and P in BiFeO3. QFeO3 (Q = Bi, P, SB) showed the maximum absorption of incident photons in the visible region. The results obtained from calculating the optical parameters suggest that these materials have a strong potential to be used in photovoltaic applications.

Enhanced Oxygen Storage Capacity of Porous CeO2 by Rare Earth Doping

CeO2 is an important rare earth (RE) oxide and has served as a typical oxygen storage material in practical applications. In the present study, the oxygen storage capacity (OSC) of CeO2 was enhanced by doping with other rare earth ions (RE, RE = Yb, Y, Sm and La). A series of Undoped and RE-doped CeO2 with different doping levels were synthesized using a solvothermal method following a subsequent calcination process, in which just Ce(NO3)3∙6H2O, RE(NO3)3∙nH2O, ethylene glycol and water were used as raw materials. Surprisingly, the Undoped CeO2 was proved to be a porous material with a multilayered special morphology without any additional templates in this work. The lattice parameters of CeO2 were refined by the least-squares method with highly pure NaCl as the internal standard for peak position calibrations, and the solubility limits of RE ions into CeO2 were determined; the amounts of reducible-reoxidizable Cen+ ions were estimated by fitting the Ce 3d core-levels XPS spectra; the non-stoichiometric oxygen vacancy (VO) defects of CeO2 were analyzed qualitatively and quantitatively by O 1s XPS fitting and Raman scattering; and the OSC was quantified by the amount of H2 consumption per gram of CeO2 based on hydrogen temperature programmed reduction (H2-TPR) measurements. The maximum [OSC] of CeO2 appeared at 5 mol.% Yb-, 4 mol.% Y-, 4 mol.% Sm- and 7 mol.% La-doping with the values of 0.444, 0.387, 0.352 and 0.380 mmol H2/g by an increase of 93.04, 68.26, 53.04 and 65.22%. Moreover, the dominant factor for promoting the OSC of RE-doped CeO2 was analyzed.

Origin of Ferroelectricity in BiFeO3-Based Solid Solutions

We investigate the origin of ferroelectricity in the BiFeO₃-LaFeO₃ system in rhombohedral R3c and tetragonal P4mm symmetries by ab initio density functional theory calculations and compare their electronic features with paraelectric orthorhombic Pnma symmetry. We show that a coherent accommodation of stereo-active lone pair electrons of Bi is the detrimental factor of ferroelectricity. A Bloch function arising from an indirect Bi_6p-Fe_3d hybridization mediated through O_2p is the primary origin of spontaneous polarization (P_s) in the rhombohedral system. In the orthorhombic system, a similar Bloch function was found, whereas a staggered accommodation of stereo-active lone pair electrons of Bi exclusively results in paraelectricity. A giant P_s reported in the tetragonal system originates from an orbital hybridization of Bi_6p and O_2p, where Fe-3d plays a minor role. The P_s in the rhombohedral system decreases with increasing La content, while that in the tetragonal system displays a discontinuous drop at a certain La content. We discuss the electronic factors affecting the P_s evolutions with La content.

Rare-earth doped BiFe0.95Mn0.05O3 nanoparticles for potential hyperthermia applications

Ionic engineering is exploited to substitute Bi cations in BiFe0.95Mn0.05O3 NPs (BFM) with rare-earth (RE) elements (Nd, Gd, and Dy). The sol-gel synthesized RE-NPs are tested for their magnetic hyperthermia potential. RE-dopants alter the morphology of BFM NPs from elliptical to rectangular to irregular hexagonal for Nd, Gd, and Dy doping, respectively. The RE-BFM NPs are ferroelectric and show larger piezoresponse than the pristine BFO NPs. There is an increase of the maximum magnetization at 300 K of BFM up to 550% by introducing Gd. In hyperthermia tests, 3 mg/ml dispersion of NPs in water and agar could increase the temperature of the dispersion up to ∼39°C under an applied AC magnetic field of 80 mT. Although Gd doping generates the highest increment in magnetization of BFM NPs, the Dy-BFM NPs show the best hyperthermia results. These findings show that RE-doped BFO NPs are promising for hyperthermia and other biomedical applications.

The Effect of Bi2O3 and Fe2O3 Impurity Phases in BiFeO3 Perovskite Materials on Some Electrical Properties in the Low-Frequency Field

Pure bismuth ferrite (BFO) and BFO with impurity phases (Bi₂O₃ or Fe₂O₃) were synthesized by the hydrothermal method. Complex dielectric permittivity (ε) and electrical conductivity (σ) were determined by complex impedance measurements at different frequencies (200 Hz–2 MHz) and temperatures (25–290) °C. The conductivity spectrum of samples, σ(f), complies with Jonscher’s universal law and the presence of impurity phases leads to a decrease in the static conductivity (σ_DC); this result is correlated with the increased thermal activation energy of the conduction in impure samples compared to the pure BFO sample. The conduction mechanism in BFO and the effect of impurity phases on σ and ε were analyzed considering the variable range hopping model (VRH). Based on the VRH model, the hopping length (R_h), hopping energy (W_h) and the density of states at the Fermi level (N(E_F)) were determined for the first time, for these samples. In addition, from ε(T) dependence, a transition in the electronic structure of samples from a semiconductor-like to a conductor-like behavior was highlighted around 465–490 K for all samples. The results obtained are useful to explain the conduction mechanisms from samples of BFO type, offering the possibility to develop a great variety of electrical devices with novel functions.

Neutron powder-diffraction study of phase transitions in strontium-doped bismuth ferrite: 1. Variation with chemical composition

We report results from a study of the crystal and magnetic structures of strontium-doped BiFeO₃using neutron powder diffraction and the Rietveld method. Measurements were obtained over a wide range of temperatures from 300-800 K for compositions between 10%-16% replacement of bismuth by strontium. The results show a clear variation of the two main structural deformations-symmetry-breaking rotations of the FeO₆octahedra and polar ionic displacements that give ferroelectricity-with chemical composition, but relatively little variation with temperature. On the other hand, the antiferromagnetic order shows a variation with temperature and a second-order phase transition consistent with the classical Heisenberg model. There is, however, very little variation in the behaviour of the antiferromagnetism with chemical composition, and hence with the degree of the structural symmetry-breaking distortions. We therefore conclude that there is no significant coupling between antiferromagnetism and ferroelectricity in Sr-doped BiFeO₃and, by extension, in pure BiFeO₃.

Structural, electrical, and magnetic study of La-, Eu-, and Er- doped bismuth ferrite nanomaterials obtained by solution combustion synthesis

In this work, the multiferroic bismuth ferrite materials Bi_0.9RE_0.1FeO₃ doped by rare-earth (RE = La, Eu, and Er) elements were obtained by the solution combustion synthesis. Structure, electrical, and magnetic properties of prepared samples were investigated by X-ray photoelectron spectroscopy, Mössbauer spectroscopy, electrical hysteresis measurement, broadband dielectric spectroscopy, and SQUID magnetometry. All obtained nanomaterials are characterized by spontaneous electrical polarization, which confirmed their ferroelectric properties. Investigation of magnetic properties at 300.0 K and 2.0 K showed that all investigated Bi_0.9RE_0.1FeO₃ ferrites possess significantly higher magnetization in comparison to bismuth ferrites obtained by different methods. The highest saturation magnetisation of 5.161 emu/g at 300.0 K was observed for the BLaFO sample, while at 2.0 K it was 12.07 emu/g for the BErFO sample. Several possible reasons for these phenomena were proposed and discussed.

Magnetic Behaviour of Perovskite Compositions Derived from BiFeO3

The phase content and sequence, the crystal structure, and the magnetic properties of perovskite solid solutions of the (1−y)BiFeO3–yBiZn0.5Ti0.5O3 series (0.05 ≤ y ≤ 0.90) synthesized under high pressure have been studied. Two perovskite phases, namely the rhombohedral R3c and the tetragonal P4mm, which correspond to the structural types of the end members, BiFeO3 and BiZn0.5Ti0.5O3, respectively, were revealed in the as-synthesized samples. The rhombohedral and the tetragonal phases were found to coexist in the compositional range of 0.30 ≤ y ≤ 0.90. Magnetic properties of the BiFe1−y[Zn0.5Ti0.5]yO3 ceramics with y < 0.30 were measured as a function of temperature. The obtained compositional variations of the normalized unit-cell volume and the Néel temperature of the BiFe1−y[Zn0.5Ti0.5]yO3 perovskites in the range of their rhombohedral phase were compared with the respective dependences for the BiFe1−yB3+yO3 perovskites (where B3+ = Ga, Co, Mn, Cr, and Sc). The role of the high-pressure synthesis in the formation of the antiferromagnetic states different from the modulated cycloidal one characteristic of the parent BiFeO3 is discussed.

Structural, dielectric, magnetic and optical properties of double perovskite oxide Sm2NiMnO6nanoparticles synthesized by a sol-gel process

Here we report on the structural, dielectric, magnetic and optical properties of double perovskite Sm₂NiMnO₆(SNMO) nanoparticles synthesized by a sol-gel method. Structural Reitveld refinements on x-ray powder diffraction data revealed that the SNMO nanoparticles crystallized in a monoclinic crystal structure withP2₁/nspace group. SEM and (HR)TEM images revealed the phase purity and single-crystalline nature of the SNMO nanoparticles. XPS spectra confirmed the presence of Sm³⁺, Ni²⁺and Mn⁴⁺ions in the SNMO nanoparticles and oxygen in the forms of lattice oxygen and the hydroxyls species. SNMO ceramics exhibited relaxor-type dielectric behavior, well fitted by modified Curie-Weiss law. Such dielectric behavior originated from the interactions of random dipoles arisen from the B-site cations disorder accompanied with the variations in local electric fields and local strain fields due to the different radii of B-site cations, and/or the virtual electrons hopping between the Ni²⁺and Mn⁴⁺cations. Magnetic data demonstrate the variations of the magnetic transitions at low temperatures and the spin glass-like behavior below 11 K, which is attributed to the spin fluctuations induced by the competing interactions between the ferromagnetic (FM) and antiferromagnetic phases. Large positive Curie-Weiss temperature (θ_p) indicates the dominant FM super-exchange interactions in the SNMO samples. The SNMO nanoparticles have a direct optical band gap of 1.42 eV, close to 1.34 eV in a single junction solar cell. That enables the SNMO nanoparticles to be useful for solar cell absorbers.

Comparative density functional studies of pristine and doped bismuth ferrite polymorphs by GGA+U and meta-GGA SCAN+U

We analyzed the effect of nonmagnetic dopants Al, Ga, Sc, and In at the Fe-site on the phase stability, structural, and electronic properties of different bismuth ferrite (BiFeO₃) polymorphs in the framework of density functional theory with the Hubbard U correction (DFT+U). We started our consideration from the determination of the magnitude of the U parameter. First, we calculated the structural, electronic, and magnetic properties of the rhombohedral R3c-G phase of BiFeO₃ within the generalized gradient approximation (GGA) and strongly constrained and appropriately normed (SCAN) meta-GGA for different values of the U. Next, we compared these results with those obtained within the parameter-free hybrid functional. After determining the optimal values of the Hubbard U parameter we analyzed the total energies between the selected BiFeO₃ polymorphs without and with dopants within both GGA+U and SCAN+U. For all dopants the concentration was 12.5% which was close to their solubility limit in BiFeO₃ under ambient conditions. We found that none of these dopants led to the structural phase transition. However, DFT+U calculations revealed that the doping of BiFeO₃ with Al and Ga reduced the energy barrier between R3c-G and Cm-C phases whereas for Sc and In the energy difference between both phases increased. For the orthorhombic phases the considered dopants do not affect the energy barrier between them and the rhombohedral phase. In addition, the ferroelectric polarization does not change after replacing the Fe atom by the dopant for the all considered BiFeO₃ polymorphs.

Nanoscale Piezoelectric Properties and Phase Separation in Pure and La-Doped BiFeO3 Films Prepared by Sol-Gel Method

Pure BiFeO₃ (BFO) and doped Bi_0.9La_0.1FeO₃ (BLFO) thin films were prepared on Pt/TiO₂/SiO₂/Si substrates by a modified sol–gel technique using a separate hydrolysis procedure. The effects of final crystallization temperature and La doping on the phase structure, film morphology, and nanoscale piezoelectric properties were investigated. La doping and higher crystallization temperature lead to an increase in the grain size and preferred (102) texture of the films. Simultaneously, a decrease in the average effective piezoelectric coefficient (about 2 times in La-doped films) and an increase in the area of surface non-polar phase (up to 60%) are observed. Phase separation on the films’ surface is attributed to either a second phase or to a non-polar perovskite phase at the surface. As compared with undoped BFO, La-doping leads to an increase in the average grain size and self-polarization that is important for future piezoelectric applications. It is shown that piezoelectric activity is directly related to the films’ microstructructure, thus emphasizing the role of annealing conditions and La-doping that is frequently used to decrease the leakage current in BFO-based materials.

Rietveld Study of the Changes of Phase Composition, Crystal Structure, and Morphology of BiFeO3 by Partial Substitution of Bismuth with Rare-Earth Ions

BiFeO3 is an interesting material due to its multiferroic properties. It attracts attention due to its potential applications in spintronics and in microelectronics for data storage, among others. Single-phase bulk material from BiFeO3 is difficult to synthesize. The kinetics of perovskite phase formation most often leads to the presence of impurity phases. It has been shown that low levels of replacement of Bi with rare earth ions lead to stabilization of the perovskite phase. In the present work, Rietveld refinement of the crystal structure based on powder X-ray diffraction patterns was applied to study the influence of partial substitution of Bi by rare-earth (RE) elements with different ionic radii on structural and morphological properties of the ferrite phase. Substitution by large RE ions was found to preserve the rhombohedral symmetry of BiFeO3, whereas substitution by smaller RE ions led to the coexistence of two polymorphic perovskite phases with rhombohedral R3c and orthorhombic Pnma symmetries. The unit cell parameters as well as the interatomic distances and angles, not only around the A cation but also around the iron ions, were influenced by the substitution. The mean crystallite and particle size decreased with the decrease of ionic radius of substituting RE ion.

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn 2Ti2NbO10 Dion-Jacobson Phases

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n = 2 systems and from proper mechanisms in n = 3 CsBi2Ti2NbO10. We report here a combined experimental and computational study on analogous n = 3 CsLn 2Ti2NbO10 (Ln = La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.

Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite

The polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal-insulator or paramagnet-ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO₃of 40 nm could be transformed to AFM SrCoO_2.5with ordered oxygen vacancy planes either vertical (V-SrCoO_2.5) or parallel (P-SrCoO_2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO_2.5are along [010] and11¯0, respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO₃and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.

Dielectric analysis and electrical conduction mechanism of La1-x Bi x FeO3 ceramics

Bulk-phase polycrystalline La_1−xBi_xFeO₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by citric sol–gel and sintering methods. The structural, morphological, and electrical properties of the resulting sol–gel solutions were investigated using various techniques. In an X-ray diffraction analysis, all samples crystallized in the orthorhombic structure with the Pbnm space group and showed an increase in lattice constant with increasing Bi content which was also confirmed by vibrational analysis. The sample surfaces and average grain sizes were examined by scanning electron microscopy. The grain distribution was non-uniform and the grain size increased with the increasing Bi content. The complex electrical conductivities and dielectric analyses of these materials were investigated as functions of frequency by impedance spectroscopy at various temperatures (75–200 °C). The frequency-dependent dielectric constant at each temperature increased with increasing Bi content. A Jonscher's power law analysis revealed that the AC and DC conductivities arose by completely different mechanisms. The temperature dependence and dielectric relaxation of the DC conductivity satisfied the Arrhenius law and decreased with increasing Bi content. The activation energy ranged from 0.20 to 0.45 eV and was similar in the conduction and relaxation mechanisms, indicating that both transport mechanisms were based on hopping phenomena. We believe that lowering the activation energy will help with the optimization of constituents as promising candidates in novel materials for future electrocatalysts.

Bulk-phase polycrystalline La_1−xBi_xFeO₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by citric sol–gel and sintering methods.

Peculiarities of the Crystal Structure Evolution of BiFeO3-BaTiO3 Ceramics across Structural Phase Transitions

Evolution of the crystal structure of ceramics BiFeO₃–BaTiO₃ across the morphotropic phase boundary was analyzed using the results of macroscopic measuring techniques such as X-ray diffraction, differential scanning calorimetry, and differential thermal analysis, as well as the data obtained by local scale methods of scanning probe microscopy. The obtained results allowed to specify the concentration and temperature regions of the single phase and phase coexistent regions as well as to clarify a modification of the structural parameters across the rhombohedral–cubic phase boundary. The structural data show unexpected strengthening of structural distortion specific for the rhombohedral phase, which occurs upon dopant concentration and temperature-driven phase transitions to the cubic phase. The obtained results point to the non-monotonous character of the phase evolution, which is specific for metastable phases. The compounds with metastable structural state are characterized by enhanced sensitivity to external stimuli, which significantly expands the perspectives of their particular use.

Enhanced electromagnon excitations in Nd-doped BiFeO3 nanoparticles near morphotropic phase boundaries

In multiferroics, electromagnons have been recognized as a noticeable topic due to their indispensable role in magnetoelectric, magnetodielectric, and magnetocapacitance effects. Here, the electromagnons of Bi_1-xNd_xFeO₃ (x = 0-0.2) nanoparticles are studied via terahertz time-domain spectroscopy, and the impacts of doping concentrations on electromagnons have been discussed. We found that the electromagnons in Bi_1-xNd_xFeO₃ nanoparticles are associated with their phase transition. The total coupling weight of electromagnons is gradually increased in polar R3c structures and then reduces in the antipolar Pbam phase, and the weight in the antipolar phase is less than that of the pure R3c phase. Interestingly, a colossal electromagnon is observed at polar-antipolar and antiferromagnetic-ferromagnetic phase boundaries. Our work offers an avenue for designing and choosing materials with better magnetodielectric and magnetocapacitance properties.

Ferromagnetic-like behavior of Bi0.9La0.1FeO3-KBr nanocomposites

We studied magnetostatic response of the Bi_0.9La_0.1FeO₃– KBr composites (BLFO-KBr) consisting of nanosized (≈100 nm) ferrite Bi_0.9La_0.1FeO₃ (BLFO) conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is rather small (less than 15 wt%) the magnetic response of the composite is very weak and similar to that observed for the BLFO (pure KBr matrix without Bi_1-xLa_xFeO₃ has no magnetic response as anticipated). However, when the fraction of KBr increases above 15%, the magnetic response of the composite changes substantially and the field dependence of magnetization reveals ferromagnetic-like hysteresis loop with a remanent magnetization about 0.14 emu/g and coercive field about 1.8 Tesla (at room temperature). Nothing similar to the ferromagnetic-like hysteresis loop can be observed in Bi_1-zLa_zFeO₃ ceramics with z ≤ 0.15, which magnetization quasi-linearly increases with magnetic field. Different physical mechanisms were considered to explain the unusual experimental results for BLFO-KBr nanocomposites, but only those among them, which are highly sensitive to the interaction of antiferromagnetic Bi_0.9La_0.1FeO₃ with ionic conductor KBr, can be relevant.

Local insight into the La-induced structural phase transition in multiferroic BiFeO3 ceramics by x-ray absorption fine structure spectroscopy

Substitution of bismuth by rare earth (RE) ions is of great technological importance to develop room-temperature BiFeO₃-based multiferroic materials. Despite this interest, many fundamental properties and the structure-property correlations of RE-doped BiFeO₃ remain poorly understood. Here we report a systematical experimental and theoretical exploration on the structural phase transition in Bi_1-x La _x FeO₃ (0  ⩽  x  ⩽  0.2) ceramics. By using x-ray absorption fine structure spectroscopy, we for the first time show that the La³⁺ dopants in fact substitute the Bi site of secondary nanosized particles with orthorhombic Pbam symmetry instead of the long-believed parental rhombohedral R3c phase at all La³⁺ doping concentrations (0.001  ⩽  x  ⩽  0.2). This homogeneously mixed two-phase compound cannot be detected by the x-ray diffraction until La content approaching x  =  0.1. The finding is further supported by complementary studies of transmission electron microscopy and thermodynamic preference, and it casts serious challenges on the prevailing assumption of La³⁺ substitution on the Bi³⁺ site in R3c structure when x  ⩽  0.1 as well as the previously proposed origin of enhanced functional properties based on morphotropic phase boundary. This new insight may ignite a revival on exploring the underlying multiferroic mechanisms in BiFeO₃-based materials and facilitate the bottom-up design of novel multifunctional devices.

Dielectric, Ferroelectric, and Magnetic Properties of Sm-Doped BiFeO₃ Ceramics Prepared by a Modified Solid-State-Reaction Method

Sm-doped BiFeO₃ (BFO) material was prepared using a modified solid-state-reaction method, which used fast heating and cooling during the sintering process. The Sm doping level varied between 1 mol % to 8 mol %. Processing parameters, such as sintering temperature and annealing temperature, were optimized to obtain high-quality samples. Based on their dielectric properties, the optimum sintering and annealing temperatures were found to be 300 °C and 825 °C, respectively. Leakage-free square-shaped ferroelectric hysteresis loops were observed in all samples. The remnant polarization was maximized in the 5 mol %-doped sample (~35 μC/cm²⁾. Furthermore, remnant magnetization was increased after the Sm doping and the 8 mol%-doped sample possessed the largest remnant magnetization of 0.007 emu/g. Our results demonstrated how the modified solid-state-reaction method proved to be an effective method for preparing high-quality BiFeO₃ ceramics, as well as how the Sm dopant can efficiently improve ferroelectric and magnetic properties.

Enhancing ferroelectric photovoltaic effect by polar order engineering

Ferroelectric materials for photovoltaics have sparked great interest because of their switchable photoelectric responses and above-bandgap photovoltages that violate conventional photovoltaic theory. However, their relatively low photocurrent and power conversion efficiency limit their potential application in solar cells. To improve performance, conventional strategies focus mainly on narrowing the bandgap to better match the solar spectrum, leaving the fundamental connection between polar order and photovoltaic effect largely overlooked. We report large photovoltaic enhancement by A-site substitutions in a model ferroelectric photovoltaic material, BiFeO₃. As revealed by optical measurements and supported by theoretical calculations, the enhancement is accompanied by the chemically driven rotational instability of the polarization, which, in turn, affects the charge transfer at the band edges and drives a direct-to-indirect bandgap transition, highlighting the strong coupling between polarization, lattice, and orbital order parameters in ferroelectrics. Polar order engineering thus provides an additional degree of freedom to further boost photovoltaic efficiency in ferroelectrics and related materials.

Strain-Driven Nanoscale Phase Competition near the Antipolar-Nonpolar Phase Boundary in Bi0.7La0.3FeO3 Thin Films

Complex-oxide materials tuned to be near phase boundaries via chemistry/composition, temperature, pressure, etc. are known to exhibit large susceptibilities. Here, we observe a strain-driven nanoscale phase competition in epitaxially constrained Bi_0.7La_0.3FeO₃ thin films near the antipolar-nonpolar phase boundary and explore the evolution of the structural, dielectric, (anti)ferroelectric, and magnetic properties with strain. We find that compressive and tensile strains can stabilize an antipolar PbZrO₃-like Pbam phase and a nonpolar Pnma orthorhombic phase, respectively. Heterostructures grown with little to no strain exhibit a self-assembled nanoscale mixture of the two orthorhombic phases, wherein the relative fraction of each phase can be modified with film thickness. Subsequent investigation of the dielectric and (anti)ferroelectric properties reveals an electric-field-driven phase transformation from the nonpolar phase to the antipolar phase. X-ray linear dichroism reveals that the antiferromagnetic-spin axes can be effectively modified by the strain-induced phase transition. This evolution of antiferromagnetic-spin axes can be leveraged in exchange coupling between the antiferromagnetic Bi_0.7La_0.3FeO₃ and a ferromagnetic Co_0.9Fe_0.1 layer to tune the ferromagnetic easy axis of the Co_0.9Fe_0.1. These results demonstrate that besides chemical alloying, epitaxial strain is an alternative and effective way to modify subtle phase relations and tune physical properties in rare earth-alloyed BiFeO₃. Furthermore, the observation of antiferroelectric-antiferromagnetic properties in the Pbam Bi_0.7La_0.3FeO₃ phase could be of significant scientific interest and great potential in magnetoelectric devices because of its dual antiferroic nature.

Perovskite BiFeO3 thin film photocathode performance with visible light activity

Perovskite materials are now an important class of materials in the application areas of photovoltaics and photocatalysis. Inorganic perovskites such as BiFeO3 (BFO) are promising photocatalyst materials with visible light activity and inherent stability. Here we report the large area sol-gel synthesis of BFO films for solar stimulated water photo oxidation. By modifying the sol-gel synthesis process we have produced a perovskite material that has p-type behaviour and a flat band potential of ∼1.15 V (versus NHE). The photocathode produces a density of -0.004 mA cm(-2) at 0 V versus NHE under AM1.5 G illumination. We further show that 0.6 μmol h(-1) of O2 was produced at an external bias of -0.5 V versus Ag/AgCl. The addition of a non-percolating conducting network of Ag increases the photocurrent to -0.07 mA cm(-2) at 0 V versus NHE (at 2% Ag loading) with an increase to 2.7 μmol h(-1) for O2 production. We attribute the enhancement in photoelectrochemical performance to increased light absorption due light scattering by the incorporated Ag particles, improved charge transfer kinetics at the Ag/BFO interface and reduced over potential losses. We support these claims by an observed shift in flat band and onset potentials after Ag modification through UV-vis spectroscopy, Mott-Schottky plots and j-v curve analysis.

Substitution driven structural and magnetic properties and evidence of spin phonon coupling in Sr-doped BiFeO3 nanoparticles

The manifestation of dimensionalities and Sr induced modifications in structural, vibrational and magnetic properties of Bi_1−xSr_xFeO₃; (x = 0–0.25) nanoparticles synthesized by a tartaric acid based sol–gel route are reported.

PZT-like structural phase transitions in the BiFeO3–KNbO3 solid solution

The solid solution between bismuth ferrite and potassium niobate indicates a similar series of phase transitions to PZT.