Exotic Magnetism in Perovskite KOsO_{3}

A new perovskite KOsO_{3} has been stabilized under high-pressure and high-temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at 230 K as shown from refining synchrotron x-ray powder diffraction (SXRD) data. The larger orbital overlap integral and the extended wave function of 5d electrons in the perovskite KOsO_{3} allow to explore physics from the regime where Mott and Hund's rule couplings dominate to the state where the multiple interactions are on equal footing. We demonstrate an exotic magnetic ordering phase found by neutron powder diffraction along with physical properties via a suite of measurements including magnetic and transport properties, differential scanning calorimetry, and specific heat, which provide comprehensive information for a system at the crossover from localized to itinerant electronic behavior.

Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 was attributed to the dynamic Jahn-Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed.

Anisotropic Thermal Expansion and a Second-Order Charge Order Transition in the Ferrimagnetic Dy2CuZnMn4O12 Perovskite with Triple A-Site Cation Ordering

Dy2CuZnMn4O12 perovskite, belonging to the A-site columnar-ordered quadruple perovskite family with the general composition of A2A'A″B4O12, was prepared by a high-pressure, high-temperature method at 6 GPa and 1500 K. Its crystal structure was studied by synchrotron powder X-ray diffraction between 100 and 800 K. The ideal cation distribution (without antisite disorder) was found to be realized within the sensitivity of the synchrotron X-ray diffraction method. Between 100 and 400 K, it crystallizes in space group Pmmn (no. 59) and has layered charge ordering of Mn3+ and Mn4+ at the B sites. Above 425 K, it crystallizes in space group P42/nmc (no. 137) with one crystallographic B site and an average Mn3.5+ oxidation state. The charge ordering transition (at TCO = 425 K) appears to be of the second order as no anomalies were found on differential scanning calorimetry curves and temperature dependence of the unit cell volume, and the orthorhombic a and b lattice parameters merge gradually. The compound demonstrates anisotropic thermal expansion with the c lattice parameter decreasing with increasing temperature above 280 K. A ferrimagnetic transition occurs at TC = 116 K with an additional, gradual rise of magnetic susceptibilities below 45 K, probably due to increases of the ordered moments of the Dy sublattices.

Cd2FeReO6: A High-TC Double Perovskite Oxide with Remarkable Tunneling Magnetoresistance

In this study, we successfully synthesized the double perovskite oxide Cd2FeReO6 by using a high-temperature and high-pressure method. The crystal structure was confirmed to belong to the P21/n space group, exhibiting approximately 68% ordering of Fe3+ and Re5+ ions at the perovskite B-site with the remaining regions showing antisite disorder. The measured Curie temperature of Cd2FeReO6 was 460 K, slightly lower than expected but still significantly above room temperature. Remarkably, Cd2FeReO6 displayed a remarkable low-field butterfly type tunneling magnetoresistance of -23% (-37% between the lowest and the largest values) at 5 K and 90 kOe, the highest among the A2FeReO6 (A = Ca, Sr, Pb, Ba) family. First-principles calculations provided insight into the origin of this observed magnetoresistance behavior, revealing Cd2FeReO6's half-metallic ferrimagnetic nature. This research extends our understanding of the double perovskite family and emphasizes its potential significance in the domains of spintronics and materials science. The exploration of differing magnetoresistance behaviors between Cd2FeReO6 and Ca2FeReO6, along with the influence of antisite disorder in Cd2FeReO6, opens intriguing avenues for further research.

Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4O12

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi4O12. This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below TN~12 K, Mn2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice.

Ferrimagnetic Ordering and Spin-Glass State in Diluted GdFeO3-Type Perovskites (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75

ABO₃ perovskite materials with small cations at the A site, especially those with ordered cation arrangements, have attracted a great deal of interest because they show unusual physical properties and deviations from the general characteristics of perovskites. In this work, perovskite solid solutions (Lu_0.5Mn_0.5)(Mn_1-xTi_x)O₃ with x = 0.25, 0.50, and 0.75 were synthesized by means of a high-pressure, high-temperature method at approximately 6 GPa and approximately 1550 K. All the samples crystallize in the GdFeO₃-type perovskite structure (space group Pnma) and have random distributions of the small Lu³⁺ and Mn²⁺ cations at the A site and Mn^4+/3+/2+ and Ti⁴⁺ cations at the B site, as determined by Rietveld analysis of high-quality synchrotron X-ray powder diffraction data. Lattice parameters are a = 5.4431 Å, b = 7.4358 Å, c = 5.1872 Å (for x = 0.25); a = 5.4872 Å, b = 7.4863 Å, c = 5.2027 Å (for x = 0.50); and a = 5.4772 Å, b = 7.6027 Å, c = 5.2340 Å (for x = 0.75). Despite a significant dilution of the A and B sublattices by non-magnetic Ti⁴⁺ cations, the x = 0.25 and 0.50 samples show long-range ferrimagnetic order below T_C = 89 K and 36 K, respectively. Mn cations at both A and B sublattices are involved in the long-range magnetic order. The x = 0.75 sample shows a spin-glass transition at T_SG = 6 K and a large frustration index of approximately 22. A temperature-independent dielectric constant was observed for x = 0.50 (approximately 32 between 5 and 150 K) and for x = 0.75 (approximately 50 between 5 and 250 K).

Abnormal Eu3+ → Eu2+ Reduction in Ca9-xMnxEu(PO4)7 Phosphors: Structure and Luminescent Properties

β-Ca₃(PO₄)₂-type phosphors Ca_9-xMn_xEu(PO₄)₇ have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca₈MnEu(PO₄)₇ was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu³⁺ → Eu²⁺ in air was observed in Ca_9-xMn_xEu(PO₄)₇ according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu³⁺ shows partial reduction and coexistence of Eu²⁺/³⁺ states. It reflects in combination of a broad band from the Eu²⁺ 4f⁶5d¹ → 4f⁷ transition and a series of sharp lines attributed to ⁵D₀ → ⁷F_J transitions of Eu³⁺. Eu²⁺/Eu³⁺ ions are redistributed among two crystal sites, M1 and M3, while Mn²⁺ fully occupies octahedral site M5 in Ca₈MnEu(PO₄)₇. The main emission band was attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ at 395 nm excitation. The abnormal quenching of Eu³⁺ emission was observed in Ca_9-xMn_xEu(PO₄)₇ phosphors with doping of the host by Mn²⁺ ions. The phenomena of abnormal reduction and quenching were discussed in detail.

K5Eu(MoO4)4 red phosphor for solid state lighting applications, prepared by different techniques

The influence of preparation techniques on the structure and luminescent properties of K5Eu(MoO4)4 (KEMO) was investigated. KEMO phosphors were synthesized by three different techniques: solid state and sol-gel (sg) methods...

High-Pressure Synthesis and Magnetic and Electrical Properties of Fe-Doped Bi3Re3O11 and Bi3Os3O11

Under high-pressure and high-temperature conditions, doped Bi₃Re₃O₁₁ and Bi₃Os₃O₁₁ with Fe up to 29 atomic % were synthesized. The crystal structures and chemical compositions of Bi₃Os_2.45Fe_0.55O₁₁ and Bi₃Re_2.13Fe_0.87O₁₁ were determined by synchrotron powder X-ray diffraction and electron probe microanalysis. Both crystal structures were explained by a KSbO₃-type model with the space group Pn3̅. Magnetic and electronic transport property measurements showed that Bi₃Os_2.45Fe_0.55O₁₁ exhibited a ferrimagnetic transition at the highest magnetic ordering temperature of 490 K in the KSbO₃-type, while Bi₃Re_2.13Fe_0.87O₁₁ exhibited a spin glassy behavior below 22 K. The magnetoresistance at 5 K and 90 kOe was almost zero for Bi₃Os_2.45Fe_0.55O₁₁, but -10% for Bi₃Re_2.13Fe_0.87O₁₁. These results suggest that KSbO₃- type 5d oxides, which exhibit only weak temperature-dependent paramagnetism to date, are a group of compounds that can be converted into spintronic materials by doping with 3d elements, leading to the development of new KSbO₃-type materials with both theoretical and practical significance.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12

Perovskite-type ABO₃ oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm₂CuMn(MnTi₃)O₁₂, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P4₂/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with T_SG = 7 K, while specific heat, in the form of C_p/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm₂CuMn(MnTi₃)O₁₂ was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu²⁺ doping drastically modified the magnetic and dielectric properties of Sm₂CuMn(MnTi₃)O₁₂ in comparison with the parent compound Sm₂MnMn(MnTi₃)O₁₂, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu²⁺ and Mn²⁺ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm₂CuMn(MnTi₃)O₁₂.

Crystal structure of the cubic double-perovskite Sr2Cr0.84Ni0.09Os1.07O6

The crystal structure of the cubic double-perovskite Sr2Cr0.84Ni0.09Os1.07O6, grown at high pressure, was solved using intensity data measured at 113 K. The Os site was modelled with a partial Ni occupancy, and the Cr site was modelled with both Os and Ni partial occupancy. The refined structure shows that this cubic form is stable at 113 K.

Triple A-Site Cation Ordering in the Ferrimagnetic Y2CuGaMn4O12 Perovskite

A new member of A-site columnar-ordered A₂A′A″B₄O₁₂ quadruple perovskites with the composition of Y₂CuGaMn₄O₁₂ was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y₂]_A[Cu_0.8Mn_0.2]_A′[Ga_0.8Mn_0.2]_A″[Mn_3.6Cu_0.2Ga_0.2]_BO₁₂. It has the space group P4₂/nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at T_C = 115 K with the ordered moment of 2.19 μ_B at each B site and 0.89 μ_B at the A′ or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A′ or A″ site are ordered in the opposite direction. Cu²⁺ doping drastically changes magnetic properties as “parent” Y₂MnGaMn₄O₁₂ just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y₂CuGaMn₄O₁₂: the lattice parameter a almost linearly decreases from 1.5 K to T_C and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

A new member of the A-site columnar-ordered quadruple perovskite A₂A′A″B₄O₁₂ family, Y₂CuGaMn₄O₁₂, was prepared at high pressure and high temperature, with triple A-site ordering and ferrimagnetic properties.

Aurivillius Phase Bi4V3O12 with d1 Magnetic Cations, Anisotropic and Negative Thermal Expansion, Multiple Structural Transitions, and Low-Dimensional Magnetism

Aurivillius phases are an important class of inorganic compounds as they often show ferroelectric properties, and some members of this family are used in nonvolatile ferroelectric memories. The majority of Aurivillius phases have nonmagnetic d⁰ cations in the perovskite block. Bi₄Ti₃O₁₂ is the best-known and extensively studied compound within this family. Here, using a high-pressure, high-temperature synthesis method, we could successfully prepare a full magnetic analogue, Bi₄V₃O₁₂, with d¹ cations. Bi₄V₃O₁₂ is unstable in air above about 520 K. However, in an inert atmosphere, Bi₄V₃O₁₂ demonstrates two first-order reversible structural transitions near 525 and 760 K. The high-temperature prototypical phase is the same in both Bi₄V₃O₁₂ and Bi₄Ti₃O₁₂ with tetragonal (T) I4/mmm symmetry and a_T = 3.85608(5) Å and c_T = 32.6920(8) Å (at 850 K) for Bi₄V₃O₁₂, while the low-temperature phases are different. Bi₄V₃O₁₂ shows anisotropic thermal expansion above 300 K and negative volumetric thermal expansion above about 700 K. Magnetic measurements showed a broad maximum near 70 K on magnetic susceptibility, indicating the presence of low-dimensional magnetism with strong antiferromagnetic interactions between V⁴⁺ ions with the Curie-Weiss temperature of about -370 K. But no long-range magnetic ordering was found in Bi₄V₃O₁₂ down to 2 K.

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents

The dielectric function and the bandgap of BiFe_0.5Cr_0.5O₃ thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO₃ and BiCrO₃. The bandgap value of BiFe_0.5Cr_0.5O₃ is lower than that of BiFeO₃ and BiCrO₃, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm⁻¹ with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe_0.5Cr_0.5O₃ thin films and BiFe_xCr_1−xO₃ polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe_0.3Cr_0.7O₃.

K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+ → Eu3+ Energy Transfer

This paper describes the influence of sintering conditions and Eu³⁺/Tb³⁺ content on the structure and luminescent properties of K₅Eu_1-xTb_x(MoO₄)₄ (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K₅Tb(MoO₄)₄ (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the β ↔ α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-K₅Eu_0.6Tb_0.4(MoO₄)₄ structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group C2/m(0β0)00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to ⁵D₄ → ⁷F_J (J = 3-6) Tb³⁺ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λ_ex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the ⁵D₄ → ⁷F₅ Tb³⁺ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λ_ex = 377 nm exhibited an intense red emission at ∼615 nm due to the ⁵D₀ → ⁷F₂ Eu³⁺ transition, thus indicating an efficient energy transfer from Tb³⁺ to Eu³⁺.

The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12

Perovskite-structure AMnO₃ manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron-phonon coupling, and Jahn-Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn₇O₁₂ were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn₇O₁₂ compounds host rich physics beyond the canonical AMnO₃ materials.

Crystal and Magnetic Structure Transitions in BiMnO3+δ Ceramics Driven by Cation Vacancies and Temperature

The crystal structure of BiMnO_3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P2₁/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO_3+δ (δ ≤ 0.14) at temperatures above 470 °C.

KTb(MoO4)2 Green Phosphor with K+-Ion Conductivity: Derived from Different Synthesis Routes

The influence of different synthesis routes on the structure and luminescent properties of KTb(MoO₄)₂ (KTMO) was studied. KTMO samples were prepared by solid-state, hydrothermal, and Czochralski techniques. These methods lead to the following different crystal structures: a triclinic scheelite-type α-phase is the result for the solid-state method, and an orthorhombic KY(MoO₄)₂-type γ-phase is the result for the hydrothermal and Czochralski techniques. The triclinic α-KTMO phase transforms into the orthorhombic γ-phase when heated at 1273 K above the melting point, while KTMO prepared by the hydrothermal method does not show phase transitions. The influence of treatment conditions on the average crystallite size of orthorhombic KTMO was revealed by X-ray diffraction line broadening measurements. The electrical conductivity was measured on KTMO single crystals. The orthorhombic structure of KTMO that was prepared by the hydrothermal method was refined using synchrotron powder X-ray diffraction data. K⁺ cations are located in extensive two-dimensional channels along the c-axis and the a-axis. The possibility of K⁺ migration inside these channels was confirmed by electrical conductivity measurements, where strong anisotropy was observed in different crystallographic directions. The evolution of luminescent properties as a result of synthesis routes and heating and cooling conditions was studied and compared with data for the average crystallite size calculation and the grain size determination. All samples' emission spectra exhibit a strong green emission at 545 nm due to the ⁵D₄ → ⁷F₅ Tb³⁺ transition. The maximum of the integral intensity emission for the ⁵D₄ → ⁷F₅ emission under λ_ex = 380 nm excitation was found for the KTMO crashed single crystal.

Temperature evolution of 3d- and 4f-electron magnetic ordering in the ferrimagnetic Mn self-doped perovskite (Yb0.667Mn0.333)MnO3

A high-pressure synthesis method was employed to prepare Mn-self-doped perovskites (R_0.667Mn_0.333)MnO₃(R= Yb, Lu) at about 6 GPa and 1670 K. Crystal and magnetic structures of (Yb_0.667Mn_0.333)MnO₃have been studied by combining neutron powder diffraction, magnetic susceptibility and specific heat measurements. Within the orthorhombic space groupPnma, magnetic cations are located on site 4c(A site, occupied by two thirds of Yb³⁺and one third of Mn²⁺) and on site 4b(B site, occupied by two thirds of Mn³⁺and one third of Mn⁴⁺). The degree of structural distortion of the MnO₆octahedra follows the general trend of (R_1-xMn_x)MnO₃compounds which shows a decrease with increasing amount of Jahn-Teller inactive Mn⁴⁺cations. Mn-Mn interactions produce a collinear ferrimagnetic structure (T_C,Mn= 106 K) with ferromagnetically ordered Mn moments at the B site being coupled antiferromagnetically with ordered Mn moments at the A site. Mn-Yb interactions induce a small but non-zero ferromagnetic Yb³⁺moment which can explain a small decrease of the magnetic susceptibility at low temperature. Yb-Yb interactions create an antiferromagnetic structure atT_N,Yb≈ 40 K. Ordered moments of the ferrimagnetic and antiferromagnetic structures are oriented perpendicular to each other within theac-plane and Yb³⁺moments contribute to both structures. The appearance of ordered Yb³⁺moments induced by Mn-Yb interactions in perovskite (Yb_0.667Mn_0.333)MnO₃is a result of the Mn self-doping on the A site and has not been observed in the orthorhombic perovskite modification (space groupPnma) of the undoped parent compound YbMnO₃, but interestingly, it also appears in the hexagonal non-perovskite modification (space groupP6₃cm) of YbMnO₃.

Solid Solutions between PbVO3 and BiCoO3

(1 - x)PbVO₃-xBiCoO₃ solid solutions with 0 ≤ x ≤ 1 were prepared at a high pressure of 5-6 GPa and a high temperature of 1223-1473 K. They adopt a polar tetragonal P4mm structure for the 0 ≤ x ≤ 0.3 and 0.75 ≤ x ≤ 1 ranges with giant tetragonal distortions and a cubic Pm3̅m structure for the 0.4 ≤ x ≤ 0.7 range. High-temperature structural studies with synchrotron X-ray powder diffraction showed that polarization, calculated by the point-charge model, and the tetragonal distortion remained nearly constant in the x = 0.8 sample from 295 K up to the decomposition temperature of about 700 K. Magnetic and differential scanning calorimetry measurements showed that the Néel temperature, T_N, nearly linearly decreased from 470 K for x = 1 to 250 K for x = 0.75 (with T_N = 395 K for x = 0.9 and T_N = 295 K for x = 0.8). Long-range magnetic ordering also takes place at T_N = 44 K for x = 0. All other samples with 0.1 ≤ x ≤ 0.7 demonstrated spin-glass-like magnetic properties and notably reduced Weiss temperatures. Effective magnetic moments estimated for the x = 0.6, 0.65, and 0.7 cubic samples gave evidence that cobalt is present in the +2 and +3 oxidation states, and Co³⁺ cations take the low-spin state.

Structural stability of CuAl2O4under pressure

Structural properties of CuAl2O4, which was recently argued to show unusual suppression of the Jahn-Teller distortions by the spin-orbit coupling, are investigated under pressures up to 6 GPa. Analysis of x-ray powder diffraction experiments shows that CuAl2O4gets unstable and decomposes onto CuO and Al2O3at pressures ∼6 GPa and temperature ∼1000 K. This finding is complemented by the density-functional theory +U+ spin-orbit coupling calculations, which demonstrate that this instability is partially driven by a (relatively) large compressibility of strongly Jahn-Teller distorted CuO.

Emergence of a Magnetostructural Dipolar Glass in the Quadruple Perovskite Dy_{1-δ}Mn_{7+δ}O_{12}

We show that a polar, pseudo-Jahn-Teller instability exists for the underbonded rare-earth A-site cations in the quadruple perovskite Dy_{1-δ}Mn_{7+δ}O_{12}, which leads to the spontaneous formation of a dipolar glass. This observation alone expands the applicability of pseudo-Jahn-Teller physics in perovskite-derived materials, for which it is typically confined to B-site cations. We demonstrate that the dipolar glass order parameter is coupled to a ferrimagnetic order parameter via strain, leading to a first order magnetostructural phase transition that can be tuned by magnetic field. This phenomenology may emerge in a broad range of perovskite-derived materials in which A-site cation ordering and octahedral tilting are mutually tied to meet the criteria of structural stability.

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.

High-Pressure Synthesis, Crystal Structures, and Properties of A-Site Columnar-Ordered Quadruple Perovskites NaRMn2Ti4O12 with R = Sm, Eu, Gd, Dy, Ho, Y

The formation of NaRMn₂Ti₄O₁₂ compounds (R = rare earth) under high pressure (about 6 GPa) and high temperature (about 1750 K) conditions was studied. Such compounds with R = Sm, Eu, Gd, Dy, Ho, Y adopt an A-site columnar-ordered quadruple-perovskite structure with the generic chemical formula A₂A'A″B₄O₁₂. Their crystal structures were studied by powder synchrotron X-ray and neutron diffraction between 1.5 and 300 K. They maintain a paraelectric structure with centrosymmetric space group P4₂/nmc (No. 137) at all temperatures, in comparison with the related CaMnTi₂O₆ perovskite, in which a ferroelectric transition occurs at 630 K. The centrosymmetric structure was also confirmed by second-harmonic generation. It has a cation distribution of [Na⁺R³⁺]_A[Mn²⁺]_A'[Mn²⁺]_A″[Ti⁴⁺₄]_BO₁₂ (to match with the generic chemical formula) with statistical distributions of Na⁺ and R³⁺ at the large A site and a strongly split position of Mn²⁺ at the square-planar A' site. We found a C-type long-range antiferromagnetic structure of Mn²⁺ ions at the A' and A″ sites below T_N = 12 K for R = Dy and found that the presence of Dy³⁺ disturbs the long-range ordering of Mn²⁺ below a second transition at lower temperatures. The first magnetic transition occurs below 8-13 K in all compounds, but the second magnetic transition occurs only for R = Dy, Sm, Eu. All compounds show large dielectric constants of a possible extrinsic origin similar to that of CaCu₃Ti₄O₁₂. NaRMn₂Ti₄O₁₂ with R = Er-Lu crystallized in the GdFeO₃-type Pnma perovskite structure, and NaRMn₂Ti₄O₁₂ with R = La, Nd contained two perovskite phases: an AA'₃B₄O₁₂-type Im3̅ phase and a GdFeO₃-type Pnma phase.

Spontaneous Rotation of Ferrimagnetism Driven by Antiferromagnetic Spin Canting

Spin-reorientation phase transitions that involve the rotation of a crystal's magnetization have been well characterized in distorted-perovskite oxides such as orthoferrites. In these systems spin reorientation occurs due to competing rare-earth and transition metal anisotropies coupled via f-d exchange. Here, we demonstrate an alternative paradigm for spin reorientation in distorted perovskites. We show that the R_{2}CuMnMn_{4}O_{12} (R=Y or Dy) triple A-site columnar-ordered quadruple perovskites have three ordered magnetic phases and up to two spin-reorientation phase transitions. Unlike the spin-reorientation phenomena in other distorted perovskites, these transitions are independent of rare-earth magnetism, but are instead driven by an instability towards antiferromagnetic spin canting likely originating in frustrated Heisenberg exchange interactions, and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies.

Study of Polycrystalline Bulk Sr3OsO6 Double-Perovskite Insulator: Comparison with 1000 K Ferromagnetic Epitaxial Films

Polycrystalline Sr₃OsO₆, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 °C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr₃OsO₆ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr₃OsO₆ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr₃OsO₆ is under the influence of interfaces. A specific heat capacity of 39.6(9) × 10^-3 J mol^-1 K^-2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr₃OsO₆ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.

Sr9In(VO4)7 as a model ferroelectric in the structural family of β-Ca3(PO4)2-type phosphates and vanadates

Sr₉In(VO₄)₇ was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca₃(PO₄)₂-type structure and crystallizes in polar space group R3c. The structural parameters of Sr₉In(VO₄)₇ were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) Å and c = 39.6170(3) Å with Z = 6. In³⁺ cations occupy the octahedral M5 site, Sr²⁺ cations occupy the M1, M2, and M3 sites of the β-Ca₃(PO₄)₂-type structure, and the M4 site remains vacant. Sr₉In(VO₄)₇ was characterized by differential thermal analysis (DTA), optical second-harmonic generation (SHG), high-temperature XRD, and dielectric measurements. All these methods prove the existence of a ferroelectric-paraelectric phase transition at T _c = 974 K. This transition is compared with a similar transition in Ca₉In(PO₄)₇ with lower T _c = 902 K. The polar-to-centrosymmetric phase transition in such compounds has a quite unique mechanism of the order-disorder type. The structural transition involves slight shifts of the M1, M2, M3 cations and the E2O₄, E3O₄ tetrahedra, while half of the E1O₄ tetrahedra (E = P or V) statistically reverse their orientation along the three-fold axis, so that the centre of symmetry appears in the structure as a whole. To invert the E1O₄ tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr²⁺ or Ca²⁺) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca₉In(PO₄)₇ corresponds to the smaller rotational vibration amplitude of the P1O₄ tetrahedron required to reverse this tetrahedra at T _c in comparison with V1O₄ in Sr₉In(VO₄)₇.

Spin-Glass Magnetic Properties of A-Site Columnar-Ordered Quadruple Perovskites Y2MnGa(Mn4-xGax)O12 with 0 ≤ x ≤ 3

Y₂MnGa(Mn_4-xGa_x)O₁₂ solid solutions were synthesized at high pressure of ∼6 GPa and high temperature of ∼1570 K for the 0 ≤ x ≤ 3 compositional range. Synchrotron X-ray and neutron powder diffraction were used to study the crystal structures and cation distributions. These solutions adopt the parent structure of the A-site columnar-ordered quadruple perovskite family with space group P4₂/nmc (No. 137). They have lattice parameters of a = 7.36095 Å and c = 7.753 84 Å (x = 0), a = 7.361 68 Å and c = 7.716 16 Å (x = 1), a = 7.360 34 Å and c = 7.67142 Å (x = 2), and a = 7.363 93 Å and c = 7.616 85 Å (x = 3) at room temperature. The x = 0 sample has a cation distribution of [Y³⁺₂]_A[Mn³⁺]_A'[Ga³⁺_0.68Mn²⁺_0.32]_A″[Mn_3.68Ga_0.32]_BO₁₂ with a preferred localization of Ga³⁺ in the tetrahedral A″ site and with a small amount of Ga³⁺ in the octahedral B site. A complete triple A-site order, [Y³⁺₂]_A[Mn³⁺]_A'[Ga³⁺]_A″[Mn³⁺_4-xGa³⁺_x]_BO₁₂, is realized for x ≥ 1. All samples demonstrate spin-glass-like magnetic properties, and the absence of a long-range magnetic order at the ground state at 1.5 K was confirmed by neutron diffraction for the x = 1 sample. First-principles calculations indicated the spin-glass-like magnetic ordering is derived from the Ga substitution to the B sites and gave evidence that the ideal cation distribution could produce robust ferromagnetism in this family of perovskites.

Valence Variations by B-Site Doping in A-Site Columnar-Ordered Quadruple Perovskites Sm2MnMn(Mn4- xTi x)O12 with 1 ≤ x ≤ 3

Sm₂MnMn(Mn_{4- x}Ti _x)O₁₂ with 1 ≤ x ≤ 3 were prepared by a high-pressure, high-temperature method at 6 GPa and about 1570-1670 K. They belong to a family of A-site columnar-ordered quadruple perovskites A₂A'A″B₄O₁₂, where A' is a site with a square-planar coordination and A″ is a site with a tetrahedral coordination. Their crystal structures were investigated using synchrotron X-ray and neutron powder diffraction. They crystallize in space group P4₂/ nmc (No. 137) with a = 7.41172 Å and c = 7.97131 Å for x = 1, a = 7.54945 Å and c = 7.76756 Å for x = 2, and a = 7.63949 Å and c = 7.70339 Å for x = 3 at 295 K. The determined charge and cation distributions are [Sm³⁺_1.88Mn²⁺_0.12]_A[Mn³⁺]_A'[Mn²⁺_0.88Sm³⁺_0.12]_A″[Mn³⁺₃Ti⁴⁺]_BO₁₂ for x = 1, [Sm³⁺_1.91Mn²⁺_0.09]_A[Mn²⁺]_A'[Mn²⁺_0.91Sm³⁺_0.09]_A″[Mn³⁺₂Ti⁴⁺₂]_BO₁₂ for x = 2, and [Sm³⁺_1.88Mn²⁺_0.12]_A[Mn²⁺_0.88Sm³⁺_0.12]_A'[Mn²⁺]_A″[Mn²⁺Ti⁴⁺₃]_BO₁₂ for x = 3. Mn and Ti are distributed randomly in one B site in all compounds with the average oxidation state changing from +3.25 to +3.5 per one B atom, and such flexibility is realized because Mn at the A' site can change its oxidation state between +2 and +3. Sm and Mn are slightly disordered between the A and A″ sites for x = 1 and 2, and between the A and A' sites for x = 3. The x = 1 sample shows spin-canted antiferromagnetic properties with T_N = 27 K, and the x = 2 sample, with T_N = 62 K. On the other hand, the x = 3 sample is a ferrimagnet, confirmed by neutron diffraction, with T_C = 40 K. The x = 3 sample shows relaxor-like dielectric properties below 220 K.

Crystal and Magnetic Structures and Properties of (Lu1- xMn x)MnO3 Solid Solutions

(Lu_{1- x}Mn _x)MnO₃ solid solutions, having the perovskite-type structure and Pnma space group, with 0 ≤ x ≤ 0.4 were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from Lu₂O₃ and Mn₂O₃. Their crystal and magnetic structures were studied by neutron powder diffraction. The degree of octahedral MnO₆ tilting decreases in (Lu_{1- x}Mn _x)MnO₃ with increasing x. Only the incommensurate (IC) spin structure with a propagation vector of k = ( k₀, 0, 0) and k₀ ≈ 0.44 remains in (Lu_0.9Mn_0.1)MnO₃ in the whole temperature range below the Neel temperature T_N = 36 K, and the commensurate noncollinear E-type structure that has been reported in the literature for undoped o-LuMnO₃ is not observed. (Lu_{1- x}Mn _x)MnO₃ samples with 0.2 ≤ x ≤ 0.4 have a ferrimagnetic structure with a propagation vector of k = (0, 0, 0) and ferromagnetic (FM) ordering of Mn³⁺ and Mn⁴⁺ cations at the B site, which are antiferromagnetically coupled to a noncollinear predominantly FM arrangement of Mn²⁺ at the A site. The ferrimagnetic Curie temperature, T_C, increases monotonically from 67 K for x = 0.2 to 118 K for x = 0.4. Magnetic and dielectric properties of (Lu_{1- x}Mn _x)MnO₃ and a composition-temperature phase diagram are also reported.

Intrinsic Triple Order in A-site Columnar-Ordered Quadruple Perovskites: Proof of Concept

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A₂ A'A''B₄ O₁₂ quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy₂ CuMnMn₄ O₁₂ (1) and Ho₂ MnGaMn₄ O₁₂ (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho₂ ]_A [Mn]_A' [Ga_0.66 Mn_0.34 ]_A'' [Mn_3.66 Ga_0.34 ]_B O₁₂ and [Dy₂ ]_A [Cu_0.73 Mn_0.27 ]_A' [Mn_0.80 Dy_0.20 ]_A'' [Mn_1.89 Cu_0.11 ]_B1 [Mn₂ ]_B2 O₁₂ . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.

High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb)

A-site-ordered quadruple perovskites RMn₇O₁₂ with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at T_N1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below T_N2 ≈ 12 K were observed in SmMn₇O₁₂ and EuMn₇O₁₂ at high magnetic fields. Very weak kinklike dielectric anomalies were observed at T_N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn₇O₁₂ shows signs of nonstoichiometry expressed as (Tb_{1- x}Mn _x)Mn₇O₁₂, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn₇O₁₂ and EuMn₇O₁₂ were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn₇O₁₂ and (Tb_0.88Mn_0.12)Mn₇O₁₂ were studied by synchrotron X-ray powder diffraction at 295 K.

Rise of A-site columnar-ordered A2A'A''B4O12 quadruple perovskites with intrinsic triple order

A-site-ordered AA'₃B₄O₁₂ quadruple perovskites (with twelve-fold coordinated A and square-planar coordinated A' sites) were discovered in 1967. Since then, there have been considerable research efforts to synthesize and characterize new members of such perovskites. These efforts have led to the discoveries of many interesting physical and chemical properties, such as inter-site charge transfer and disproportionation, giant dielectric constant, multiferroic properties, reentrant structural transitions and high catalytic activity. The first member of A-site columnar-ordered A₂A'A''B₄O₁₂ quadruple perovskites (with ten-fold coordinated A, square-planar coordinated A' and tetrahedrally coordinated A'' sites), CaFeTi₂O₆, was discovered in 1995, and for 19 years it was the only representative of this family. In the last few years, A₂A'A''B₄O₁₂ perovskites have experienced rapid growth. Herein, we present a brief overview of the recent developments in this field and highlight an under-investigated status and great potential of A₂A'A''B₄O₁₂, which can be prepared mainly at high pressure and high temperature. The presence of the A'' site gives an additional degree of freedom in designing such perovskites. The A₂A'A''B₄O₁₂ perovskites are discussed in comparison with well-known AA'₃B₄O₁₂ perovskites.

Mn Self-Doping of Orthorhombic RMnO3 Perovskites: (R0.667Mn0.333)MnO3 with R = Er-Lu

Orthorhombic rare-earth trivalent manganites RMnO₃ (R = Er-Lu) were self-doped with Mn to form (R_0.667Mn_0.333)MnO₃ compositions, which were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from R₂O₃ and Mn₂O₃. The average oxidation state of Mn is 3+ in (R_0.667Mn_0.333)MnO₃. However, Mn enters the A site in the oxidation state of 2+, creating the average oxidation state of 3.333+ at the B site. The presence of Mn²⁺ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron powder X-ray diffraction. (R_0.667Mn_0.333)MnO₃ crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu_0.667Mn_0.333)MnO₃ at 293 K, and they are isostructural with the parent RMnO₃ manganites. Compared with RMnO₃, (R_0.667Mn_0.333)MnO₃ exhibits enhanced Néel temperatures of about T_N1 = 106-110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about T_N2 = 9-16 K. (Tm_0.667Mn_0.333)MnO₃ exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K.

Charge and orbital orders and structural instability in high-pressure quadruple perovskite CeCuMn6O12

We prepared a quadruple perovskite CeCuMn6O12 under high-pressure and high-temperature conditions at 6 GPa and about 1670 K and investigated its structural, magnetic and transport properties. CeCuMn6O12 crystallizes in space group Im-3 above T CO  =  297 K; below this temperature, it adopts space group R-3 with the 1:3 (Mn4+:Mn3+) charge and orbital orders. Unusual compressed Mn3+O6 octahedra are realized in CeCuMn6O12 similar to CaMn7O12 with the  -Q 3 Jahn-Teller distortion mode. Below about 90 K, structural instability takes place with phase separation and the appearance of competing phases; and below 70 K, two R-3 phases coexist. CeCuMn6O12 exhibits a ferromagnetic-like transition below T C  =  140 K, and it is a semiconductor with the magnetoresistance reaching about  -40% at 140 K and 70 kOe. We argued that the valence of Ce is  +3 in CeCuMn6O12 with the Ce3+([Formula: see text])([Formula: see text])O12 charge distribution in the charge-ordered R-3 phase and Ce3+([Formula: see text])([Formula: see text])O12 in the charge-disordered Im-3 phase.

Five-Fold Ordering in High-Pressure Perovskites RMn3O6 (R = Gd-Tm and Y)

Cation and anion ordering plays an important role in the properties of materials, in particular, in the properties of perovskite materials. Here we report on unusual 5-fold cation/charge ordering in high-pressure-synthesized (at 6 GPa and ∼1670 K) RMn₃O₆ perovskites with R = Gd-Tm and Y. R³⁺, Mn²⁺, and Mn³⁺ cations are ordered at the A site in two separate chains consisting of R³⁺ and alternating Mn²⁺ (in tetrahedral coordination) and Mn³⁺ (in square-planar coordination), while Mn³⁺ and mixed-valent Mn³⁺/Mn⁴⁺ are ordered at the B site in layers. The ordering can be represented as [R³⁺Mn²⁺_0.5Mn³⁺_0.5]_A[Mn³⁺Mn^3.5+]_BO₆. The triple cation ordering observed at the A site is very rare, and the layered double-B-site ordering is also scarce. RMn₃O₆ compounds crystallize in space group Pmmn with a = 7.2479(2) Å, b = 7.4525(3) Å, and c = 7.8022(2) Å for DyMn₃O₆ at 213 K, and they are structurally related to CaFeTi₂O₆. They are prone to nonstoichiometry, R_1-δMn₃O_6-1.5δ, where δ = -0.071 to -0.059 for R = Gd, δ = 0 for R = Dy, δ = 0.05-0.1 for R = Ho and Y, and δ = 0.12 for R = Er and Tm. They show complex magnetic behaviors with several transition temperatures, and their magnetic properties are highly sensitive to the δ values.

A layered wide-gap oxyhalide semiconductor with an infinite ZnO2 square planar sheet: Sr2ZnO2Cl2

A new layered perovskite zinc oxychloride, Sr₂ZnO₂Cl₂, with a square planar geometry was successfully synthesized using a high pressure method.

Solid Solutions between BiMnO3 and BiCrO3

Solid solutions BiMn_1-xCr_xO₃ (0 ≤ x ≤ 1) have been prepared at 6 GPa and 1370-1620 K. Their structural properties have been studied with synchrotron X-ray powder diffraction, and their physical properties have been investigated by dc/ac magnetic, specific heat, dielectric, and differential scanning calorimetry measurements. A magnetic phase diagram of BiMn_1-xCr_xO₃ is established. A phase with orbital ordering observed in BiMnO₃ is suppressed at x > 0.1, accompanied by a drop in the ferromagnetic Curie temperature T_C from 101 K for x = 0 to 76 K for x = 0.15 and sharp changes in the lattice parameters. The T_C value monotonically decreases up to x = 0.3 (with T_C = 53 K). For intermediate compositions with x = 0.4, 0.5, spin-glass magnetic properties are found at 28 and 24 K, respectively. The Néel temperature T_N linearly increases from 36 K for x = 0.6 to 111 K for x = 1.0. A spin-reorientation transition is observed at 61 K for x = 0.9 and 72 K for x = 1.0. Re-entrant spin-glass transitions are also observed for samples with x = 0.3, 0.6, 0.7 by ac susceptibility at low temperatures. At high temperatures, a structural phase transition from C2/c to Pnma symmetry is observed for all compositions with a monotonic change of the phase transition temperature. The magnetic phase diagram from the BiMnO₃-rich side (x ≤ 0.5) resembles a phase diagram of BiMn_1-xSc_xO₃ solid solutions, indicating that the nature of substituting cations (magnetic or nonmagnetic) is not crucial for doped BiMnO₃.

Spin-Driven Multiferroic Properties of PbMn7O12 Perovskite

We synthesize PbMn7O12 perovskite under high-pressure (6 GPa) and high-temperature (1373 K) conditions and investigate its structural, magnetic, dielectric, and ferroelectric properties. We find that PbMn7O12 exhibits rich physical properties from interplay among charge, orbital, and spin degrees of freedom and rich structural properties. PbMn7O12 crystallizes in space group R3̅ near room temperature and shows a structural phase transition at TCO = 397 K to a cubic structure in space group Im3̅; the Im3̅-to-R3̅ transition is associated with charge ordering. Below TOO = 294 K, a structural modulation transition associated with orbital ordering takes place. There are two magnetic transitions with Néel temperatures of TN1 = 83 K and TN2 = 77 K and probably a lock-in transition at TN3 = 43 K (on cooling). There is huge hysteresis on specific heat (between ∼37 and 65 K at 0 Oe), dielectric constant (between ∼20 and 70 K at 0 Oe), and dc and ac magnetic susceptibilities around the lock-in transition. Sharp dielectric constant, dielectric loss, and pyroelectric current anomalies are observed at TN2, indicating that electric polarization is developed at this magnetic transition, and PbMn7O12 perovskite is a spin-driven multiferroic. Polarization of PbMn7O12 is measured to be ∼4 μC/m(2). Field-induced transitions are detected at ∼63 and ∼170 kOe at 1.6-2 K; similar high-magnetic field properties are also found for CdMn7O12, CaMn7O12, and SrMn7O12. PbMn7O12 exhibits a quite small magnetodielectric effect, reaching approximately -1.3 to -1.7% at 10 K and 90 kOe.