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.

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.

A-Site Columnar-Ordered Perovskite CaZnV2O6 as a Pauli-Paramagnetic Metal.

In this study, we successfully synthesized a novel A-site columnar-ordered perovskite CaZnV2O6. This compound features a square-planar-coordinated Zn2+ disorder, which is the same characteristic as the centrosymmetric paraelectric CaMnTi2O6. Unlike CaMnTi2O6, which shows a centrosymmetric paraelectric-noncentrosymmetric ferroelectric transition, CaZnV2O6 retains Pauli-paramagnetic metallicity arising from itinerant V4+ d1 electrons and centrosymmetry down to 5 K. Based on analogous compounds, we expect CaZnV2O6 to provide a new playground for the electronic and magnetic states of V4+.

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.

La4Ga2S8O3: A Rare-Earth Gallium Oxysulfide with Disulfide Ions.

Band gap engineering using multiple anions is an established approach to novel photocatalysts that exhibit suitable band gap energies for water splitting and high photocorrosion resistance. However, few studies have been conducted on photocatalysts with polyanions, including polychalcogenide ions. Here, we present a new quaternary gallium oxysulfide with disulfide pairs (S2)2-, La4Ga2S8O3, grown out of a KI molten salt. Single-crystal X-ray diffraction analysis revealed that the oxysulfide crystallizes in the orthorhombic space group Pbcn with lattice constants of a = 18.3330(6) Å, b = 13.0590(5) Å, and c = 5.9022(3) Å. In the crystal structure, the GaS4-based zigzag chains and OLa4-based fluorite-like strips are independently arranged in two dimensions, which alternately stack via the disulfide pairs along the third direction. The oxysulfide is a direct-type semiconductor with a band gap of 2.45 eV. First-principles calculations combined with X-ray photoemission spectroscopy measurements show that S 3p states derived from the disulfide pairs dominate the valence band maximum and conduction band minimum, and these band-edge positions are suitable for the oxidation and reduction of water. Our comprehensive study based on the electronic structure suggests that the disulfide pairs make La4Ga2S8O3 a potential photocatalyst for water splitting under visible-light irradiation.

Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4 O12.

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)MnMnTi4 O12 . 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.

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 Bi3Os2.45Fe0.55O11 and Bi3Re2.13Fe0.87O11 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 Bi3Os2.45Fe0.55O11 exhibited a ferrimagnetic transition at the highest magnetic ordering temperature of 490 K in the KSbO3-type, while Bi3Re2.13Fe0.87O11 exhibited a spin glassy behavior below 22 K. The magnetoresistance at 5 K and 90 kOe was almost zero for Bi3Os2.45Fe0.55O11, but -10% for Bi3Re2.13Fe0.87O11. 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.

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[Cu0.8Mn0.2]A'[Ga0.8Mn0.2]A″[Mn3.6Cu0.2Ga0.2]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 TC = 115 K with the ordered moment of 2.19 µB at each B site and 0.89 µB 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 TC 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.

La3 Ga3 Ge2 S3 O10 : An Ultraviolet Nonlinear Optical Oxysulfide Designed by Anion-Directed Band Gap Engineering.

Chalcogenide-containing compounds have been widely studied as infrared nonlinear optical (NLO) materials. However, they have never been applied in the ultraviolet (UV) region owing to the high energy levels of chalcogen anions, leading to band gap narrowing. We report the synthesis of a new UV NLO oxysulfide La3 Ga3 Ge2 S3 O10 with an exceptionally wide band gap of 4.70 eV due to from the unique anion-ordered frameworks comprising 1D 1 ∞ [(Ga3/5 Ge2/5 )3 S3 O3 ] triangular tubes and 0D (Ga3/5 Ge2/5 )2 O7 dimers of corner-sharing (Ga/Ge)S2 O2 and (Ga/Ge)O4 tetrahedra, respectively. Second harmonic generation (SHG) measurements revealed that La3 Ga3 Ge2 S3 O10 was phase matchable with twice the SHG response of KH2 PO4 . The results of theoretical calculations suggest that the strong SHG response is mainly attributable to the S-3p and O-2p orbitals in the occupied states. The anion-directed band-gap engineering may give insights into the application of NLO oxychalcogenides in the UV regions.

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.

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 TN = 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.

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

Y2MnGa(Mn4-xGax)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″[Mn3.68Ga0.32]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(Mn4- xTi 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 TN = 27 K, and the x = 2 sample, with TN = 62 K. On the other hand, the x = 3 sample is a ferrimagnet, confirmed by neutron diffraction, with TC = 40 K. The x = 3 sample shows relaxor-like dielectric properties below 220 K.

Growth of Black Phosphorus Nanobelts and Microbelts.

Black phosphorus nanobelts are fabricated with a one-step solid-liquid-solid reaction method under ambient pressure, where red phosphorus is used as the precursor instead of white phosphorus. The thickness of the as-fabricated nanobelts ranges from micrometers to tens of nanometers as studied by scanning electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate that the nanobelts have the composition and the structure of black phosphorus, transmission electron microscopy reveals a typical layered structure stacked along the b-axis, and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy analysis demonstrates the doping of bismuth into the black phosphorus structure. The nanobelt can be directly measured in scanning tunneling microscopy in ambient conditions.

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' [Ga0.66 Mn0.34 ]A'' [Mn3.66 Ga0.34 ]B O12 and [Dy2 ]A [Cu0.73 Mn0.27 ]A' [Mn0.80 Dy0.20 ]A'' [Mn1.89 Cu0.11 ]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.

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

(Lu1- xMn 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 (Lu1- xMn 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 (Lu0.9Mn0.1)MnO3 in the whole temperature range below the Neel temperature TN = 36 K, and the commensurate noncollinear E-type structure that has been reported in the literature for undoped o-LuMnO3 is not observed. (Lu1- xMn 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, TC, increases monotonically from 67 K for x = 0.2 to 118 K for x = 0.4. Magnetic and dielectric properties of (Lu1- xMn x)MnO3 and a composition-temperature phase diagram are also reported.

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 (R0.667Mn0.333)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 (R0.667Mn0.333)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. (R0.667Mn0.333)MnO3 crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu0.667Mn0.333)MnO3 at 293 K, and they are isostructural with the parent RMnO3 manganites. Compared with RMnO3, (R0.667Mn0.333)MnO3 exhibits enhanced Néel temperatures of about TN1 = 106-110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about TN2 = 9-16 K. (Tm0.667Mn0.333)MnO3 exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K.

High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV2O4, FeV2O4, and MnCr2O4: Crystal Chemistry of AB2O4 Postspinel Compounds.

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr2O4. At 1200-1800 °C, MgV2O4 Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2O3, and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2O4 Sp transforms to CT-type FeV2O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B3+O6 octahedra (B3+ = V3+ and Cr3+) running parallel to one of orthorhombic cell axes. A relatively large A2+ cation (A2+ = Mg2+, Fe2+, and Mn2+) occupies a tunnel-shaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen-A2+ cation distances of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eight-coordinated) sites for the A2+ cations, respectively. A relationship between cation sizes of VIIIA2+ and VIB3+ and crystal structures (CF- and CT-types) of A2+B23+O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+B23+O4 crystallize in wide ionic radius ranges of 0.9-1.4 Å for VIIIA2+ and 0.55-1.1 Å for VIB3+, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIIIA2+ and 0.6-0.65 Å for VIB3+. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A2+ cation than that of CF-type.

Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2.

We report the successful synthesis of three new Ruddlesden-Popper-type scandium oxychloride perovskites, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2, by conventional solid-state reaction. Small single crystals of Sr2ScO3Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/ nmm space group ( a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO5Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO5 pyramid. These structural features in the oxychloride are different from those of octahedral ScO5F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr2ScO3F. Rietveld refinements of the neutron powder diffraction data of Sr3Sc2O5Cl2 ( I4/ mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba3Sc2O5Cl2 ( I4/ mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO5 pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr2ScO3Cl with the exception that the ScO5 pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr2ScO3Cl indicate the strong covalency of the Sc-O bonds but almost nonbonding interaction between Sc and Cl ions.

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 TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 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 (Tb1- xMn 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 (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.