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 <i>Pmmn</i> with <i>a</i> = 7.2479(2) Å, <i>b</i> = 7.4525(3) Å, and <i>c</i> = 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

Anion Order-to-Disorder Transition in Layered Iron Oxyfluoride Sr₂FeO₃F Single Crystals

Controlling the distribution of mixed anions around a metal center is a long-standing subject in solid state chemistry. We successfully obtained single crystals of an iron-based layered perovskite compound, Sr₂FeO₃F, by utilizing a high-pressure and high-temperature technique. The phase prepared at 1300 °C and 3 GPa crystallized in tetragonal space group <i>P</i>4/<i>nmm</i> with O/F atoms at the apical sites being ordered. However, a temperature of 1800 °C and a pressure of 6 GPa resulted in partial O/F site disordering. The degree of anion disordering, which was 5%, showed that the anion-ordered arrangement was quite robust, in sharp contrast to that of Sr₂BO₃F (B = Co or Ni) with the fully disordered state. ⁵⁷Fe Mössbauer spectroscopy measurements revealed no large difference in Néel temperatures between the two phases, but the phase prepared under the latter condition exhibited a quasi-continuous distribution of hyperfine fields caused by O/F site disordering. We discuss the mechanism of the anion order-to-disorder transition observed in related oxyfluoride perovskites

Promising Approach to Achieving a Large Exchange Bias Effect in Bulk Materials with Small Cooling Fields

The exchange bias effect is pivotal in semiconductor technology, particularly for magnetic recording and spin valve devices. However, conventional materials that rely on interfaces present manufacturing challenges. This study focuses on the exchange bias effect in bulk materials without interfaces. A novel magnetic material, Cd2FeOsO6, was synthesized, exhibiting a strong exchange bias effect at low-cooling magnetic fields. This study offers insights into materials with pronounced exchange bias and a unique mechanism. Cd2FeOsO6 demonstrates ferrimagnetic ordering and hard magnetism below 285 K. Notably, a substantial 10 kOe exchange bias arises with a weak 80 Oe magnetic field. This effect is likely due to partial ordering and strong spin–orbit coupling of the ligand of Os. These findings highlight the potential of double perovskite materials for notable exchange bias effects at room temperature with modest cooling fields. Leveraging 5d element properties advances bulk materials with enhanced exchange bias traits

High-Pressure Synthesis, Crystal Structure, and Magnetic Properties of Sr₂MnO₃F: A New Member of Layered Perovskite Oxyfluorides

We have successfully synthesized Sr₂MnO₃F, a new layered perovskite oxyfluoride with a <i>n</i> = 1 Ruddlesden–Popper-type structure using a high-pressure, high-temperature method. Structural refinements against synchrotron X-ray diffraction data collected from manganese oxyfluoride demonstrated that it crystallizes in a tetragonal cell with the space group <i>I</i>4/<i>mmm</i>, in which the Mn cation is located at the octahedral center position. This is in stark contrast to the related oxyhalides that have square-pyramidal coordination such as Sr₂MO₃X (M = Fe, Co, Ni; X = F, Cl) and Sr₂MnO₃Cl. There was no evidence of O/F site order, but close inspection of the anion environment centered at the Mn cation on the basis of bond-valence-sum calculation suggested preferential occupation of the apical sites by the F ion with one oxide ion in a random manner. Magnetic susceptibility and heat capacity measurements revealed an antiferromagnetic ordering at 133 K (=<i>T</i>_N), which is much higher than that of the chloride analogue with corrugated MnO₂ planes (<i>T</i>_N = 80 K)

Neutron Diffraction Study of Unusual Phase Separation in the Antiperovskite Nitride Mn₃ZnN

The antiperovskite Mn₃ZnN is studied by neutron diffraction at temperatures between 50 and 295 K. Mn₃ZnN crystallizes to form a cubic structure at room temperature (C1 phase). Upon cooling, another cubic structure (C2 phase) appears at around 177 K. Interestingly, the C2 phase disappears below 140 K. The maximum mass concentration of the C2 phase is approximately 85% (at 160 K). The coexistence of C1 and C2 phase in the temperature interval of 140–177 K implies that phase separation occurs. Although the C1 and C2 phases share their composition and lattice symmetry, the C2 phase has a slightly larger lattice parameter (Δ<i>a</i> ≈ 0.53%) and a different magnetic structure. The C2 phase is further investigated by neutron diffraction under high-pressure conditions (up to 270 MPa). The results show that the unusual appearance and disappearance of the C2 phase is accompanied by magnetic ordering. Mn₃ZnN is thus a valuable subject for study of the magneto-lattice effect and phase separation behavior because this is rarely observed in nonoxide materials

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 <i>P</i>4/<i>nmm</i> space group (<i>a</i> = 4.08066(14) Å, <i>c</i> = 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 (<i>I</i>4/<i>mmm</i>: <i>a</i> = 4.107982(5) Å, <i>c</i> = 23.58454(7) Å) and Ba₃Sc₂­O₅Cl₂ (<i>I</i>4/<i>mmm</i>: <i>a</i> = 4.206920(5) Å, <i>c</i> = 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

Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Structural properties of a quadruple perovskite BiMn₇O₁₂ were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above <i>T</i>₁ = 608 K, BiMn₇O₁₂ crystallizes in a parent cubic structure with space group <i>Im</i>3̅. Between 460 and 608 K, BiMn₇O₁₂ adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as <i>I</i>2/<i>m</i>(o)), and orbital order appears below <i>T</i>₁. Below <i>T</i>₂ = 460 K, BiMn₇O₁₂ is likely to exhibit a transition to space group <i>Im</i>. Finally, below about <i>T</i>₃ = 290 K, a triclinic distortion takes place to space group <i>P</i>1. Structural analyses of BiMn₇O₁₂ are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the <i>Im</i> and <i>P</i>1 models is very small, and this fact can explain why the <i>Im</i> to <i>P</i>1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMn₇O₁₂ is in striking contrast with that of LaMn₇O₁₂ and could be caused by effects of the Bi³⁺ lone electron pair

Mn Self-Doping of Orthorhombic RMnO₃ Perovskites: (R_0.667Mn_0.333)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 <i>Pnma</i> with <i>a</i> = 5.50348(2) Å, <i>b</i> = 7.37564(1) Å, and <i>c</i> = 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 <i>T</i>_N1 = 106–110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about <i>T</i>_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

High-Pressure Synthesis, Crystal Structures, and Magnetic Properties of 5d Double-Perovskite Oxides Ca₂MgOsO₆ and Sr₂MgOsO₆

Double-perovskite oxides Ca₂MgOsO₆ and Sr₂MgOsO₆ have been synthesized under high-pressure and high-temperature conditions (6 GPa and 1500 °C). Their crystal structures and magnetic properties were studied by a synchrotron X-ray diffraction experiment and by magnetic susceptibility, specific heat, isothermal magnetization, and electrical resistivity measurements. Ca₂MgOsO₆ and Sr₂MgOsO₆ crystallized in monoclinic (<i>P</i>2₁/<i>n</i>) and tetragonal (<i>I</i>4/<i>m</i>) double-perovskite structures, respectively; the degree of order of the Os and Mg arrangement was 96% or higher. Although Ca₂MgOsO₆ and Sr₂MgOsO₆ are isoelectric, a magnetic-glass transition was observed for Ca₂MgOsO₆ at 19 K, while Sr₂MgOsO₆ showed an antiferromagnetic transition at 110 K. The antiferromagnetic-transition temperature is the highest in the family. A first-principles density functional approach revealed that Ca₂MgOsO₆ and Sr₂MgOsO₆ are likely to be antiferromagnetic Mott insulators in which the band gaps open, with Coulomb correlations of ∼1.8–3.0 eV. These compounds offer a better opportunity for the clarification of the basis of 5d magnetic sublattices, with regard to the possible use of perovskite-related oxides in multifunctional devices. The double-perovskite oxides Ca₂MgOsO₆ and Sr₂MgOsO₆ are likely to be Mott insulators with a magnetic-glass (MG) transition at ∼19 K and an antiferromagnetic (AFM) transition at ∼110 K, respectively. This AFM transition temperature is the highest among double-perovskite oxides containing single magnetic sublattices. Thus, these compounds offer valuable opportunities for studying the magnetic nature of 5d perovskite-related oxides, with regard to their possible use in multifunctional devices

High-Pressure Synthesis, Crystal Structure, and Semimetallic Properties of HgPbO₃

The crystal structure of HgPbO₃ was studied using single-crystal X-ray diffraction and powder synchrotron X-ray diffraction. The structure was well characterized as a centrosymmetric model with a space group of <i>R</i>-3<i>m</i> [hexagonal setting: <i>a</i> = 5.74413(6) Å and <i>c</i> = 7.25464(8) Å] rather than as a noncentrosymmetric model as was expected. It was found that Pb⁴⁺ is octahedrally coordinated by six oxygen atoms as usual, while Hg²⁺ is coordinated by three oxygen atoms in a planar manner, this being a very rare coordination of Hg in a solid-state material. The magnetic and electronic transport properties were investigated in terms of the magnetic susceptibility, magnetization, Hall coefficient, and specific heat capacity of polycrystalline HgPbO₃. Although HgPbO₃ has a carrier concentration (=7.3–8.5 × 10²⁰ cm^–3) that is equal to that of metallic oxides, the very weak temperature dependence of the electrical resistivity (residual-resistivity ratio ∼1.5), the significant diamagnetism (= –1.02 × 10^–4 emu mol^–1 at 300 K) that is in the same order of that of Bi powder and the remarkably small Sommerfeld coefficient [=1.6(1) × 10^–3 J mol^–1 K^–2] implied that it is semimetallic in nature. HgPbO₃ does not have a cage structure; nevertheless, at temperatures below approximately 50 K, it clearly exhibits phonon excitation of an anharmonic vibrational mode that is as significant as those of RbOs₂O₆. The mechanism of the anharmonic mode of the HgPbO₃ has yet to be identified, however