Solid Solutions of Pauli-Paramagnetic CaCu₃V₄O₁₂ and Antiferromagnetic CaMn₃V₄O₁₂

Solid solutions of Pauli-paramagnetic CaCu₃V₄O₁₂ and antiferromagnetic CaMn₃V₄O₁₂ were prepared by a high-pressure synthesis technique. All samples crystallized in the A-site-ordered perovskite structure with isovalent Cu²⁺ and Mn²⁺ ions at the square-planar A′ site. The V ion at the B site kept a charge state close to +4 in all of the solid solutions, and the electrons of V were delocalized and contributed to the metallic properties. The substitution of Mn²⁺ for Cu²⁺ in CaCu₃V₄O₁₂, where both Cu and V electrons were delocalized, produced the <i>S</i> = ⁵/₂ localized moments, and the spins at the Mn site interacted antiferromagnetically. Spin-glass-like magnetic behaviors due to the random distribution of Cu/Mn ions at the A′ site were observed at intermediate compositions of the solid solution, whereas the antiferromagnetic transition was observed at the end composition CaMn₃V₄O₁₂

High-Pressure Synthesis, Crystal Structure, and Unusual Valence State of Novel Perovskite Oxide CaCu₃Rh₄O₁₂

A novel perovskite oxide, CaCu₃Rh₄O₁₂, has been synthesized under high-pressure and high-temperature conditions (15 GPa and 1273 K). Rietveld refinement of synchrotron X-ray powder diffraction data indicates that this compound crystallizes in a cubic AA′₃B₄O₁₂-type perovskite structure. Synchrotron X-ray absorption and photoemission spectroscopy measurements reveal that the Cu and Rh valences are nearly trivalent. The spectroscopic analysis based on calculations suggests that the appropriate ionic model of this compound is Ca²⁺Cu^∼2.8+₃Rh^∼3.4+₄O₁₂, as opposed to the conventional Ca²⁺Cu²⁺₃Rh⁴⁺₄O₁₂. The uncommon valence state of this compound is attributed to the relative energy levels of the Cu 3d and Rh 4d orbitals, in which the large crystal-field splitting energy of the Rh 4d orbitals is substantial

Geometrical Spin Frustration of Unusually High Valence Fe⁵⁺ in the Double Perovskite La₂LiFeO₆

A double perovskite-structure oxide La₂LiFeO₆ with unusually high-valence Fe⁵⁺ was synthesized using a high-pressure technique. The Li⁺ and Fe⁵⁺ ions at the B site in the rhombohedral <i>R</i>3̅ perovskite structure are ordered in a rock salt manner, and the resultant tetrahedral network of Fe⁵⁺ gives geometrical spin frustration, which is consistent with a large frustration index <i>f</i> (|θ|/<i>T</i>_N) ≈ 10. Mg²⁺ substitution for Li⁺ produces Fe⁴⁺ from some Fe⁵⁺ and changes the magnetic properties. The Weiss temperature is increased from −119 to 21 K by the substitution of only 1%, significantly decreasing the frustration index. The geometrical frustration of the Fe⁵⁺ spin sublattice cannot be tolerant for even a very small amount of Fe⁴⁺ disturbance

Pd²⁺-Incorporated Perovskite CaPd₃<i>B</i>₄O₁₂ (<i>B</i> = Ti, V)

Novel <i>A</i>-site ordered perovskites CaPd₃Ti₄O₁₂ and CaPd₃V₄O₁₂ were synthesized under high-pressure and high-temperature of 15 GPa and 1000 °C. These compounds are the first example in which a crystallographic site in a perovskite-type structure is occupied by Pd²⁺ ions with a 4d⁸ low spin configuration. The ionic models for these compounds were determined to be Ca²⁺Pd²⁺₃Ti⁴⁺₄O₁₂ and Ca²⁺Pd²⁺₃V⁴⁺₄O₁₂ by structural refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission, and soft X-ray absorption spectroscopy. Magnetic susceptibility, electrical resistivity, and specific heat measurements demonstrated diamagnetic insulating behavior for CaPd₃Ti₄O₁₂ in contrast to the Pauli-paramagnetic metallic nature of CaPd₃V₄O₁₂

Pd²⁺-Incorporated Perovskite CaPd₃<i>B</i>₄O₁₂ (<i>B</i> = Ti, V)

Novel <i>A</i>-site ordered perovskites CaPd₃Ti₄O₁₂ and CaPd₃V₄O₁₂ were synthesized under high-pressure and high-temperature of 15 GPa and 1000 °C. These compounds are the first example in which a crystallographic site in a perovskite-type structure is occupied by Pd²⁺ ions with a 4d⁸ low spin configuration. The ionic models for these compounds were determined to be Ca²⁺Pd²⁺₃Ti⁴⁺₄O₁₂ and Ca²⁺Pd²⁺₃V⁴⁺₄O₁₂ by structural refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission, and soft X-ray absorption spectroscopy. Magnetic susceptibility, electrical resistivity, and specific heat measurements demonstrated diamagnetic insulating behavior for CaPd₃Ti₄O₁₂ in contrast to the Pauli-paramagnetic metallic nature of CaPd₃V₄O₁₂

Valence Transitions in Negative Thermal Expansion Material SrCu₃Fe₄O₁₂

The valence states of a negative thermal expansion material, SrCu₃Fe₄O₁₂, are investigated by X-ray absorption and ⁵⁷Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr²⁺Cu^∼2.4+₃Fe^∼3.7+₄O₁₂. The valence states continuously transform to Sr²⁺Cu^∼2.8+₃Fe^∼3.4+₄O₁₂ upon cooling to ∼200 K, followed by a charge disproportionation transition into the Sr²⁺Cu^∼2.8+₃Fe³⁺_∼3.2Fe⁵⁺_∼0.8O₁₂ valence state at ∼4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu₃Fe₄O₁₂ can be distinguished from the first-order charge-transfer phase transitions (3Cu²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺) in Ln³⁺Cu²⁺₃Fe^3.75+₄O₁₂ (Ln = trivalent lanthanide ions)

<i>B</i>‑Site Deficiencies in <i>A</i>‑site-Ordered Perovskite LaCu₃Pt_3.75O₁₂

An <i>A</i>-site-ordered perovskite LaCu₃Pt_3.75O₁₂ was synthesized by replacing Ca²⁺ with La³⁺ in a cubic quadruple <i>AA</i>′₃<i>B</i>₄O₁₂-type perovskite CaCu₃Pt₄O₁₂ under high-pressure and high-temperature of 15 GPa and 1100 °C. In LaCu₃Pt_3.75O₁₂, 1/16 of <i>B</i>-site cations are vacant to achieve charge balance. The <i>B</i>-site deficiencies were evidenced by crystal structure refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission spectroscopy, and soft X-ray absorption spectroscopy, leading to the ionic model La³⁺Cu²⁺₃Pt⁴⁺_3.75O^2–₁₂. Magnetic susceptibility data for this compound indicated a spin-glass-like behavior below <i>T</i>_g = 3.7 K, which is attributed to disturbance of the antiferromagnetic superexchange interaction by the <i>B</i>-site deficiencies

Suppression of Intersite Charge Transfer in Charge-Disproportionated Perovskite YCu₃Fe₄O₁₂

A novel iron perovskite YCu₃Fe₄O₁₂ was synthesized under high pressure and high temperature of 15 GPa and 1273 K. Synchrotron X-ray and electron diffraction measurements have demonstrated that this compound crystallizes in the cubic <i>AA</i>′₃<i>B</i>₄O₁₂-type perovskite structure (space group<i> Im</i>3̅, No. 204) with a lattice constant of <i>a</i> = 7.30764(10) Å at room temperature. YCu₃Fe₄O₁₂ exhibits a charge disproportionation of 8Fe^3.75+ → 3Fe⁵⁺ + 5Fe³⁺, a ferrimagnetic ordering, and a metal-semiconductor-like transition simultaneously at 250 K, unlike the known isoelectronic compound LaCu₃Fe₄O₁₂ that currently shows an intersite charge transfer of 3Cu²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺, an antiferromagnetic ordering, and a metal–insulator transition at 393 K. This finding suggests that intersite charge transfer is not the only way of relieving the instability of the Fe^3.75+ state in the <i>A</i>³⁺Cu²⁺₃Fe^3.75+₄O₁₂ perovskites. Crystal structure analysis reveals that bond strain, rather than the charge account of the <i>A</i>-site alone, which is enhanced by large <i>A</i>³⁺ ions, play an important role in determining which of intersite charge transfer or charge disproportionation is practical

Suppression of Intersite Charge Transfer in Charge-Disproportionated Perovskite YCu₃Fe₄O₁₂

A novel iron perovskite YCu₃Fe₄O₁₂ was synthesized under high pressure and high temperature of 15 GPa and 1273 K. Synchrotron X-ray and electron diffraction measurements have demonstrated that this compound crystallizes in the cubic <i>AA</i>′₃<i>B</i>₄O₁₂-type perovskite structure (space group<i> Im</i>3̅, No. 204) with a lattice constant of <i>a</i> = 7.30764(10) Å at room temperature. YCu₃Fe₄O₁₂ exhibits a charge disproportionation of 8Fe^3.75+ → 3Fe⁵⁺ + 5Fe³⁺, a ferrimagnetic ordering, and a metal-semiconductor-like transition simultaneously at 250 K, unlike the known isoelectronic compound LaCu₃Fe₄O₁₂ that currently shows an intersite charge transfer of 3Cu²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺, an antiferromagnetic ordering, and a metal–insulator transition at 393 K. This finding suggests that intersite charge transfer is not the only way of relieving the instability of the Fe^3.75+ state in the <i>A</i>³⁺Cu²⁺₃Fe^3.75+₄O₁₂ perovskites. Crystal structure analysis reveals that bond strain, rather than the charge account of the <i>A</i>-site alone, which is enhanced by large <i>A</i>³⁺ ions, play an important role in determining which of intersite charge transfer or charge disproportionation is practical

Glassy Distribution of Bi³⁺/Bi⁵⁺ in Bi_1–<i>x</i>Pb_<i>x</i>NiO₃ and Negative Thermal Expansion Induced by Intermetallic Charge Transfer

The valence distribution and local structure of Bi_1–<i>x</i>Pb_<i>x</i>NiO₃ (<i>x</i> ≤ 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi³⁺ and Bi⁵⁺ was observed for all the samples, but it was a long-ranged one with distinct crystallographic sites in the <i>P</i>1̅ triclinic structure for <i>x</i> ≤ 0.15, while the ordering was short-ranged for <i>x</i> = 0.20 and 0.25. An intermetallic charge transfer between Bi⁵⁺ and Ni²⁺, leading to large volume shrinkage, was observed for all the samples upon heating at ∼500 K