Covalency Competition in the Quadruple Perovskite CdCu₃Fe₄O₁₂

Cadmium ions (Cd²⁺) are similar to calcium ions (Ca²⁺) in size, whereas the Cd²⁺ ions tend to form covalent bonds with the neighboring anions because of the high electronegativity. The covalent Cd–O bonds affect other metal–oxygen bonds, inducing drastic changes in crystal structures and electronic states. Herein, we demonstrate high-pressure synthesis, crystal structure, and properties of a new quadruple perovskite CdCu₃Fe₄O₁₂. This compound exhibits an electronic phase transition accompanying a charge disproportionation of Fe ions without charge ordering below ∼200 K, unlike charge-disproportionation transition with rock-salt-type charge ordering for CaCu₃Fe₄O₁₂. First-principle calculations and Mössbauer spectroscopy display that covalent Cd–O bonds effectively suppress the Fe–O bond covalency, resulting in an electronic state different from that of CaCu₃Fe₄O₁₂. This finding proposes covalency competition among constituent metal ions dominating electronic states of complex metal oxides

Inverse Charge Transfer in the Quadruple Perovskite CaCu₃Fe₄O₁₂

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu₃Fe₄O₁₂ undergoes an “inverse” electron charge transfer in which valence electrons move from B-site Fe to A′-site Cu ions (∼3Cu^∼2.4+ + 4Fe^∼3.65+ → ∼3Cu^∼2.2+ + 4Fe^∼3.8+) simultaneously with a charge disproportionation transition (4Fe^∼3.8+ → ∼2.4Fe³⁺ + ∼1.6Fe⁵⁺), on cooling below 210 K. The direction of the charge transfer for CaCu₃Fe₄O₁₂ is opposite to those reported for other perovskite oxides such as BiNiO₃ and ACu₃Fe₄O₁₂ (A = Sr²⁺ or the large trivalent rare-earth metal ions), in which the electrons move from A/A′-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds

Inverse Charge Transfer in the Quadruple Perovskite CaCu₃Fe₄O₁₂

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu₃Fe₄O₁₂ undergoes an “inverse” electron charge transfer in which valence electrons move from B-site Fe to A′-site Cu ions (∼3Cu^∼2.4+ + 4Fe^∼3.65+ → ∼3Cu^∼2.2+ + 4Fe^∼3.8+) simultaneously with a charge disproportionation transition (4Fe^∼3.8+ → ∼2.4Fe³⁺ + ∼1.6Fe⁵⁺), on cooling below 210 K. The direction of the charge transfer for CaCu₃Fe₄O₁₂ is opposite to those reported for other perovskite oxides such as BiNiO₃ and ACu₃Fe₄O₁₂ (A = Sr²⁺ or the large trivalent rare-earth metal ions), in which the electrons move from A/A′-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds

Room-Temperature Pressure-Induced Nanostructural CuInTe₂ Thermoelectric Material with Low Thermal Conductivity

A room-temperature high-pressure synthesis method is proposed as an alternative way to induce nanoscale structural disorder in the bulk thermoelectric CuInTe₂ matrix. This disorder stems from the coexistence of distinct domains with different degrees and geometries of disorder at Cu/In cation sites. The lattice thermal conductivity of high-pressure-treated CuInTe₂ is substantially less than that of hot-pressed CuInTe₂. The Debye–Callaway model reveals that the reduced lattice thermal conductivity is mainly attributed to disorder at the Cu/In cation sites and stacking faults, which were probably created during formation of the high-pressure-treated phases. This study demonstrates that room-temperature high-pressure synthesis can produce a radical change in the crystal structure and physical properties of conventional thermoelectric materials

AgCu₃V₄O₁₂: a Novel Perovskite Containing Mixed-Valence Silver ions

A novel silver-containing perovskite, AgCu₃V₄O₁₂, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group <i>Im</i>3̅), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valence-sum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag⁺ and Ag³⁺, unlike the case of well-known perovskite-type AgNbO₃ and AgTaO₃ containing only Ag⁺ ions. We discuss metallic conduction in relation to electronic structure calculations

AgCu₃V₄O₁₂: a Novel Perovskite Containing Mixed-Valence Silver ions

A novel silver-containing perovskite, AgCu₃V₄O₁₂, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group <i>Im</i>3̅), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valence-sum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag⁺ and Ag³⁺, unlike the case of well-known perovskite-type AgNbO₃ and AgTaO₃ containing only Ag⁺ ions. We discuss metallic conduction in relation to electronic structure calculations

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

<i>A</i>‑Site-Ordered Perovskite MnCu₃V₄O₁₂ with a 12-Coordinated Manganese(II)

A novel cubic perovskite MnCu₃V₄O₁₂ has been synthesized at a high pressure and high temperature of 12 GPa and 1373 K. This compound crystallizes in the <i>A</i>-site-ordered perovskite structure (space group <i>Im</i>3̅) with lattice constant <i>a</i> = 7.26684(10) Å at room temperature. The most notable feature of this compound lies in the fact that the Mn²⁺ ion is surrounded by 12 equidistant oxide ions to form a regular icosahedron; the situation of Mn²⁺ is unprecedented for the crystal chemistry of an oxide. An anomalously large atomic displacement parameter <i>U</i>_iso= 0.0222(8) Ų is found for Mn²⁺ at room temperature, indicating that the thermal oscillation of the small Mn²⁺ ion in a large icosahedron is fairly active. Magnetic susceptibility and electric resistivity measurements reveal that 3d electrons of Mn²⁺ ions are mainly localized, while 3d electrons in Cu²⁺ and V⁴⁺ ions are delocalized and contribute to the metallic conduction

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