Charge-Order Melting in Charge-Disproportionated Perovskite CeCu3Fe4O12

High-Pressure Synthesis of Semiconducting PbCu3Mn4O12 with Near-Room-Temperature Ferrimagnetic Order

A transition-metal oxide of PbCu3Mn4O12 was prepared at 1523 K and 10 GPa. An A-site-ordered quadruple perovskite structure with the space group Im3̅ is assigned for this compound. Based on bond-valence-sum calculations and X-ray absorption spectroscopy, the charge combination is determined to be PbCu32+Mn44+O12. Due to Cu2+(↑)-Mn4+(↓) antiferromagnetic coupling, a near-room-temperature ferrimagnetic phase transition is observed at approximately 287 K. PbCu3Mn4O12 exhibits a semiconducting electric transport property with the energy band gap Eg ≈ 0.2 eV. In addition, considerable low-field magnetoresistance effects are observed at lower temperatures. This study provides an intrinsic near-room-temperature ferrimagnetic semiconductor that exhibits potential applications in next-generation spintronic devices.

A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce)

A novel quadruple perovskite oxide CeCu₃Co₄O₁₂ has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu₃Co₄O₁₂ series (A = Ca, Y, Ce) from Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ to Y³⁺Cu³⁺₃Co³⁺₄O₁₂ to Ce⁴⁺Cu^2.67+₃Co³⁺₄O₁₂, where the electrons are doped in the order from B-site (Co^3.25+ → Co³⁺) to A'-site (Cu³⁺ → Cu^2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO₃-type perovskite and quadruple perovskites CaCu₃B₄O₁₂ (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na_1-xLa_xMn₃Ti₄O₁₂ and A'- and B-site electron doping for AMn₃V₄O₁₂ (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.

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.

Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5

A Sr analogue of Ca_0.5Bi_0.5FeO₃, Sr_0.5Bi_0.5FeO₃, containing unusually high valence Fe^3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe^3.5+ by a single charge disproportionation (CD) transition (Fe^3.5+ → 0.75Fe³⁺ + 0.25Fe⁵⁺) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca_0.5Bi_0.5FeO₃. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R3̅c crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi³⁺ ions in Sr_0.5Bi_0.5FeO₃ do not act as countercations accepting oxygen holes as they do in Ca_0.5Bi_0.5FeO₃, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi⁵⁺ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe³⁺ spins align antiferromagnetically and one-fourth of the Fe³⁺ spins are randomly replaced by Fe⁵⁺ spins coupled ferromagnetically with the neighboring Fe³⁺ spins.

Successive Charge Transitions of Unusually High-Valence Fe3.5+ : Charge Disproportionation and Intermetallic Charge Transfer

A perovskite-structure oxide containing unusually high-valence Fe^3.5+ was obtained by high-pressure synthesis. Instability of the Fe^3.5+ in Ca_0.5 Bi_0.5 FeO₃ is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.