Os Doping Suppressed Cu-Fe Charge Transfer and Induced Structural and Magnetic Phase Transitions in LaCu 3 Fe 4- x Os x O 12 ( x = 1 and 2).

B-site Os-doped quadruple perovskite oxides LaCu ₃ Fe _{4- x} Os _x O ₁₂ ( x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu ₃ Fe ₄ O ₁₂ experiences Cu-Fe intermetallic charge transfer that changes the Cu ³⁺ /Fe ³⁺ charge combination to Cu ²⁺ /Fe ^3.75+ at 393 K, in the Os-doped samples, the Cu and Fe charge states are found to be constant 2+ and 3+, respectively, indicating the complete suppression of charge transfer. Correspondingly, Os ⁶⁺ and mixed Os ^4.5+ valence states are determined by X-ray absorption spectroscopy for x = 1 and x = 2 compositions, respectively. The x = 1 sample crystallizes in an Fe/Os disordered structure with the Im 3̅ space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn 3̅, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu ²⁺ (↑)Fe ³⁺ (↑)Os ^4.5+ (↓) ferrimagnetic ordering near 520 K. This work provides a rare example for 5d substitution-suppressed intermetallic charge transfer as well as induced structural and magnetic phase transitions with high spin ordering temperature.