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⁺($C{u}^{2+}{Mn}_2^{3+}$)(${Mn}_3^{3+}M{n}^{4+}$)O₁₂ charge distribution in the charge-ordered R-3 phase and Ce³⁺($C{u}^{2+}{Mn}_2^{3+}$)(${Mn}_4^{3.25+}$)O₁₂ in the charge-disordered Im-3 phase. (paper)