High pressure synthesis and crystal structure of a new series of perovskite-like compounds CMn7O12 (C = Na,Ca, Cd,Sr, La,Nd)

A new series of perovskite-like compounds CMn₇O₁₂ have been synthesized under high pressure and high temperature conditions. C is a large divalent or trivalent cation such as Ca, Cd, Sr, La and Nd. The structures of the quenched materials have been determined from powder X-ray data. They are distortions of the NaMn₇O₁₂ cubic structure. The C²⁺Mn³⁺₃](Mn³⁺₃Mn⁴⁺)O₁₂ compounds are trigonal ($\text{R}\text{3}$). The C²⁺ and Mn³⁺ as well as the Mn³⁺ and Mn⁴⁺ cations are ordered on the corresponding A and B sites of the perovskite structure, respectively. The C³⁺Mn³⁺₃] (Mn³⁺₄)O₁₂ compounds are monoclinic ($\text{I}\text{2}\text{m}$). In these compounds the order exists only in the A sites. It is shown that the lower symmetry may be the result of a cooperative Jahn-Teller effect of the Mn³⁺ cations occupying the B sites.