(Ca 0.5 Mn 0.5 ) 2 MnTeO 6 - An Anomalously Stable High-Pressure Double Perovskite.

High pressure high temperature treatments of the composition CaMnMnTeO 6 are found to yield only an A 2 BB'O 6 -type double perovskite (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 , rather than a AA'BB'O 6 double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO 6 and CaMnMnWO 6 with similar cation sizes. Double perovskite (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 adopts a monoclinic structure in space group P2 1 /n with a framework of highly tilted MnO 6 and TeO 6 octahedra enclosing disordered Ca 2+ and Mn 2+ cations. Magnetic measurements show that (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K.