P-T phase diagram of NaMn7O12, a double manganese perovskite-like oxide

We defined the P-T phase diagram of NaMn7O12, a high-pressure phase, with the double perovskite structure. The effects of the reaction parameters, of the precursor and of the reagent non-stoichiometry on the product yield were systematically investigated and the results are reported. The syntheses were carried out in a multi-anvil apparatus under high-pressure and high-temperature conditions (up to 80 kbar and up to 1200 1C, respectively). The reagents were either an oxide mixture or a pre-reacted citrate precursor. Excess of Na with respect to the compound stoichiometry was added. The reaction products were identified and semi-quantitatively evaluated by powder X-ray diffraction. Under optimized conditions, better than 98% single-phase NaMnOpowders were obtained; in a few experiments, crystals as large as 150 mm could be mechanically separated. The structure of NaMn7O12 was refined by powder and single-crystal X-ray diffraction and found to be cubic (Im3 spatial group) with a ¼ 7.312A ? .