P-T phase diagram and single crystal structural refinement of NaMn7O12

The double perovskite NaMn7O12 was synthesized by a solid state reaction carried out under several GPa at a relatively low temperatures. The stability field in the P-T phase diagram was estimated to be located at 3-8 GPa and 430-900 °C. The synthesis was carried out in a multi-anvil apparatus. The reaction pressure and temperature were systematically varied within the 10−4-8 GPa and 400-1100 °C ranges, respectively. The reaction yields were optimized by using a MnNa citrate precursor, and by optimizing the Na concentration in the reagents. The best yields were obtained when the precursors or the mixtures contained 5% excess Na. The products and the relative amounts were evaluated by X-ray powder diffraction. One-hundred micron crystals were obtained in specific experiments. NaMn7O12 crystallizes in the cubic Im3 space group, with a=7.312 Å and Z=2. The structure was refined by single crystal X-ray diffraction. It confirmed that the Mn(2) sites (A′ position of the double perovskite structure with general formula AA3′B4O12) are entirely occupied by Mn3+ while the Mn(1) sites (B position) are occupied disorderly by Mn3+ and Mn4+ in a 50/50 ratio. The Na Debye-Waller factor close to zero seems to indicate the presence of intergrowths. On the other hand the bond valence sum for the Na site yields 1.22 v.u. This unrealistic value indicates that the icosahedral site is too small to accommodate the Na cations at ambient pressure, which is in agreement with the findings that NaMn7O12 is a high pressure phase, metastably retained at ambient pressure.