Crystal chemistry of complex orthophosphates with framework structure containing group IV d-transition metals and sodium

The phosphates Na5M(PO4)3 (M = Ti, Zr, Hf) of NZP type structure were synthesized by solid-state procedure and precipitating methods. Their thermal behavior and heat capacity were studied in the range 7–650 K. Solid-to-solid phase transitions were observed for the phosphates Na5Zr(PO4)3 (406.9 K) and Na5Hf(PO4)3 (516.5 K). The crystal structure of a novel phosphate, Na5Hf(PO4)3, at 298.15 K was refined by the Rietveld method. The basis of the structure is the framework built up of corner-sharing NaO6- and HfO6-octahedra (with ordered distribution of Na and Hf fragments) and PO4-tetrahedra. Cavities in the framework are occupied mainly by Na+ cations and small part of Hf4+ cations. Structure and phase formation data on the phosphates of alkali and group IV d-transition metals are generalized.

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