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

Abstract

The phosphates Na 5M(PO 4) 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 Na 5Zr(PO 4) 3 (406.9 K) and Na 5Hf(PO 4) 3 (516.5 K). The crystal structure of a novel phosphate, Na 5Hf(PO 4) 3, at 298.15 K was refined by the Rietveld method. The basis of the structure is the framework built up of corner-sharing NaO 6- and HfO 6-octahedra (with ordered distribution of Na and Hf fragments) and PO 4-tetrahedra. Cavities in the framework are occupied mainly by Na + cations and small part of Hf 4+ cations. Structure and phase formation data on the phosphates of alkali and group IV d-transition metals are generalized.