Synthesis, phase formation, and thermal expansion of sulfate phosphates with the NaZr2(PO4)3 structure

Abstract

The NaFeZr(PO4)2SO4 and Pb2/3FeZr(PO4)7/3(SO4)2/3 sulfate phosphates with the NaZr2(PO4)3 (NZP) structure were synthesized and studied using X-ray diffraction, electron microprobe analysis, IR spectroscopy, and simultaneous differential thermal and thermogravimetric analysis. The phase formation and thermal stability of the compounds were studied by powder X-ray diffraction and DTA–TG. The Pb2/3FeZr(PO4)7/3(SO4)2/3 structure was refined by full-profile analysis. The structure framework is composed of randomly occupied (Fe,Zr)O6 octahedra and (P,S)O4 tetrahedra; the Pb2+ ions occupy extra-framework sites. The thermal expansion of Pb2/3FeZr(PO4)7/3(SO4)2/3 in the temperature range from–120 to 200°C was studied by temperature X-ray diffraction. In terms of the average linear coefficient of thermal expansion (αav = 1.7 × 10–6°C–1), this compound can be classified as having low expansion. The combination of different tetrahedral anions (a phosphorus and a smaller sulfur one) in the NZP resulted in a decrease in the framework size and cavities and enabled the preparation of low-expansion sulfate phosphate with a smaller extra-framework cation (cheap Pb) instead of larger cations (Cs, Ba, Sr) used most often in the monoanionic phosphates.