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

Abstract

The NaFeZr(PO₄)₂SO₄ and Pb_2/3FeZr(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 Pb_2/3FeZr(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 Pb_2/3FeZr(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 (α_av = 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.