Synthesis, structure and thermal expansion of new complex phosphates of titanium and metals in the +2 oxidation state

Abstract

The phosphates of the composition Ca0.5+xMxTi2-x(PO4)3 (M − Ni, Mg) were synthesized by a co-precipitating of salts in aqueous solutions followed by thermal treatment stages. The studied series formed limited NZP solid solutions within the regions 0 ≤ x ≤ 0.5 (Ni) and 0 ≤ x ≤ 1 (Mg). The Rietveld refinement of the Ca1.2Mg0.7Ti1.3(PO4)3 structure showed that its basis is framework formed by TiO6 and (Mg/Ti)O6 octahedra (together with PO4 tetrahedra), and the Ca2+ ions are situated in the cavities. Thermal expansion of the phosphates depending on their chemical composition was characterized in the temperature range from 173 to 473 K. There was a gradual decrease in the linear thermal expansion coefficients with x growth, so that average linear coefficients varied from (11.0–2.6)∙10−6 K−1. Low average thermal expansion coefficient and its low anisotropy allow the studied phosphate ceramics to withstand temperature changes.