Thermal expansion of NASICON materials

Abstract

Our and others’ results on the thermal expansion of compounds and the crystal-chemical information about their structure have been used to properly select compositions of new NASICON materials with small thermal expansion and low thermal expansion anisotropy: MxM1−x′Ti2(PO4)3 (M and M′ are alkali metals, 0 ≤ x ≤ 1), M″FeTi(PO4)3 (M″ is an alkaline-earth metal), and Ca0.5(1 + x)FexTi2 − x(PO4)3 (0 ≤ x ≤ 1). Such materials have been synthesized through salt coprecipitation from aqueous solutions and their thermal expansion has been assessed by high-temperature X-ray diffraction. We have determined their unit-cell parameters as functions of temperature and correlated their thermal expansion parameters and composition. The K0.5Rb0.5Ti2(PO4)3 and Rb0.8Cs0.2Ti2(PO4)3 materials obtained combine small volumetric thermal expansion and near-zero thermal expansion anisotropy.