Crystal structure solution and high-temperature thermal expansion in NaZr₂(PO₄)₃-type materials

The NaZr₂P₃O₁₂ family of materials have shown low and tailorable thermal expansion properties. In this study, SrZr₄P₆O₂₄ (SrO·4ZrO₂·3P₂O₅), CaZr₄P₆O₂₄ (CaO·4ZrO₂·3P₂O₅), MgZr₄P₆O₂₄ (MgO·4ZrO₂·3P₂O₅), NaTi₂P₃O₁₂ [½(Na₂O·4TiO₂·3P₂O₅)], NaZr₂P₃O₁₂ [½(Na₂O·4ZrO₂·3P₂O₅)], and related solid solutions were synthesized using the organic-inorganic steric entrapment method. The samples were characterized by in-situ high-temperature X-ray diffraction from 25 to 1500°C at the Advanced Photon Source and National Synchrotron Light Source II. The average linear thermal expansion of SrZr₄P₆O₂₄ and CaZr₄P₆O₂₄ was between -1 × 10^-6 per °C and 6 × 10^-6 per °C from 25 to 1500°C. The crystal structures of the high-temperature polymorphs of CaZr₄P₆O₂₄ and SrZr₄P₆O₂₄ with R3c symmetry were solved by Fourier difference mapping and Rietveld refinement. This polymorph is present above ∼1250°C. This work measured thermal expansion coefficients to 1500°C for all samples and investigated the differences in thermal expansion mechanisms between polymorphs and between compositions.