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 [¹₂(Na₂O‧4TiO₂‧3P₂O₅)], NaZr₂P₃O₁₂ [¹₂(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 x 10^-6 per ^◦C and 6 x 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.