Crystal structure solution and high-temperature thermal expansion in NaZr 2 (PO 4 ) 3 -type materials.

The NaZr2 P3 O12 family of materials have shown low and tailorable thermal expansion properties. In this study, SrZr4 P6 O24 (SrO·4ZrO2 ·3P2 O5 ), CaZr4 P6 O24 (CaO·4ZrO2 ·3P2 O5 ), MgZr4 P6 O24 (MgO·4ZrO2 ·3P2 O5 ), NaTi2 P3 O12 [½(Na2 O·4TiO2 ·3P2 O5 )], NaZr2 P3 O12 [½(Na2 O·4ZrO2 ·3P2 O5 )], 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 SrZr4 P6 O24 and CaZr4 P6 O24 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 CaZr4 P6 O24 and SrZr4 P6 O24 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.