Crystal structures, phase transitions and thermal expansion properties of NaZr2(PO4)3–SrZr4(PO4)6 solid solutions

Abstract

Crystal structure evolution and temperature-dependent phase transition of solid solutions are much desired for the understanding of the optimization of functional properties. Herein, the NASICON-type NaZr₂(PO₄)₃–SrZr₄(PO₄)₆ solid solutions with the formula Na_(2−2x)Sr_x[ ]_xZr₄(PO₄)₆ (0 ≤ x ≤ 1), where [ ] represents the vacancy, were prepared by the sol–gel method, and their crystal structures, phase transitions and thermal expansion properties were investigated in detail. In the range of x = 0.3–0.35, there is a reversible structural phase transition Rc ↔ R and the different structural models of the R phase and Rc phase were built to better understand the phase transition mechanism. We determined the phase transition boundary between the Rc and R structures and predicted the phase transition temperature of Na_(2−2x)Sr_x[ ]_xZr₄(PO₄)₆ with any x for further investigation of controlled physical properties. The results indicated that Na_0.5Sr_0.75[ ]_0.75Zr₄(PO₄)₆ showed a near zero thermal expansion in the temperature range of 450–600 K, which can find potential applications.