Pb0.5 + xMgxZr2 – x(PO4)3(x = 0, 0.5) Phosphates: Structure and Thermodynamic Properties

Abstract

Abstract—Crystalline Pb0.5 +xMgxZr2 –x(PO4)3 (x = 0, 0.5) phosphates of NaZr2(PO4)3 (NZP) structural type were synthesized. The heat capacity of Pb0.5Zr2(PO4)3 was measured by adiabatic vacuum and differential scanning calorimetry (DSC) within the temperature range 8–660 K. The studied phosphates were found to experience a reversible phase transition in the region 256–426 K. According to the results of Rietveld structural study, this transition occurred due to an increase in disorder of lead cation positions in cavities of the NZP structure. The measurements of PbMg0.5Zr1.5(PO4)3 heat capacity in the temperature range 195–660 K showed that it experienced a similar phase transition at 255–315 K. Based on the measured experimental data, the thermodynamic functions of Pb0.5Zr2(PO4)3, such as $$C_{p}^{0}(T),$$ [H0(T) – H0(0)], S0(T), and [G0(T) – H0(0)] were calculated for the temperature range 0–660 K. The standard formation enthalpy of Pb0.5Zr2(PO4)3 was determined at 298.15 K.