Abstract—Crystalline Pb_{0.5 +x}Mg_xZr_{2 –x}(PO₄)₃ (x = 0, 0.5) phosphates of NaZr₂(PO₄)₃ (NZP) structural type were synthesized. The heat capacity of Pb_0.5Zr₂(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 PbMg_0.5Zr_1.5(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 Pb_0.5Zr₂(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 Pb_0.5Zr₂(PO₄)₃ was determined at 298.15 K.

中文翻译：

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Pb 0.5 + x Mg x Zr 2 – x（PO 4）3（x = 0，0.5）磷酸盐：结构和热力学性质

摘要_—合成了NaZr ₂（PO ₄）₃（NZP）结构类型的晶体Pb _{0.5 + x} Mg _x Zr _{2 – x}（PO ₄）₃（x = 0，0.5）磷酸盐。Pb _0.5 Zr ₂（PO ₄）₃的热容通过绝热真空和差示扫描量热法（DSC）在8–660 K的温度范围内进行测量。发现所研究的磷酸盐在256–426 K的区域内经历了可逆的相变。根据Rietveld结构研究的结果，过渡发生是由于NZP结构空腔中铅阳离子位置的紊乱增加所致。在195–660 K的温度范围内对PbMg _0.5 Zr _1.5（PO ₄）₃热容的测量表明，它在255–315 K处经历了相似的相变。基于测得的实验数据，Pb _0.5 Zr的热力学函数₂（PO ₄）₃，例如\（C_ {p} ^ {0}（T），\） [ H ⁰（T）– H ⁰（0）]，S ⁰（T）和[ G ⁰（T）– H ⁰（ [0）]的计算范围为0-660K。确定Pb _0.5 Zr ₂（PO ₄）₃的标准形成焓为298.15K 。