Enthalpies of formation of lead zirconate titanate (PZT) solid solutions

The formation energetics and heats of mixing of lead zirconate titanate (PZT) PbZr_xTi_1-xO₃ solid solutions have been investigated using high-temperature oxide melt solution calorimetry. While the heat content (H₉₇₃-H₂₉₈) is almost constant (80-82 kJ/mol) for x = 0.15-1.0, its value for PbTiO₃ (x = 0.0) is larger (86.15 ± 0.55 kJ/mol). Linear variation in the enthalpy of drop solution, ΔH_ds, and the enthalpy of formation from the oxides, ΔH_f,ox (298K), is observed in the tetragonal region (0≤x≤ 0.5) and continues smoothly into part of the rhombohedral region (0.5≤x≤ 0.65). Fitting the data in the tetragonal phase region (0≤x≤ 0.5) to a straight line enables an extrapolation to give the heat of the virtual transformation PbTiO₃ (tetragonal → orthorhombic) as 24.0 ± 3.9 kJ/mol. An extrapolation from the rhombohedral region gives 33.3 ± 3.6 kJ/mol for ΔH of the virtual transition PbZrO₃ (orthorhombic or rhombohedral → tetragonal). The transition enthalpy between orthorhombic and rhombohedral PbZrO₃ is zero within experimental error. The enthalpy of formation from the oxides, ΔH_{f, ox}(298 K), varies from -36.7 ± 2.7 kJ/mol for PbTiO₃ to 0.7 ± 4.3 kJ/mol for PbZrO₃. The ΔH_f,ox(973 K) values agree with previously reported ΔH_f,ox values measured in alkali borate solvent at 1073 K. Fitting ΔH_mix(973 K) to a regular solution model yields an interaction parameter, W, of 39.8 ± 4.6 kJ/mol. However, structural complexity makes the regular solution model only a crude physical description.