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Thermodynamic studies of a polycrystalline sodium niobate (NaNbO 3 ) under equilibrium conditions by the method of Knudsen effusion mass spectrometry (KEMS) were conducted in the temperature range 1250–1485 K. The vaporization of pure Na 2 O(c) was investigated at temperatures of 900–1165 K. Thermodynamic quantities of sublimation enthalpy, activity, and entropy were derived from the partial pressures of gaseous Na, Na 2 O, and O 2 over the sodium niobate and pure sodium oxide reference samples. The sublimation enthalpy of Na from NaNbO 3 (c) at T =1370 K was Δ sub H 1370 K 0 ( Na ) = 465 ± 9 kJ mol − 1 . Heat capacity measurements under constant pressure ( C p ( T ) 0 ) by Dynamic Scanning Calorimetry (DSC) in the temperature range 298–1450 K resulted in six polynomial equations for each branch of the five transitions. Below the first phase transition at 639 K the C p ( T ) 0 values increased from 97 to 123 J mol −1 K −1 . Beyond this transition and up to the third one at 792 K, C p ( T ) 0 gently inclined from about 120 to a maximum value of 123 J mol −1 K −1 . Finally, beyond the fifth transition at 907 K it approximated to an almost constant value of 115 J mol −1 K −1 . The enthalpy and the entropy values were calculated from the C p ( T ) 0 polynomials and from the vaporization studies. The enthalpy of the five transitions in sodium niobate, Δ trs H 298 0 ( NaNbO 3 ( c ) ) were measured by drop calorimetry. Deduced from empirical and experimental approaches, the heat of reaction for oxides was Δ r H 298 0 ( NaNbO 3 ( c ) ) = − 94.3 ± 7 kJ mol −1 and the heat of formation was Δ f H 298 0 ( NaNbO 3 ( c ) ) = − 1250.8 ± 7 kJ mol −1 . The thermodynamic activity of Na 2 O in sodium niobate was temperature dependent, and its values varied from 2.6×10 −3 to 8.3×10 −4 in the range 1100–1250 K. NaNbO 3 (c) has a long-term stability in the temperature range including the transition to cubic phase at 907 K.

Thermodynamic studies of a single crystal lead titanate (PbTiO3) under equilibrium conditions by the method of Knudsen effusion mass spectrometry (KEMS) were conducted in the temperature range 950–1210 K. The vaporization of pure PbO(c) was investigated at temperatures of 810–1040 K. Thermodynamic quantities of sublimation enthalpy, activity, and entropy were derived from the partial pressures of gaseous Pb, PbO, and O2 over the lead titanate and pure lead oxide reference samples. The sublimation enthalpy of Pb from PbTiO3(c) at T=1080 K was ΔsubH1080K0(Pb)=(340±11)kJmol−1 and for PbO was ΔsubH1080K0=(251±12)kJmol−1. Heat capacity measurements under constant pressure (Cp(T)0) by dynamic scanning calorimetry (DSC) in the temperature range 298–1273 K resulted in two polynomial equations. The Cp(T)0 values increased from 106 to 139±0.2J mol−1 K−1 below the ferroelectric–paraelectric phase transition at Tc=763 K, and Cp(T)0≅(112±0.3) J mol−1 K−1 was almost constant above Tc. The low values of entropy, ΔtrsS=(2.1±0.9) J mol−1 K−1 and enthalpy, ΔtrsH=(1.57±0.7) kJ mol−1, and the λ (lambda)-shape of the ferro-paraelectric phase change in the Cp(T)0 function suggest that the transition can be classified as a first order one with a tendency to a continuous one. The enthalpy and the entropy values were calculated from the Cp(T)0 polynomials and from the vaporization studies. Deduced from empirical and experimental approaches, the heat of reaction was ΔrH2980(PbTiO3(c))=−(26.4±1.8)kJmol−1 and the heat of formation was ΔfH2980(PbTiO3(c))=−(1190.4±3.9)kJmol−1. The thermodynamic activity of Pb and PbO in lead titanate was temperature dependent and their values were a=(8.67±2.9)10−4, and (9.96±3.6)10−3, respectively, at a temperature of 1000 K. Near the ferroelectric–paraelectric phase change at 770 K, PbTiO3(c) has a long-term stability.