Vaporization and caloric studies on sodium niobate

Abstract 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.