Thermodynamic vaporization studies on Mn-doped sodium niobate

Abstract

Thermodynamic studies of a Mn-doped single crystal sodium niobate (NaNbO3:xMn) under equilibrium conditions by the method of Knudsen Effusion Mass Spectrometry (KEMS) were conducted in the temperature range 1220–1500 K. The vaporization of pure Na2O(c) as a reference was investigated at temperatures of 810–1090 K. Thermodynamic quantities of sublimation enthalpy, Gibb's energy for reactions, entropy, and activity were derived from the partial pressures of gaseous Na and O2 over the sodium niobate and pure sodium oxide reference samples. The sublimation enthalpy of Na from nominal NaNbO3:0.1%Mn at T = 1350 K was ΔsubH1350K0(Na)=(348±5)kJmol−1 The enthalpy and the entropy values were calculated from the vaporization studies. Deduced from empirical and experimental approaches, the heat of reaction ΔrH2980(NaNbO3:0.1%Mn(c)) = −(75±7)kJmol−1and the heat of formation ΔfH2980(NaNbO3:0.1%Mn(c)) = −(1242±7)kJmol−1were determined. The thermodynamic activity of Na in sodium niobate was temperature-dependent, and at a temperature of 1200 K, a = 3.2·10−2 for nominalNaNbO3:0.1%Mn(c). All collected values are given inside the article.