Thermodynamic studies on the systems M–Te–O (M = Nd, Sm)

Abstract

The standard Gibbs energies of formation of Nd2TeO6 and M6TeO12 (where M = Nd, Sm) were determined from vapour pressure measurements. The vapour pressure of TeO2(g) was measured by employing thermogravimetry-based transpiration technique. The temperature dependence of the vapour pressure of TeO2(g) over the mixtures Nd2TeO6+Nd6TeO12, generated by the incongruent vapourisation reaction, 3Nd2TeO6(s) → Nd6TeO12(s)+2TeO2(g)+O2(g), was measured in the temperature range 1,408–1,495 K. Similarly, the vapour pressure of TeO2(g) over the mixtures M6TeO12+M2O3 (where M = Nd, Sm), generated by the incongruent vapourisation reaction, M6TeO12(s) → 3M2O3(s)+TeO2(g)+½O2(g), was measured in the temperature range 1,703–1,773 and 1,633–1,753 K for Nd6TeO12(s) and Sm6TeO12(s), respectively. Enthalpy increments of M2TeO6(s) (where M = Nd, Sm) were determined by inverse drop calorimetric method in the temperature range 573–1,273 K. The thermodynamic functions, viz., heat capacity, entropy and free energy functions, were derived from the measured values of enthalpy increments. A mean value of −2,426.2 ± 0.6 and −2,417.9 ± 1.1 kJ mol−1 was obtained for \( \Updelta_{\text{f} } H_{298}^{\text{o}} \)(Nd2TeO6, s) and \( \Updelta_{\text{f}} H_{298}^{\text{o}} \)(Sm2TeO6, s), respectively, by combining the value of \( \Updelta_{\text{f}} G^{\text{o}} \)(Nd2TeO6, s) and \( \Updelta_{\text{f}} G^{\text{o}} \)(Sm2TeO6, s) derived from vapour pressure data and the free energy functions derived from the drop calorimetric data.