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.