Thermodynamics of Vaporization in the Li2O–Ta2O5 system: The Standard Enthalpies of Formation of Lithium Tantalates

Vaporization in the Li2O–Ta2O5 system is studied using the Knudsen effusion method in combination with mass-spectrometric analysis of the gas phase in the temperature range of 1570–1860 K. Saturated vapor above the system is found to consist of lithium atoms, oxygen molecules, and lithium oxide Li2O molecules over the entire range of condensed phase compositions. The absolute partial pressures are calculated, and a p–x section of the complete p–T–x phase diagram at T = 1753 K is constructed. The experimental data made it possible to calculate the standard enthalpies of a number of heterophase reactions and the standard enthalpies of formation of lithium tantalates using the second and third laws of thermodynamics: $${{\Delta }_{f}}H_{{{\text{298}}}}^{^\circ }$$ (Li3TaO4) = –2074.6 ± 62.9 kJ/mol, $${{\Delta }_{f}}H_{{{\text{298}}}}^{^\circ }$$ (LiTaO3) = –1403.5 ± 57.4 kJ/mol, and $${{\Delta }_{f}}H_{{{\text{298}}}}^{^\circ }$$ (LiTa3O8) = –3466.4 ± 47.3 kJ/mol. Activities of the components at T = 1473 K and the standard enthalpy of formation of lithium heptatantalate ( $${{\Delta }_{f}}H_{{{\text{298}}}}^{^\circ }$$ (Li7TaO6) = –3383.0 kJ/mol) are estimated from the p–x section of the phase diagram.