An update on the thermodynamics of Ta2O5

Using a solid-state electrochemical cell incorporating yttria-doped thoria (YDT) as the electrolyte and a mixture of (Mn + MnO) as the reference electrode, standard Gibbs free energy of formation of beta-Ta2O5 has been determined as a function of temperature in the range (1000 to 1300) K. The solid-state electrochemical cell used can be represented as (-)Pt,Ta +Ta2O5//(Y2O3)ThO2//Mn + MnO, Pt(+) Combining the reversible e.m.f. of the cell with recent data on the free energy of formation of MnO, standard Gibbs free energy of formation of Ta2O5 from Ta metal and diatomic oxygen gas (O-2) in the temperature range (1000 to 1300) K is obtained: Delta fG degrees +/- 0.35/(kJ.mol(-1)) = -2004.376 + 0.40445(T/K). Because of the significant solid solubility of oxygen in tantalum, a small correction for the activity of Ta in the metal phase in equilibrium with Ta2O5 is applied. An analysis of the results obtained in this study and other free energy data reported in the literature by the "third law" method suggests the need for refining data for Ta2O5 reported in thermodynamic compilations. Used in the analysis is a revised value for standard entropy of Ta2O5 based on more recent low-temperature heat capacity measurements. An improved set of thermodynamic properties of ditantalum pentoxide (Ta2O5) are presented in the temperature range (298.15 to 2200) K. (C) 2008 Elsevier Ltd. All rights reserved.