An update on the thermodynamics of Ta₂O₅

Abstract

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 β-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 (O₂) in the temperature range (1000 to 1300) K is obtained: Δ_fG∘±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.