Third Law Entropy of Barium Molybdate

Abstract

The isobaric heat capacities, C_p,m^°, for BaMoO₄ at 2–300 K were measured by the relaxation method. The third law entropy, S_m^°, was determined via the Debye-Einstein function into which the Debye temperature, Θ_D, as physico-chemical constant and a thermal expansion term were incorporated. Θ_D was determined from C_p,m^° at very low temperatures. The obtained thermodynamic properties were: S_m^° (BaMoO₄(cr), 298.15 K)/J K⁻¹ mol⁻¹ = 152.69 ± 1.53; Θ_D(BaMoO₄(cr))/K = 295 ± 3. The phase stability of BaMoO₄ was discussed on the basis of its standard Gibbs energy of formation, Δ _fG_m^°. It was derived by combining S_m^° determined in this study with the reference data of the standard enthalpy of formation, Δ _fH_m^°. The thermodynamic properties obtained in the present study can be used for evaluating the hierarchy for formation of the yellow phase related-substances in nuclear waste glasses.