Thermodynamic Properties for Nd₂(MoO₄)₃ Formed in the Nuclear Fuel Waste Glasses

Abstract

The thermodynamic properties for Nd₂(MoO₄)₃ were investigated. Nd₂(MoO₄)₃ is one of the end member of the yellow phases which are known as hygroscopic harmful phases in the nuclear fuel waste glasses. The standard molar entropy, , at 298.15 K of Nd₂(MoO₄)₃ was determined by measuring its isobaric heat capacities, , from 2 K via the fitting functions including the Debye-Einstein formula and electronic- as well as magnetic terms. The Neel temperature, T_N, estimated by extrapolating the magnetic-term in the fitting function. Its standard Gibbs energy of formation, , was determined by combining datum with the standard enthalpy of formation, , which were estimated from ones for Ce₂(MoO₄)₃ and Sm₂(MoO₄)₃. The unknown standard Gibbs energy of solution, , at 298.15 K of Nd₂(MoO₄)₃ was predicted from the reference data for MoO₄²⁻(aq) and Nd³⁺(aq). The obtained thermodynamic values are as follows:

The data obtained in the present work are expected to be useful for geochemical simulations of the diffusion of radioactive elements through underground water.

Fig. 2 Experimental molar heat capacities, C^°_p,m, for Nd₂(MoO₄)₃ at 2–300 K (open circle) and ones calculated from the fitting functions (eqs. (4)–(7), solid line), compared with C^°_p,m(17/4 MoO₃) (solid circle) and C^°_p,m(17/5 Nd₂O₃) (solid triangle) at 298.15 K. Solid rectangle shows C^°_p,m at 298.15 K on the basis of the Neumann-Kopp law.