A calorimetric and thermodynamic investigation of uranyl molybdate UO2MoO4

A thermophysical and thermochemical calorimetric investigation of UO2MoO4 was undertaken. The UO2MoO4 was synthesized by solid-state reaction of MoO3 and UO3 and characterized by X-ray powder diffraction and X-ray fluorescence methods. The temperature of fusion, Tfus = (1212 ± 2) K, and the enthalpy of fusion, ΔfusH° (1212 K, UO2MoO4, cr.) = (16 ± 1) kJ · mol−1, were determined using DTA methods. The low-temperature heat capacity, Cp°(Т), behaviour of UO2MoO4 was measured using adiabatic calorimetry from T = (6 to 301) K and the third-law entropy determined. S° = (192.8 ± 0.8) J · mol−1 · K−1 at T = 298.15 K is obtained. The enthalpy of formation of UO2MoO4 was determined using HF-solution calorimetry giving ΔfH°(298 K, UO2MoO4, cr.) = −(1985 ± 8) kJ · mol−1. From these data, the Gibbs energy of formation ΔfG°(298 K, UO2MoO4, cr.) = −(1836 ± 8) kJ · mol−1 is calculated. Smoothed Cp°(Т) values between T = (0 and 300) K are presented along with values for S° and the functions [H°(T) − H°(0)] and [G°(T) − H°(0)].