A calorimetric and thermodynamic investigation of Cs6[(UO2)2(MoO4)3(MoO5)] and calculated phase behaviour in the system (Cs2MoO4 + UO3 + H2O)

Abstract

A calorimetric investigation of the thermodynamic properties of cesium uranyl molybdate, Cs6[(UO22MoO43MoO5), was undertaken. The investigated phase was synthesized by high-temperature solid-state reaction from a mixture of cesium nitrate, molybdenum (VI) oxide and gamma uranium (VI) oxide. The synthetic product was characterized by X-ray powder diffraction and X-ray fluorescence and they show a successful synthesis giving better than 98% of the intended phase. Low-temperature molar heat capacity, Cp,mo, was measured using adiabatic calorimetry between T = 12.3 and 324.3 K. The data were well behaved and do not show any indication of a phase transition or a Cp,mo anomaly. Using the Cp,mo(T) data, the standard molar third-law entropy at T = 298.15 K, Smo, is calculated as 987 ± 4 J∙K−1∙mol−1. The molar enthalpy of formation of Cs6[UO22MoO43MoO5) was determined using HF-acid solution calorimetry giving: ΔfHmo(T=298.15K,Cs6[UO22MoO43MoO5),cr)=-7873±12kJ∙mol-1. These new calorimetric results, together with thermodynamic data from the literature, are used to calculate the molar Gibbs free energy of formation, ΔfGmo, giving: ΔfGmo(T=298.15K,Cs6[UO22MoO43MoO5),cr)=-7309±13kJ∙mol-1. Best-fit and smoothed Cp,moT values for Cs6[UO22MoO43MoO5) between 0 K and T = 324 K are presented in table form, along with values for Smo and the functions [HmoT-Hmo0] and [ΦmoT-Φmo0]. The amounts and behaviour of a number of different crystalline phases in the system “Cs6[UO22MoO43MoO5)" in aqueous solution as a function of pH are calculated using a Gibbs-free-energy-minimization approach. The pH-crystalline phase stability fields in the Cs2MoO4+UO3+H2O system, with and without CO2, are also calculated.