A calorimetric and thermodynamic investigation of cesium uranyl tungstate Cs8[(UO2)4(WO4)4(WO5)2

Abstract

A calorimetric investigation on the thermodynamic properties of cesium uranyl tungstate, Cs8UO24WO44WO52, was undertaken. In addition, model calculations on its thermodynamic behaviour in aqueous solution were made. Cs8UO24WO44WO52 was synthesized for the first time by high-temperature solid-state reaction from a mixture of cesium nitrate, tungsten (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. The low-temperature heat capacity, Cp,mo, was measured using adiabatic calorimetry from T = 5 K to 329 K. Using the Cp,mo(T) data, the third law entropy at T = 298.15 K, Smo, is calculated as (1514 ± 12) J∙K−1∙mol−1. The enthalpy of formation of Cs8UO24WO44WO52 was determined using HF-acid solution calorimetry giving ΔfHmo(T=298.15K,Cs8UO24WO44WO52,cr)=-(13,163±25kJ)·mol-1. The new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, ΔfGmo, giving: ΔfGmo(T=298.15K,Cs8UO24WO44WO52,cr)=-(12,254±28)J·mol-1. Best-fit and smoothed Cp,moT values for Cs8UO24WO44WO52 between 0 K and T = 320 K are presented in table form, along with values for Smo and the functions [HmoT-Hmo0] and [ΦmoT-Φmo0]. The stability of Cs8UO24WO44WO52 in contact with aqueous solutions and as a function of pH is calculated. These data were compared with thermodynamic results of previous studies performed on other uranyl tungstates, i.e., A2[UO22WO5O]cr with A = K and Rb, phases. An analysis indicates that Cs2[UO22WO5)O is the most stable phase yet investigated in the family of alkali-metal uranyl tungstates.