An experimental calorimetric and a DFT + U study of the thermodynamic properties of [formula omitted] and [formula omitted

Abstract

A calorimetric and thermodynamic investigation of two alkali-metal uranyl tungstates, Cs4UO24WO5W2O8O2(cr) and Cs4[UO27WO53O3](cr), was undertaken. Both phases were synthesized by solid-state sintering of a mixture of cesium nitrate, tungsten (VI) oxide and gamma-uranium (VI) oxide at high temperature. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The enthalpies of formation of both phases were determined using HF-solution calorimetry giving ΔfHmoT=298.15K,Cs4UO24WO5W2O8O2,cr=-9239±17kJ·mol-1 andΔfHmoT=298.15K,Cs4UO27WO53O3,cr=-13008±23kJ·mol-1. The low-temperature heat capacity, Cp,mo, was measured using adiabatic calorimetry from T = 5 K to 324 K for Cs4UO24WO5W2O8O2cr and from T = 5 K to 325 K for Cs4UO27WO53O3cr. Using these Cp,moT data, the third law entropy at T = 298.15 K, Smo, is calculated as (931 ± 4) J∙K−1∙mol−1 for Cs4UO24WO5W2O8O2cr and (1262 ± 8) J∙K−1∙mol−1 for Cs4UO27WO53O3cr. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, ΔfGmo, for both phases giving: ΔfGmoT=298.15K,Cs4UO24WO5W2O8O2,cr=-8623±18kJ·mol-1 and ΔfGmoT=298.15K,Cs4UO27WO53O3,cr=-12171±24kJ·mol-1. Smoothed Cp,moT values between 0 K and 320 K are presented, along with values for Smo and the functions HmoT-Hmo0 and GmoT-Hmo0, for both phases. Based on these data, the enthalpy of the intercalation reaction Cs4UO24WO5W2O8O2cr+3UO3cr,γ→Cs4UO27WO53O3cr at 298 K was determined as -98±29kJ·mol-1. A value of -90.3kJ·mol-1 at 0 K was obtained from DFT + U calculations. The results show the potential of using DFT + U simulations to understand solid-state reactions involving crystalline phases having 5d- and 5f-elements.