Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

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 deltafHmoT=298.15K,Cs4UO24WO5W2O8O2,cr=-9239+-17kJ*mol-1 anddeltafHmoT=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, deltafGmo, for both phases giving: deltafGmoT=298.15K,Cs4UO24WO5W2O8O2,cr=-8623+-18kJ*mol-1 and deltafGmoT=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,gamma-greater thanCs4UO27WO53O3cr 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.