An experimental calorimetric and a DFT plus U study of the thermodynamic properties of Cs4[(UO2)4(WO5)(W2O8)O2] and Cs4[(UO2)7(WO5)3O3]

A calorimetric and thermodynamic investigation of two alkali-metal uranyl tungstates, Cs-4[(UO2)(4)(WO5)(W2O8)O-2] (cr) and Cs-4[(UO2)(7)(WO5)(3)O-3](cr), was undertaken. Both phases were synthesized by solid-state sintering of a mixture of cesium nitrate, tungsten (VI) oxide and gammauranium (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 Delta H-f(m)0 (T = 298.15 K,Cs-4[(UO2)(4)(WO5)(W2O8)O-2], cr) = -(9239 +/- 17) kJ . mol(-1) and Delta(fH)m(0)(T = 298.15 K, Cs-4[(UO2)(7)(WO5)(3)O-3] , cr) = -(13008 +/- 23) kJ . mol(-1). The low-temperature heat capacity, Cp,m(0) was measured using adiabatic calorimetry from T = 5 K to 324 K for Cs-4[(UO2)(4)(WO5)(W2O8)O-2] (cr) and from T = 5 K to 325 K for Cs-4[(UO2)(7)(WO5)(3)O-3](cr). Using these C-p,m(0) (T) data, the third law entropy at T= 298.15 K, S-m(0) is calculated as (931 4) J.K-1 .mol(-1) for Cs-4[(UO2)(4)(WO5)(W2O8)O-2] (cr) and (1262 +/- J.K-1. mol(-1) for Cs-4[(UO2)(7)(WO5)(3)O-3] (cr). These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Delta(f) G(m)(0), for both phases giving: Delta(f)G(m)(0) (T = 298.15 K, Cs-4[(UO2)(4)(WO5)(W2O8)O-2], cr) = -(8623 +/- 18) kJ . mol(-1) and Delta(f)G(m)(0) (T = 298.15 K, Cs-4 [(UO2)(7)(WO5)(3)O-3], cr) = -(12171 +/- 24) kJ . mol(-1). Smoothed C-p,m(0) (T) values between 0 K and 320 K are presented, along with values for S-m(0) and the functions [H-m(0)(T) - H-m(0)(0)] and [G(m)(0)(T) - H-m(0)(0)], for both phases. Based on these data, the enthalpy of the interca- lation reaction Cs-4[(UO2)(4)(WO5)(W2O8)O-2](cr) + 3UO(3)(cr, gamma) -> Cs-4[(UO2)(7) (WO5)(3)O-3] (cr) at 298 K was determined as -(98 +/- 29) kJ . mol(-1). A value of -90.3 kJ . 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. (C) 2019 Elsevier Ltd.