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

A calorimetric investigation on the thermodynamic properties of cesium uranyl tungstate, Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)], was undertaken. In addition, model calculations on its thermodynamic behaviour in aqueous solution were made. Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)] 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, C-p,m(o), was measured using adiabatic calorimetry from T= 5 K to 329 K. Using the C-p,m(o) (T) data, the third law entropy at T = 298.15 K, S-m(o), is calculated as (1514 +/- 12) J.K.mol(-1). The enthalpy of formation of Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)] was determined using HF-acid solution calorimetry giving Delta H-f(m)o (T = 298.15 K, Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)], cr) = -(13, 163 +/- 25 kJ).mol(-1). The new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Delta(f)G(m)(o), giving: Delta(f)G(m)(o) (T = 298.15 K, Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)], cr) = -(12,254 +/- 28) J.mol(-1). Best-fit and smoothed C-p,m(o)(T) values for Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)] between 0 K and T= 320 K are presented in table form, along with values for S-m(o) and the functions [H-m(o)(T) - H-m(o)(0)] and [Phi(o)(m)(T) - Phi(o)(m)(0)]. The stability of Cs-8[(UO2)(4)(WO4)(4)(WO5)(2)] 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., A(2)[(UO2)(2)(WO5)O](cr) with A=K and Rb, phases. An analysis indicates that Cs-2[(UO2)(2)(WO5)O] is the most stable phase yet investigated in the family of alkali-metal uranyl tungstates. (C) 2019 Elsevier Ltd.