Thermodynamic properties and behaviour of A2[(UO2)(MoO4)2] compounds with A = Li, Na, K, Rb, and Cs

A thermodynamic investigation of five alkali-metal uranyl molybdates of the general formula A(2)[(UO2)(MoO4)(2)], where A = Li, Na, K, Rb, and Cs, was undertaken. The various phases were synthesized by solid-state reaction of ANO(3), with A = Li, Na, K, Rb, or Cs, MoO3 and gamma-UO3. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The low-temperature heat capacity, C-p degrees, was measured using adiabatic calorimetry from T = (6 to 335) K. Based on these data, the third law entropy at T = 298.15 K, S degrees, is (345 +/- 1) J . K (1) . mol (1) for LiA(2)[(UO2)(MoO4)(2)], (373 +/- 1) J . 1 mol (1) for NaA(2)[(UO2)(MoO4)(2)], (390 +/- 1) J . K (1) . mol (1) for KA(2)[(UO2)(MoO4)(2)], (377 +/- 1) J . K (1) . mol (1) for RbA(2)[(UO2)(MoO4)(2)] and (394 +/- 1) J . K (1) . mol (1) for CsA(2)[(UO2)(MoO4)(2)]. The enthalpy of formation of LiA(2)[(UO2)(MoO4)(2)] was determined using HF solution calorimetry giving DfH +/-(T = 298 K, Li2[(UO2)( MoO4) 2], cr) = -(3456 +/- 9) kJ . mol (1). Using these new experimental results, together with literature data, the Gibbs free energy of formation of each compound was calculated, giving: Delta(f)G degrees (T = 298 K, LiA(2)[(UO2)(MoO4)(2)], cr) =-(3204 +/- 9) kJ.mol (1), Delta(f)G degrees (T = 298 K, Na2[(UO2)(MoO4) 2], cr) =-(3243 +/- 2) kJ . mol (1), Delta(f)G degrees (T = 298 K, KA(2)[(UO2)(MoO4)(2)], cr) =-(3269 +/- 3) kJ . mol (1), Delta(f)G degrees (T = 298 K, RbA(2)[(UO2)(MoO4)(2)], cr) = -(3262 +/- 3) kJ . mol (1), and Delta(f)G degrees (T = 298 K, CsA(2)[(UO2)(MoO4)(2)], cr) = -(3259 +/- 3) kJ . mol(1). Smoothed C-p degrees (T) values between T = (0 and 320) K are presented along with values for S - and the functions [H degrees(T)-H degrees(0)] and [G degrees(T)-H degrees(0)]. Delta S-f, Delta H-f, and Delta(f)G behaviour at T = 298 K of the A(2)[(UO2)(MoO4)(2)] group is analysed as a function of the molar volumes of the different phases. The phase relations of KA(2)[(UO2)(MoO4)(2)] in aqueous solution, in the presence and absent of carbon dioxide at T = 298 K, were studied by thermodynamic model calculations. (C) 2014 Elsevier Ltd. All rights reserved.