A calorimetric and thermodynamic investigation of A2[(UO2)2(MoO4)O2] compounds with AÂ =Â K and Rb and calculated phase relations in the system (K2MoO4Â +Â UO3Â +Â H2O)

A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A2[(UO2)2(MoO4)O2], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K2[(UO2)2(MoO4)O2] was determined using HF-solution calorimetry giving ΔfH° (T = 298 K, K2[(UO2)2(MoO4)O2], cr) = −(4018 ± 8) kJ · mol−1. The low-temperature heat capacity, Ср°, was measured using adiabatic calorimetry from T = (7 to 335) K for K2[(UO2)2(MoO4)O2] and from T = (7 to 326) K for Rb2[(UO2)2(MoO4)O2]. Using these Ср° values, the third law entropy at T = 298.15 K, S°, is calculated as (374 ± 1) J · K−1 · mol−1 for K2[(UO2)2(MoO4)O2] and (390 ± 1) J · K−1 · mol−1 for Rb2[(UO2)2(MoO4)O2]. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, ΔfG°, for both phases giving: ΔfG° (T = 298 K, K2[(UO2)2(MoO4)O2], cr) = (−3747 ± 8) kJ · mol−1 and ΔfG° (T = 298 K, Rb2[(UO2)2(MoO4)], cr) = −3736 ± 5 kJ · mol−1. Smoothed Ср°(Т) values between 0 K and 320 K are presented, along with values for S° and the functions [H°(T) − H°(0)] and [G°(T) − H°(0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K2MoO4 + UO3 + H2O) system with and without CO2 at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach.