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

Abstract

A thermodynamic investigation of five alkali-metal uranyl molybdates of the general formula A2[(UO2)(MoO4)2], where A=Li, Na, K, Rb, and Cs, was undertaken. The various phases were synthesized by solid-state reaction of ANO3, with A=Li, Na, K, Rb, or Cs, MoO3 and γ-UO3. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The low-temperature heat capacity, Ср°, was measured using adiabatic calorimetry from T=(6 to 335)K. Based on these data, the third law entropy at T=298.15K, S°, is (345±1)J·K−1·mol−1 for Li2[(UO2)(MoO4)2], (373±1)J·K−1·mol−1 for Na2[(UO2)(MoO4)2], (390±1)J·K−1·mol−1 for K2[(UO2)(MoO4)2], (377±1)J·K−1·mol−1 for Rb2[(UO2)(MoO4)2] and (394±1)J·K−1·mol−1 for Cs2[(UO2)(MoO4)2]. The enthalpy of formation of Li2[(UO2)(MoO4)2] was determined using HF solution calorimetry giving ΔfH°(T=298K, 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: ΔfG°(T=298K, Li2[(UO2)(MoO4)2], cr)=−(3204±9)kJ·mol−1, ΔfG°(T=298K, Na2[(UO2)(MoO4)2], cr)=−(3243±2)kJ·mol−1, ΔfG°(T=298K, K2[(UO2)(MoO4)2], cr)=−(3269±3)kJ·mol−1, ΔfG°(T=298K, Rb2[(UO2)(MoO4)2], cr)=−(3262±3)kJ·mol−1, and ΔfG°(T=298K, Cs2[(UO2)(MoO4)2], cr)=−(3259±3)kJ·mol−1. Smoothed Ср°(Т) values between T=(0 and 320)K are presented along with values for S° and the functions [H°(T)−H°(0)] and [G°(T)−H°(0)]. ΔfS, ΔfH, and ΔfG behaviour at T=298K of the A2[(UO2)(MoO4)2] group is analysed as a function of the molar volumes of the different phases. The phase relations of K2[(UO2)(MoO4)2 in aqueous solution, in the presence and absent of carbon dioxide at T=298K, were studied by thermodynamic model calculations.