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

Highlights: • We determined the low temperature heat capacity of Cs₈ [(UO₂)₄(WO₄)₄(WO₅)₂]. • We determined enthalpy of formation of Cs₈ [(UO₂)₄(WO₄)₄(WO₅)₂]. • We calculated Δ_fG° (298 K) of Cs₈ [(UO₂)₄(WO₄)₄(WO₅)₂]. • A thermodynamic modeling with Cs₈[(UO₂)₄(WO₄)₄(WO₅)₂] was performed. -- Abstract: A calorimetric investigation on the thermodynamic properties of cesium uranyl tungstate, ${\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right)$, was undertaken. In addition, model calculations on its thermodynamic behaviour in aqueous solution were made. ${\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right)$ 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, $C_{p,\text{m}}^{\text{o}}$, was measured using adiabatic calorimetry from T = 5 K to 329 K. Using the $C_{p,\text{m}}^{\text{o}}\left(T\right)$ data, the third law entropy at T = 298.15 K, $S_{\text{m}}^{\text{o}}$, is calculated as (1514 ± 12) J∙K⁻¹∙mol⁻¹. The enthalpy of formation of ${\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right)$ was determined using HF-acid solution calorimetry giving ${\Delta }_{\text{f}}{H}_{\text{m}}^{\text{o}}(T=298.15\text{K},{\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right),\text{cr})=-(13\text{,}163\pm 25\text{kJ})·{\text{mol}}^{-1}.$ The new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, ${\Delta }_{\text{f}}{G}_{\text{m}}^{\text{o}}$, giving: ${\Delta }_{\text{f}}{G}_{\text{m}}^{\text{o}}(T=298.15\text{K},{\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right),\text{cr})=-(12,254\pm 28)\text{J}·{\text{mol}}^{-1}$. Best-fit and smoothed $C_{p,\text{m}}^{\text{o}}\left(T\right)$ values for ${\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right)$ between 0 K and T = 320 K are presented in table form, along with values for $S_{\text{m}}^{\text{o}}$ and the functions $[H_{\text{m}}^{\text{o}}\left(T\right)-H_{\text{m}}^{\text{o}}\left(0\right)]$ and $[{\Phi }_{\text{m}}^{\text{o}}\left(T\right)-{\Phi }_{\text{m}}^{\text{o}}\left(0\right)]$. The stability of ${\text{Cs}}_8\left({\left({\text{UO}}_2\right)}_4{\left({\text{WO}}_4\right)}_4{\left({\text{WO}}_5\right)}_2\right)$ 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., ${\text{A}}_2\left[{\left({\text{UO}}_2\right)}_2\left({\text{WO}}_5\right)\text{O}\right]\left(\text{cr}\right)$ with A = K and Rb, phases. An analysis indicates that ${\text{Cs}}_2[{\left({\text{UO}}_2\right)}_2\left({\text{WO}}_5)\text{O}\right)$ is the most stable phase yet investigated in the family of alkali-metal uranyl tungstates.