Energetics of a Uranothorite (Th1-xUxSiO4) Solid Solution.

High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO4)x-(ThSiO4)1-x, solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and high-temperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO4, and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5. [ABSTRACT FROM AUTHOR]