Four thallium(I) uranates(VI), their preparation, structure and properties

Four thallium(I) uranates(VI), Tl ₄UO ₅, Tl ₂UO ₄, Tl ₂U ₂O ₇ and Tl ₂U ₃O ₁₀, were prepared and their IR, and, for the first time, their Raman, electronic and X-ray absorption (EXAFS) spectra have been measured. These uranates are thermally stable under nitrogen up to 660 °C but above this temperature they decompose and their uranium content increases, due to loss of Tl ₂ O. The thermal stability of these uranates increases with decreasing thallium content. Comparison with the reported values for the standard enthalpies of formation of the corresponding alkali metal uranates, which are essentially cation independent, allows suggested values for the above uranates, respectively, of −2437, −1900, −3182, and −4437 kJ mol ⁻¹ . Structural information and atom arrangements were obtained from analysis of powder XRD and EXAFS spectroscopy measurements. U–O distances, both primary and secondary, were obtained from EXAFS measurements and these were essentially identical with those calculated from vibrational IR spectra.