Studies of layered uranium(VI) compounds. II. Thermal stability of hydrogen uranyl phosphate and arsenate tetrahydrates

By means of accurate TGA, we have shown that the proton-conducting solid electrolytes HUO ₂PO ₄·4H ₂O (HUP) and HUO ₂AsO ₄·4H ₂O (HUAs) are more correctly represented as {HUO ₂P }/{AsO ₄}·(A-x) H ₂O , where A is assumed to be the integral number 4. The water deficiency x was estimated to be less than 0.03 at 295°K, but increased respectively by less than 0.1 and 0.05 when HUP and HUAs were heated in air to approximately 315°K. At higher temperatures, dehydration of HUP under isothermal conditions produced, over a temperature interval of less than 2°K, a lower hydrate phase having approximately 1.5 waters of crystallization. Equilibrium dehydration temperatures were established for HUP over a range of humidities, and varied from 297°K at 0.4 mm Hg water vapor pressure, to 318°K at 16 mm Hg pressure. An explanation of the apparently high standard molar enthalpy for dehydration is suggested in terms of small losses of a phosphorus-containing compound accompanying the water loss. The overall phase relationships of HUP are discussed.