X-ray and thermal studies of mixed valent uranium molybdates

Thermal expansion behavior of U2MoO8, β-UMo2O8, UMo5O16, UMo7O22 and UMo10O32 was studied in vacuum between 298 K and 873 K using high temperature X-ray diffractometer. The compounds were synthesized by solid state route, reacting UO2, MoO2 and/or MoO3 in required molar proportions in sealed evacuated quartz ampoules at desired temperatures. Studies of oxidation behavior of all mixed valent uranium molybdates were carried out using thermogravimetry in dry air up to 973 K. Oxidation states of uranium and molybdenum were proposed based on their oxidation behavior. X-ray diffraction data of all the uranium molybdates were indexed on orthorhombic system. From ambient to 873 K, lattice parameters and cell volume of U2MoO8, β-UMo2O8 and UMo7O22 showed positive thermal expansion whereas UMo5O16 and UMo10O32 showed negative axial as well as volume thermal expansion up to 873 K.