Crystal Chemical and Thermodynamic Study on CaUO 4- x, (Ca 0.5Sr 0.5)UO 4- x, and α-SrUO 4- x ( x = 0 ∼ 0.5)

Abstract

A crystallographic and thermochemical study of CaUO 4- x , (Ca 0.5Sr 0.5)UO 4- x , and αSrUO 4- x with x values ranging from nearly 0 to 0.5 was carried out. As the crystal radius of the divalent cations increases, a (rhomb) of nearly stoichiometric compounds increases, accompanied by an increase in the cell volume, whereas α(rhomb) decreases. With nonstoichiometry, the lattice parameters of (Ca 0.5Sr 0.5)UO 4- x and α-SrUO 4- x discontinuously change at 4 ∼ x = 3.79 and 3.77, respectively, although each compound is a single phase in the entire range from 4 - x = 4.00 to 3.50. For CaUO 4- x , the products were two-phase mixtures between 4 ∼ x = 3.98 and 3.70. These crystal structures are discussed taking into account the lattice parameters and interatomic distances. In the nonstoichiometric crystals, oxygen vacancies exist on the O II sites but not on the O I sites. The enthalpy of formation of α-SrUO 4- x was measured for several specimens with different x values by solution calorimetry, and was expressed in a second-order polynomial of x by least-squares calculation. This quantity and its derivative, i.e., partial molar enthalpy of oxygen, suggest the change of defect species with x and the existence of repulsive vacancy-vacancy interaction.