Synthesis and study of hexauranates M III [(UO 2 ) 6 O 4.5 (OH) 6 ]·7H 2 O (M III —Nd, Sm, Eu, Gd, Dy)

Previously unknown hexauranates MIII[(UO2)6O4.5(OH)6]·7H2O (MIII—Nd, Sm, Eu, Gd, Dy) were obtained by reaction of UO3·2.25H2O with aqueous solutions of MIII(NO3)3 under hydrothermal conditions at 200 °C. Composition and structure of these compounds were defined, processes of dehydration and thermal decomposition were studied, the state in aqueous solutions was investigated. Solubility data were used for calculation of solubility products and standard formation Gibbs functions of MIII[(UO2)6O4.5(OH)6]·7H2O. The theoretical solubility curves of studied compounds depending on pH were calculated, U(VI) and M(III) speciation diagrams both in aqueous solutions and equilibrium solid phases were plotted.