Pentavalent uranium complex oxides: A case study on double perovskites Ba2REU5+O6 (RE = La, Nd, Sm)

Abstract

Abstract Pentavalent uranium double perovskite compounds with the formula Ba 2 REUO 6 (RE = La, Nd, Sm) were synthesized using solid state reaction method. All the three compounds are iso-structural and crystallize in monoclinic system (space-group P2 1 /n) at room temperature with rock-salt type chemical ordering between RE and U cations. The crystal structure of Ba 2 LaUO 6 was refined using powder x-ray and neutron diffraction data and was chosen as a prototype model for this series. High temperature x-ray diffraction and neutron diffraction measurements show that Ba 2 LaUO 6 compound retains monoclinic structure (P2 1 /n) from 6 K to 1073 K. Local environment around uranium atom was probed using extended X-ray absorption fine structure (EXAFS) measurement carried out on Ba 2 LaUO 6 compound. The average valency of uranium determined from modified Davies-Gray titration method was found to be +5. The valency of uranium in these compounds was further confirmed by bond valence sum (BVS) calculation and X-ray absorption near edge structure (XANES). Magnetization study shows that all the three compounds have paramagnetic behaviour in the temperature range of 5 K–300 K. Magnetization data show a weak magnetic moment of 0.4 μB for U 5+ ion in La compound. Electron paramagnetic resonance (EPR) signal recorded at room temperature gave a signal with two distinct g values corresponding to 2.24 and 2.31. As reported in the literature, the expected EPR signal at g ≈ 0.7 for isolated U 5+ octahedra was not observed in these compounds. Even in absence of U(V)-U(V) magnetic interaction EPR spectra show signal with g > 2 which is a new finding in EPR study of uranium compounds. The nature of EPR spectra is correlated to octahedral distortion around uranium. Thermogravimetry (TG) was used to study the oxidation behaviour of these compounds and also to explore the possibility of formation of a new mixed valence compounds during oxidation. The weight gain observed in thermogram show the expected oxidation of U 5+ to U 6+ followed by decomposition to form two phase mixture.