Synthesis and characterization of neptunyl(VI) and plutonyl(VI) orthosilicates

Abstract

Summary (NpO2)2SiO4·2H2O (I) and (PuO2)2SiO4·2H2O (II) have been prepared by hydrothermal synthesis technique. Chemical composition of (I) was attributed to the compound from its isostructurality with (UO2)2SiO4·2H2O; crystal unit cell parameters of (NpO2)2SiO4·2H2O have been refined as orthorhombic, a=8.365(5)Å, b=11.089(5)Å, c=18.640(9)Å with the space group Fddd. (PuO2)2SiO4·2H2O has not been found isostructural with uranyl(VI) and neptunyl(VI) analogues. Chemical composition of (II) was determined by chemical analysis and proved by the formula volume calculated from powder X-ray data. Indexing of X-ray powder pattern of the prepared compound gave the tetragonal unit cell with a=6.912(3), c=18.563(3)Å, possible space group I41/amd. Visible-near IR and IR absorption spectra of the prepared compounds have been measured in NaCl and KBr. f–f absorption band of PuO2 2+ in the visible spectrum of crystal (PuO2)2SiO4·2H2O is not splitted but its maximum is essentially shifted (up to 858nm) to the long wave region in comparison with hydrated PuO2 2+ ions. Visible-near IR absorption spectrum of crystal (NpO2)2SiO4·2H2O is close to that of aqueous neptunyl(VI) solution. Thermal behavior of synthesized compounds has been studied up to 800°C in air.