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(NpO 2 ) 2 SiO 4 ·2H 2 O (I) and (PuO 2 ) 2 SiO 4 ·2H 2 O (II) have been prepared by hydrothermal synthesis technique. Chemical composition of (I) was attributed to the compound from its isostructurality with (UO 2 ) 2 SiO 4 ·2H 2 O; crystal unit cell parameters of (NpO 2 ) 2 SiO 4 ·2H 2 O 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 . (PuO 2 ) 2 SiO 4 ·2H 2 O 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  I 4 1 / amd . Visible-near IR and IR absorption spectra of the prepared compounds have been measured in NaCl and KBr.  f – f  absorption band of PuO 2  2+  in the visible spectrum of crystal (PuO 2 ) 2 SiO 4 ·2H 2 O is not splitted but its maximum is essentially shifted (up to 858nm) to the long wave region in comparison with hydrated PuO 2  2+  ions. Visible-near IR absorption spectrum of crystal (NpO 2 ) 2 SiO 4 ·2H 2 O is close to that of aqueous neptunyl(VI) solution. Thermal behavior of synthesized compounds has been studied up to 800°C in air.

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