ИСТИНА

Studies of actinide contraction and the level of participation of valence orbitals in covalent bonds in the series of actinide compounds are still the subject of discussion. The 5f orbitals of the lighter actinides can be very active up to Am; after that, they are highly compressed and are considered as the main orbitals, similar to their 4f analogues, the lanthanides. In this work we performed experimental (SC-XRD, Raman-IR Spectroscopy, XANES, thermal stability) investigations and ab initio XANES calculations for different clusters of atoms and local density of states for isostructural double sulfates of trivalent actinides СsAn(SO4)2, An = U, Np, Pu, Am. The synthesis of CsU(SO4)2·4H2O was performed using electrochemical reduction and CsNp(SO4)2·4H2O was prepared with Jones reducer. Pu was reduced by chemical reduction and Am is stable in trivalent state. Uranium double sulphate is described below as an example. The results of Raman spectroscopy (fig. 1a) indicate the presence of sulfate anions (990 cm-1): mono- and bidentate - with the uranium cation. Peaks combination near 410 cm-1 (T2g) and 490 cm-1 (LO), is typical for defective uranium dioxide, but here are shifted to the left by more than 30 cm-1. The XANES spectrum at the U L3-edge (fig. 1b) has a shift of the white line to lower energies by 5 eV compared to the spectrum of tetravalent uranium, and the shapes of its post-edge structures differ from those of uranium dioxide and uranyl salt: the second feature is closer to the edge total absorption by 1-2 eV compared with the spectrum of dioxide. The shoulder to the right of the white line, characteristic of uranyl salts, is also missing in the spectrum of CsU(SO4)2·4H2O.