Effect of the chemical state of uranium atom on the energy of spin-orbital splitting of its inner orbitals

Abstract

Summary Chemical shifts (ChSh) of nine emission lines of the uranium L-series in uranium oxides UO2+ x (x=0-1) with respect to UO2 were studied by using a precise crystal-diffraction X-ray spectrometer. ChSh of Lα 1,α 2 uranium and thorium lines in solid solutions yUO2·(1-y )ThO2 (y=0:1) were measured with respect to UO2 and ThO2, respectively. The changes in energy of spin-orbital splitting (SOS) — Δδnl ± of inner nl-orbitals of the uranium atom were calculated from the data of ChSh of spin-doublet lines. For UO2+ x oxides, a linear decrease in Δδnl ± values with increasing degree of uranium oxidation was found. Sign inversion of Δδnl ± for uranium levels was found on passing to solid solutions. No change in the SOS energy of inner thorium levels was detected. The values of Δδnl ± were found to correlate with the experimental values of the effective magnetic moment of uranium in oxides. On the basis of the comparison of experimental Δδnl ± values with Dirac–Hartree–Fock atomic calculations, it was concluded that the observed variations in Δδnl ± values are due to the redistribution of electron and spin density between the 5f 7/2- and 5f 5/2-levels of the fine structure of the uranium atom without changes in atomic charge state. On the basis of the hypothesis of intraatomic relativistic U5f 7/2↔U5f 5/2 transition, a model of paramagnetic moment formation on the uranium atom in uranium dioxide was proposed.