Spin-density study of UCl4 - discovery of covalency effects

Neutron experiments have been performed to measure the induced spin-density distribution in single crystals of uranium tetrachloride at low temperature. A structural refinement has also established the atomic positions and thermal parameters at low temperature and these are in agreement with earlier studies. The spin-density distribution, determined with polarized neutrons, shows small but significant deviations from that expected from the single-ion crystal-field model. Fourier difference maps show a small depletion of the 5f density near the uranium site and a small positive increase of about 1 A from the uranium site and principally along the shorter U-Cl bonds. We ascribe this to population of a 6d (x2-y2) orbital. No significant spin density is associated with the chlorine site. Model calculations are presented which qualitatively account for the observed effect and involve electron transfer from the 5f to the 6d antibonding orbital formed as a result of a covalent interaction between the U 6d and Cl p states. This is the first direct observation of covalency in systems involving 5f 6d electrons. (orig.)