Analysis of the absorption spectra of complex pentavalent actinide halides: LiUF/sub 6/,. cap alpha. -NaUF/sub 6/, and CsUF/sub 6/

Absorption spectra of polycrystalline samples of LiUF/sub 6/, ..cap alpha..-NaUF/sub 6/, and CsUF/sub 6/ have been recorded at 4, 77, and 298 K. This group of compounds has a common sixfold U--F coordination that approaches an octahedral site symmetry for LiUF/sub 6/ but exhibits increasing trigonal distortion (D/sub 3d/ symmetry) along the indicated series. Spectra have been systematically interpreted as consisting of sequences of vibronic transitions coupled to well-defined electronic excited states. Crystal-field calculations have been performed.