Vibrational spectra of alkali metal (Li, Na and K) uranates and consequent assignment of uranate ion site symmetry

Vibrational (Raman and infrared) spectra of nine alkali metal (lithium, sodium and potassium) uranates have been measured in the infrared range of 4000-250 cm(-1) and the Raman shift range of 1100-50 cm(-1). The Raman spectra of sodium and potassium uranates, and lithium polyuranates are reported for the first time. From these spectra the site symmetries of intrinsic uranate groups are deduced by comparing the number of observed and resolved bands with the number predicted by the various site symmetries subgroups possible for each uranate crystal symmetry. The following could then be assigned: C-2h for Li2UO4; D-2h for alpha-Na2UO4; D-4h for K2UO4; D-2 for Na2U2O7 and C-2h for K2U2O7. For the Lithium polyuranates, Li2O . 1.6UO(3), Li2O . 1.75UO(3), Li2U2O7 and Li2U3O10, as their crystal symmetries are not fully known, no definite conclusions concerning uranate site symmetry could be drawn, except that primitive monoclinic symmetry point groups must be excluded. (C) 1998 Elsevier Science B.V. All rights reserved.