A Raman spectroscopic study of the uranyl phosphate mineral bergenite

Abstract

Raman spectroscopy at 298 and 77 K of bergenite has been used to characterise this uranyl phosphate mineral. Bands at 995, 971 and 961 cm-1 (298 K) and 1006, 996, 971, 960 and 948 cm-1 (77 K) are assigned to the ν1 (PO4)3- symmetric stretching vibration. Three bands at 1059, 1107 and 1152 cm-1 (298 K) and 1061, 1114 and 1164 cm-1 (77 K) are attributed to the the ν3 (PO4)3- antisymmetric stretching vibrations. Two bands at 810 and 798 cm-1 ( 298 K) and 812 and 800 cm-1 (77 K) are attributed to the ν1 symmetric stretching vibration of the (UO2)2+ units. Bands at 860 cm-1 (298 K) and 866 cm-1 (77 K) are assigned to the ν3 antisymmetric stretching vibrations of the (UO2)2+ units. U-O bond lengths in uranyls, calculated using the wavenumbers of the ￮1 and ￮3 (UO2)2+ vibrations with empirical relations by Bartlett and Cooney, are in agreement with the X-ray single crystal structure data. Bands at (444, 432, 408 cm-1) (298 K), and (446, 434, 410 and 393 cm-1) (77 K) are assigned to the split doubly degenerate ￮2 (PO4)3- in-plane bending vibrations. The band at 547 cm-1 (298 K) and 549 cm-1 (77 K) are attributed to the ￮4 (PO4)3- out-of-plane bending vibrations. Raman bands at 3607, 3459, 3295 and 2944 cm-1 are attributed to water stretching vibrations and enable the calculation of hydrogen bond distances of 3.2, 2.847, 2.740 and 2.637 Å. These bands prove the presence of structurally nonequivalent hydrogen bonded water molecules in the structure of bergenite.