A Raman spectroscopic study of the uranyl phosphate mineral bergenite

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 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. UO bond lengths in uranyls, calculated using the wavenumbers of the ν1 and ν3(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 ν2(PO4)3− in-plane bending vibrations. The band at 547 cm−1 (298 K) and 549 cm−1 (77 K) are attributed to the ν4(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.