Raman spectroscopic characterization of a synthetic, non-stoichiometric Cu–Ba uranyl phosphate

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Crystals of phases belonging to the autunite group (general formula X2+(UO2)2(X5+O4)2·nH2O), specifically the uranyl phosphates (X5+ = P) metauranocircite (X2+ = Ba2+), metatorbernite (X2+ = Cu2+) and a barian metatorbenite phase (X2+ = Cu2+/Ba2+), have been synthesized in a silica gel medium and characterized by Raman spectroscopy. The Raman spectra showed bands in the range 750–1100 cm-¹, which were attributed to the v1 and v3 (PO4)³- and (UO2)²+ stretching vibrations. By using the wavenumbers of the most intense and well defined v1 (PO4)³- vibration, the U–O bonds lengths were calculated for the three uranyl phosphate minerals. The results are in good agreement with previous single crystal structure analysis data. Bands in the spectra from 350 to 700 cm-¹ were attributed to the (PO4)³- bending modes. Moreover, in the range 70–350 cm-¹, two groups of bands could be defined. The first group, with vibrations at lower wavenumbers, was attributed to the lattice modes and the second group, from 150 to 350 cm-¹, was assigned to the v2 (UO2)²+ bending mode. Finally, in the case of the barian metatorbernite, bands in the range 1500–3800 cm-¹ were assigned to the OH stretching and the m2 bending vibrations of water molecules. In this phase, all the vibrations show bandshifts when compared to the vibrations in metatorbernite. These bandshifts can be related to transitional Cu–O and Ba–O bond lengths.
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