Vibrational study of hydrogenated uranyl phosphates hydrates (HUP)
Abstract HUO 2 PO 4 , 4H 2 O(HUP) is among the best fast proton conductors and is used as a solid electrolyte in sensors or in electrochromic displays. In the present work, HUP, ammonium and potassium isomorphs were prepared in the form of powders, oriented polycristalline transparent disks or crystals, and were studied by X Ray analysis, IR and Raman spectroscopies. Raman and IR spectra have been recorded in the 77 K to 350 K temperature range. Skeleton (PO 4 , UO 2 ) vibrations as well as external modes or vibrations involving hydrogen atoms have been tentatively assigned using isotopic shifts (H/D or 16 O /18 O ). Different degrees of disorder are present in the structure : the UO 2 ν 1 stretching mode is a particularly intense and narrow Raman band ; on the other hand a very broad band is observed at 115 cm −1 ( δν 1 2 =25 cm −1 ) at room temperature in HUP. This band narrow and gives rise to three modes (90, 100 and 114 cm −1 respectively) below the ferroelectric phase transition. Vibrational modes of protonic species are assigned using isotopic shift and H 3 O + /K + substitution : i.e. the ν 4 mode of H 3 O + is observed at about 1750 cm −1 (IR) and the ν 2 mode of water is observed at 1640 cm −1 . A broad diffusion observed on IR spectra of NHUP could be associated to the presence of a strong hydrogen bond.