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The crystals BeSO4 4H20 and BeSO,, 4D20 were investigated by Raman scattering (4000 to 4 cm-') and both crystals exhibited TO-LO splittings, a confirmation of their piezoelectric lattice. Distinctive oblique modes, with an E and Bz mixed character were observed. Effective dynamical charges were calculated. An electroptic effect was displayed.Les cristaux BeSO4, 4H20 et BeSO,, 4D20 CtudiLs par diffusion Raman (4000 a 4 cm-') montrent tous deux des Lclatements TO-LO qui confirment leur structure a caractbre piezoklectrique. Des modes obliques distincts montrent un caractbre mixte B z et E. Des charges dynamiques effectives ont Ct6 calculkes. Un effet Clectrooptique a 6th mis en kvidence,

The piezo-electric crystals BeSO,.4 H 2 0 and BeSO4*4D20 were investigated by Raman scattering (4000 to 4 cm-') and infrared spectroscopy (4000 to 30 cm-I). Normal modes (without longitudinal modes) were observed and attributed. Modes arising from tetrahedral Be(H,O), groups, as shown unambiguously by deuterium substitution, permit us to calculate the force constants F,(Be-OH,) = 3.21 mdyn A-' and F,/r2(H20-Be-OH2) = 0.285 mdyn A-l. Water internal modes and low frequency modes were related to the structure with two hydrogen bonds. Les cristaux piezo-ilectriques BeSO4.4H,O et BeS04-4D20 ont Cti CtudiCs par diffusion Raman (4000 a 4 cm-') et spectroscopie infrarouge (4000 B 30 cm-'). Les modes normaux (sans les modes longitudinaux) ont 6tC observCs et attribub. Les modes relatifs aux groupements tttraedriques Be(H20)4, ce qui est dCmontrC par la substitution du deuttrium, nous ont permis de calculer des constantes de force F,(Be-OH)= 3.21 mdyn A-' et F , / r 2 ( H , 0 -Be -OH2) = 0.285 mdyn A-'. Les modes internes des molkcules d'eau et les modes de basse frkquence ont t t t relits B la structure qui comporte deux liaisons hydrogbne.