The infrared and Raman spectra of disordered paraelectric and ordered ferroelectric phases of CsH2PO4and CsD2PO4single crystals have been investigated at various temperatures in the 4000ndash;10 and 4000ndash;0 cmminus;1range, respectively. An assignment of lattice and internal vibrations in terms of symmetry species and approximate type of motion is given. Most of the external vibrations of the paraelectric phase follow the selection rules of theC2hfactor group symmetry, whereas the internal modes do not obey the mutual exclusion rule. The Raman spectra of the ferroelectric phase show TOhyphen;LO splitting up to 50 cmminus;1for a number of lattice and internal vibrations which is correlated with infrared intensities. Disordered short and ordered long hydrogen bonds are distinguished spectroscopically and the OH stretching and bending frequencies are correlated with xhyphen;ray and neutron diffraction data. A large positive isotope effect on the short hydrogen bond is manifested by a low (1.13)ngr;OH/ngr;OD frequency ratio. The short OHsdot;sdot;sdot;O, but not ODsdot;sdot;sdot;O, bond contracts at higher temperature as shown by the increased hydrogen bond and decreased OH stretching frequency. The Raman intensity of the 74 cmminus;1mode and 507 cmminus;1LO band plot vs temperature gives a curve of the order parameter type. The halfhyphen;width of the OHsdot;sdot;sdot;O Raman band increases three times more steeply with increasing temperature than that of the ODsdot;sdot;sdot;O band which is ascribed mainly to a larger activation energy of the proton jumps along the ODsdot;sdot;sdot;O bond.
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