Vibrational study of CsH2PO4 and CsD2PO4 single crystals

Abstract

The infrared and Raman spectra of disordered paraelectric and ordered ferroelectric phases of CsH₂PO₄ and CsD₂PO₄ single crystals have been investigated at various temperatures in the 4000–10 and 4000–0 cm⁻¹ range, 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 the C_2h factor group symmetry, whereas the internal modes do not obey the mutual exclusion rule. The Raman spectra of the ferroelectric phase show TO‐LO splitting up to 50 cm⁻¹ for 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 x‐ray and neutron diffraction data. A large positive isotope effect on the short hydrogen bond is manifested by a low (1.13)νOH/νOD frequency ratio. The short OH⋅⋅⋅O, but not OD⋅⋅⋅O, bond contracts at higher temperature as shown by the increased hydrogen bond and decreased OH stretching frequency. The Raman intensity of the 74 cm⁻¹ mode and 507 cm⁻¹LO band plot vs temperature gives a curve of the order parameter type. The half‐width of the OH⋅⋅⋅O Raman band increases three times more steeply with increasing temperature than that of the OD⋅⋅⋅O band which is ascribed mainly to a larger activation energy of the proton jumps along the OD⋅⋅⋅O bond.