Central mode in hyper-Raman spectra of the quasi-one-dimensional hydrogen-bonded ferroelectricCsH2PO4

A central mode of the quasi-one-dimensional hydrogen-bonded ferroelectric ${\mathrm{CsH}}_2$ ${\mathrm{PO}}_4$ (CDP) is found in the paraelectric phase by hyper-Raman scattering, for the first time. From the temperature and polarization dependences of the spectra, the observed central mode is assigned to be a soft mode which is related to the ferroelectric phase transition of CDP. From line-shape analysis, it is concluded that the central mode is caused by the polarization fluctuation of ${\mathrm{PO}}_4$ ions. Temperature dependences of the static susceptibility and the relaxation time are obtained from the intensity and the half-width of this central mode, respectively. Both temperature dependences are consistently explained by the quasi-one-dimensional Ising model.