Disappearance of a Vibrational Mode in the Ferroelectric Phase Transition of KH2PO4

Abstract

With the aid of a cold-neutron spectrometer, the time-of-flight spectra of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ single crystals were measured with different orientations of the momentum exchange vector with respect to the crystallographic axis. In the case where the momentum exchange vector points mainly in the direction of the hydrogen bond, the temperature dependence of the spectrum was investigated. It was found that below the Curie temperature a band mode of 50 ${\mathrm{cm}}^{-1\mathrm{}}$ disappears, and the mean square displacement of the protons in the direction of their bonds is strongly lowered.