Anomalous LO-phonon broadening at the phase transition in KD2PO4

Abstract Anomalous broadening of the peak at E = 28 meV in KH 2 PO 4 was recently observed by incoherent neutron scattering experiments upon the ferroelectric phase transition. On the other hand, coherent neutron scattering experiments with pulse neutron source revealed the existence of an optical phonon mode around 30 meV in KD 2 PO 4 , but the nature of the optical mode was still unclear. In order to investigate the character of this particular phonon mode, coherent neutron scattering experiment was performed at JRR-3M of JAERI. We found that the character of the mode at about 30 meV was mainly that of LO-phonon with slight TO-character, and the width of the peak showed a remarkable change as a function of temperature. This phonon mode seems to couple strongly the motion of the hydrogen atom.