ANOMALOUS LO-PHONON BROADENING AT THE PHASE-TRANSITION IN KD2PO4

Anomalous broadening of the peak at E = 28 meV in KH2PO4 was recently observed by incoherent neutron scattering experiments upon the ferroel ectric phase transition. On the other hand, coherent neutron scatterin g experiments with pulse neutron source revealed the existence of an o ptical phonon mode around 30 meV in KD2PO4, but the nature of the opti cal mode was still unclear. In order to investigate the character of t his 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 o f temperature. This phonon mode seems to couple strongly with the moti on of the hydrogen atom.