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.