Inelastic Neutron Scattering Study of Ferroelectric Phase Transition in Lithium Heptagermanate (Li_2Ge_7O_(15))

摘要

Phonon dispersions in ferroelectric lithium heptagermanate Li_2Ge_7O_(15) (LGO) were measured by inelastic neutron scattering in the vicinity of the transition temperature T_c = 283.5 K. The (300) reflections show more pronounced temperature dependence than the (500) reflections. The energy of a peak corresponding to the Brillouin zone-center phonon, around 0.5 meV at 297.0 K, decreases as the temperature decreases. Then, this inelastic component disappears into the central component at 288.0 K, just above T_c. The present phonon shows the typical soft mode feature and shows good accordance with the B_(1u) mode observed by hyper-Raman and FTIR measurements. It is confirmed that the ferroelectric phase transition in LGO is associated with the soft optical phonon at the zone center. Moreover, elastic diffuse scattering is observed below 0.1 meV. This implies that there exists a critical phenomenon at energies below than 0.1 meV.