Inelastic Neutron Scattering Study of Ferroelectric Phase Transition in Lithium Heptagermanate (Li₂Ge₇O₁₅)

Abstract

Phonon dispersions in ferroelectric lithium heptagermanate Li₂Ge₇O₁₅ (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.

Keywords:

• Ferroelectrics
• Soft phonon mode
• Critical slowing-down
• Inelastic neutron scattering
• Li₂Ge₇O₁₅

Acknowledgments

The authors wish to thank Y. Iwata for the discussion on neutron measurements and M. Horioka for the fruitful discussions on dielectric critical slowing down. They also wish to thank A. Sawada and Y. Ishibashi for the discussions on the transfer phenomena of the soft mode characteristics during structural phase transitions.The measurements were performed by using the neutron spectrometers 4G and C1-1 in JRR-3M at Tokai.