Neutron Diffraction Study of Structural Phase Transition in Ferroelectric Li₂Ge₇O₁₅

Structural change in Li 2 Ge 7 O 15 crystal associated with the ferroelectric phase transition at 283.5 K has been investigated by neutron diffraction method. Comparison between the high- and the low-temperature structures proves that the change within a framework composed of corner-sharing [GeO 4 ] and [GeO 6 ] polyhedra is ascribed mainly to cooperatively combined tilting motion of the tetrahedra. This phenomenon may seem to be a soft phonon mode previously observed by the Raman scattering measurement. Behavior of Li + ions in channels formed by the framework has also been examined. It is shown quantitatively by using split-atom method that one of the two Li + ions in a broader channel seems to behave rather `order-disorder' nature motion along the channel direction in the vicinity of the transition temperature.