Neutron Diffraction Study of Ferroelectric Transition in Li 2 Ge 7 O 15

The crystal structure of Li2Ge7O15 in its ferroelectric state has been determined and refined as well with an R-value of 0.063 for 1202 neutron diffraction data. The crystal belongs to the orthorhmbic system with space group C2V5-Pbc21 and unit cell dimensions a=7.36, b=16.66 and c=9.69 A (at 77 K). Atomic displacements at the phase transition (283.5 K) are evaluated in comparison with the paraelectric structure determined by Vollenkle et al., in 1970. Consequently, tiltings of GeO4 tetrahedra closely related to the soft phonon mode observed by Raman scattering and far-infrared measurements have clearly been observed.