Far-infrared and Raman studies of the ferroelectric phase transition in LiNaGe4O9

Polarized far-infrared reflectivity and Raman scattering spectra of LiNaGe4 O9 single crystals were studied between 15 and 300 K. Even if the ferroelec tric phase transition in this material is of order-disorder type and critic al relaxation was estimated from our data in the region 0.13 to 13 cm(-1), pronounced phonon softening in four different symmetry spectra was observed near T-c. This indicates changes in interatomic bonds caused by local Li a nd Na atom ordering and accompanying shifts of GeO polyhedra even above T-c . Phonon anomalies observed about 45 K below T-c in the E parallel to a far -infrared spectra might suggest another phase transition, but no correspond ing dielectric anomaly was observed in previous dielectric measurements.