Phonon anomalies near the ferroelectric phase transition in LiNaGe4O9

Lattice dynamics of LiNaGe4O9 was studied by means of far-infrared (FIR) and Raman spectroscopy from room temperature down to 15 K. Pronounced softening of phonon modes above T-c and hardening below T-c was found in the FIR spectra for all three polarizations and also in the a(cb)c Raman spectra, New modes observed in FIR and Raman spectra below T-c correspond to the modes active in Raman and Fm. spectra in the paraelectric phase respectively, in agreement with the factor group analysis, It is well known from structural studies that the phase transition is of the order-disorder type. Our results indicate that Li atom ordering affects also the phonons of four different symmetries.