Dielectric Dispersion in Ferroelectric Li2-xNaxGe4O9(x≈1)

The dielectric dispersion in the ferroelectric Li 2- x Na x Ge 4 O 9 ( x ≈1) single crystals has been measured in the frequency range from 120 Hz to 10 MHz around the transition temperature T C =109 K. The Debye-type dispersion was observed in the MHz region. The relaxation time increases abruptly as T C is approached from above, which shows the critical slowing-down of dielectric response. The relaxation time is 1.5×10 -8 sec at 0.5 K above T C . The softening of the optical phonon has not been observed by Raman scattering measurement. The ferroelectric phase transition of Li 2- x Na x Ge 4 O 9 ( x ≈1) is the typical order-disorder one. The origin of the polydispersive nature and the sample dependence of the relaxation time are discussed.