Relaxational Dynamics of the Improper Ferroelectric K3BiCl6 · 2 KC1 · KH3F4

Results are presented of the investigation of the dielectric and acoustic properties of K3BiCl6 · · 2 KC1 · KH3F4 crystal which undergoes an improper ferroelectric phase transition at Tc = 130K. In the paraelectric phase the dielectric permittivity does not depend on temperature or frequency. In the ferroelectric phase relaxational dielectric dispersion of the Debye type occurs along the polar c-axis in the frequency range 105 to 1011 Hz. It is characterized by two relaxation times, τd = 1.3 × 10−9 s and τ 1.5 × 10−11 s. The dispersion in the interval 105 to 109 Hz is caused by the domain walls and produces the main contribution Δεd = 40 to the static dielectric per-mittivity of multi-domain crystals and the changes in the ultrasound velocity and attenuation. The dielectric contribution of the ferroelectric dispersion in the frequency range 1 to 80 GHz is Δε = 10 and ε∞ = 10.