Title: TEMPERATURE AND PRESSURE EFFECT ON THE ANISOTROPY OF DIELECTRIC PERMEABILITY IN KDP AND DKDP CRYSTALS
Author(s): V.M.Kedyulich, A.G.Slivka, E.I.Gerzanich, A.M.Guivan, P.M.Lukach (Uzhgorod National University, 32 Voloshina Str., 88000 Uzhgorod, Ukraine)

Temperature dependences of longitudinal and transverse dielectric permeability of KDP and DKDP crystals are studied at different values of hydrostatic pressure in order to determine the pressure behaviour of the isotropic point for these crystals. The isotropic point temperature in KDP crystals at atmospheric pressure is $T_i=186$~K at the measuring field frequency $1$~kHz and $T_i=176$~K at the frequency of $1$~MHz. In DKDP crystals the isotropic point is achieved at the temperature $T_i=300$~K ($1$~kHz) and $T_i=253$~K ($1$~MHz). The hydrostatic pressure increase results in the reduction of the isotropic point temperature with the pressure coefficients $\partial T_i /\partial p = -4.3$~K/kbar for KDP and $\partial T_i /\partial p =-2.9$~K/kbar for DKDP. The analysis of the experimental results in the framework of the Blinc-\v{Z}ek\v{s} pseudospin formalism has shown a good agreement between the calculated and the experimentally obtained temperature of the isotropic point for KDP crystals.

Key words: ferroelectrics, dielectric permeability, hydrostatic pressure, isotropic point
PACS: 77.80.Bh, 77.22.Ch, 77.84.Fa