Abstract

Purpose. The electric field is conjugated to the spontaneous polarization and leads to a shift of the phase transition point (PT) and change the type of phase transition. Thus, we can determine type of phase transition, value of critical field for materials with first order phase transition by the dependence of temperature of maximum of dielectic permittivity T_m(E) and its value ε_m(E).
Methods. Studied CuInP₂S₆ crystals were made by the chemical transport reactions, and Ag_0.05Cu_0.95InP₂S₆ crystals by Bridgman technique. Dependencies of real part of the dielectric permittivity ε(Т) were obtained in the dynamic cooling mode with a temperature rate of 0.5 K/min at the measurement field frequency of 1 MHz.
Results. Studies of real part of dielectric permittivity ε of CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ crystals with different values of external electric field were performed. In the region of small values of field intensity Е<0.4 kV/cm and Е<0,9 kV/cm for CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ respectively increase maximum of dielectric permittivity ε_m(E) is observed. With increase of field intensity up to 0<Е<0.8 kV/cm and 0<Е<1.25 kV/cm for CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ respectively a linear dependence of field value dependence T_m(E) of maximums on temperature dependences is observed with coefficients dT_m/dE=1.0 Kcm/kV and dT_m/dE=0.45 Kcm/kV. Note, that with higher field intensity values it is not linear.
Discussions. Behaviour of the maximum of dielectric permittivity on temperature dependences ε(Т) and temperature behavior T_m(E) of CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ crystals are typical for first order phase transitions with observation of the critical point. The critical point is realized in CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ with the field intensities Е=0.8 Kcm/kV and Е=1.25 Kcm/kV respectively.
Conclusions. Studies of influence of external electric field on temperature behavior of real part of dielectric permittivity confirms first order phase transition type of CuInP₂S₆ and Ag_0.05Cu_0.95InP₂S₆ crystals. Isomorphic substitution of copper ions to silver in Ag_xCu_1-xInP₂S₆ solid solutions leads to strengthening of characteristics of second order PT and increase of distance from virtual tricritical point on x,T- diagram

Keywords: ferrielectrics, phase transitions, electric field, dielectric properties, critical field

Suggested citation