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 Tm(E) and its value εm(E).
Methods. Studied CuInP2S6 crystals were made by the chemical transport reactions, and Ag0.05Cu0.95InP2S6 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 Ag0.05Cu0.95InP2S6 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 Ag0.05Cu0.95InP2S6 respectively increase maximum of dielectric permittivity εm(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 Ag0.05Cu0.95InP2S6 respectively a linear dependence of field value dependence Tm(E) of maximums on temperature dependences is observed with coefficients dTm/dE=1.0 Kcm/kV and dTm/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 Tm(E) of CuInP2S6 and Ag0.05Cu0.95InP2S6 crystals are typical for first order phase transitions with observation of the critical point. The critical point is realized in CuInP2S6 and Ag0.05Cu0.95InP2S6 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 Ag0.05Cu0.95InP2S6 crystals. Isomorphic substitution of copper ions to silver in AgxCu1-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