Baric transformations of anomalies of dielectric permeability of CuСr_0,5In_0,5P₂S₆ layered crystals

Abstract

Purpose: The investigation of the nature of the dipole glass state in ferroelectrics has become one of the most interesting and important areas in solid state physics in recent years.  Layered crystals of solid solutions based on CuCrP₂S₆ and compounds are very convenient materials for studying the effects of freezing relaxation dynamics caused by dipole disordering. With the change of the chemical composition of solid solutions, we can observe a transformation from dipole ordering with a long-range order, (a ferroelectric CuInP₂S₆in and an anti-ferroelectric in CuCrP₂S₆) to a state of dipole glass with a relaxation behavior caused by the "freezing" of ferroactive ions in the crystal lattice. Based on the investigations of dielectric spectroscopy (10 mHz-3 GHz) in mixed Cu(In_xCr_1-x)P₂S₆ crystals, it was found that the phase changing point between the ferroelectric phase and the dipole glass phase lies between x=0.5 and 0.7 , and the corresponding point between the state of the dipole glass and the anti-ferroelectric phase is between x=0.4 and 0.2. The main aim of this study is to investigate the effect of high hydrostatic pressure on the temperature dependences of dielectric properties of CuСr_0,5In_0,5P₂S₆ crystals, in which the state of dipole glass occurs at atmosphere pressure according to the phase x, T-diagram .

Methods: Studied CuСr_0,5In_0,5P₂S₆ crystals were grown by the gas transport reaction method. For the dielectric measurement polished plate like crystals were used. All measurements were performed in a direction perpendicular to the layers. The complex dielectric permittivity ε∗ was measured using an HP4262A capacitance  bridge  at  the  frequency  1 MHz. All measurements have been performed on cooling and heating  mode with a temperature rate 0.5 K/min. Silver paste has been used for contacting. Hydrostatic pressure was applied a high-pressure chamber, its value being controlled within ±1MPa. Scanning electron microscopy (SEM) studies combined with energy dispersive X-ray spectroscopy (EDX) were performed using a SEM JEOL 7000F microscope.

Results: Studies of a complex dielectric permittivity ε* of CuСr_0,5In_0,5P₂S₆ crystals with different values of hydrostatic pressure were performed in a wide temperature range 77 K <T <400K and a pressure of 0 MPa <p <600 MPa. In the pressure region p<300MPa for CuСr_0,5In_0,5P₂S₆ crystals, no anomaly in the static dielectric permittivity indicating a polar phase transition can be detected down to the lowest temperature. The maximum of the real part of the dielectric permittivity shifts to higher temperatures with increasing pressure along with the imaginary part of the peak in the whole frequency range. These anomalies of dielectric properties are due to the existence of a state of dipole glass. In the region of pressures p> 300MPa, two anomalies of the real part of the dielectric constant are revealed. One, at lower temperatures. corresponds to the transition to the state of a dipole glass, the other-at higher temperatures, is a ferroelectric phase transition induced by hydrostatic pressure. The Value of the baric temperature shift of the ferroelectric phase transition for CuСr_0,5In_0,5P₂S₆ crystals induced by high hydrostatic pressure dTc/dp=0,18 K/MPa is close to the similar value of dTc/dp=0,21 K/MPa and dTc/dp =0,29 K/MPa for crystals CuInP₂S₆ and CuСr_0,3In_0,7P₂S₆, respectively.

Conclusions: Based on the studies of temperature dependences of dielectric permittivity of layered crystals at high hydrostatic pressure,  the baric behavior of the temperatures of the anomalies of dielectric permittivity of crystals CuСr_0,5In_0,5P₂S₆ is established. Hydrostatic pressure induces the appearance of an anomaly of dielectric constant, which corresponds to a ferroelectric phase transition. p,T  phase diagram of these crystalls was built, and pressure coefficients  for the phase transition temperature shift were determined. The phase transition temperature in crystals CuСr_0,5In_0,5P₂S₆ determined by extrapolation of the baric dependence of the temperature of the ferroelectric phase transition to atmospheric pressure is equal to T_c=162K. The  phase x,T diagram of Cu(In_xCr_1-x)P₂S₆ solid solutions was built. The concentration shift of the temperature of the ferroelectric phase transition for solid solutions of Cu(In_xCr_1-x)P₂S₆ can be well described by the equation Т_с(х)=314,6-3,5×х+0,009×х² (х, mol.%)