Baric transformations of anomalies of dielectric permeability of CuСr<sub>0,5</sub>In<sub>0,5</sub>P<sub>2</sub>S<sub>6</sub> 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 CuCrP 2 S 6 and compounds are very convenient materials for studying the effects of 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 CuInP 2 S 6 in and an anti-ferroelectric in CuCrP 2 S 6 ) 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 x Cr 1-x )P 2 S 6 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 0,5 In 0,5 P 2 S 6 crystals, in which the state of dipole glass occurs at atmosphere pressure according to the phase x , T -diagram . Methods: Studied CuСr 0,5 In 0,5 P 2 S 6 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 e∗ 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 e* of CuСr 0,5 In 0,5 P 2 S 6 crystals with different values of hydrostatic pressure were performed in a wide temperature range 77 K 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 0,5 In 0,5 P 2 S 6 crystals induced by high hydrostatic pressure d Tc /d p =0,18 K/MPa is close to the similar value of d Tc /d p =0,21 K/MPa and d Tc /d p =0,29 K/MPa for crystals CuInP 2 S 6 and CuСr 0,3 In 0,7 P 2 S 6 , 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 0,5 In 0,5 P 2 S 6 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 0,5 In 0,5 P 2 S 6 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 x Cr 1-x )P 2 S 6 solid solutions was built. The concentration shift of the temperature of the ferroelectric phase transition for solid solutions of Cu(In x Cr 1-x )P 2 S 6 can be well described by the equation Т с ( х )=314,6-3,5× х +0,009× х 2 ( х, mol.%)