Yu. M. Vysochanskii

Soft modes and phonon interactions in studied by means of neutron scattering

This paper presents an inelastic neutron scattering study of the proper ferroelectric and elastic neutron scattering results on the satellite diffraction pattern which characterizes the modulated phase. The temperature dependences of the satellite intensities and modulation wavevector are in fair agreement with results from previous x-ray experiments. Close to the incommensurate-to-ferroelectric transition temperature , an unexpected intensity overshoot is observed, similar to that seen in birefringence and dilatation experiments. The relationship between the lattice dynamics and the observed phase transition sequence is examined. The dispersion of the ferroelectric soft optic phonon (-polarization) and of the acoustic phonons is followed along the and -directions. In the ferroelectric phase, the TO mode shows a considerable softening as the incommensurate phase is approached from below. In the paraelectric and incommensurate phases, the response from the TO (-polarization) and TA ( strain) branches has been investigated via a series of constant-q scans in the -direction (approximately the modulation wavevector direction). The combined inelastic line-shapes, as observed in a number of non-equivalent Brillouin zones, could all be analysed in terms of a coupled-mode damped harmonic oscillator model. In addition, a diverging, resolution-limited, central peak is observed close to . It is suggested that the TO-TA coupling lies at the origin of the incommensurate instability. A phenomenological free energy is developed, in the continuum approximation, in which the TO-TA interaction is included via a pseudo-Lifshitz term of the type .

Asymmetric phase diagram of mixed CuInP2(SxSe1 x)6 crystals

In this article mixed CuInP2(SxSe1−x)6 crystals were investigated by broadband dielectric spectroscopy (20 Hz 3 GHz). The complete phase diagram has been obtained from these results. The phase diagram of investigated crystals is strongly asymmetric the decreasing of ferroelectric phase transition temperatures in CuInP2(SxSe1−x)6 is much more flat with small admixture of sulphur then with small admixture of selenium. In the middle part of the phase diagram (x=0.4-0.9) the dipolar glass phase has been observed. In boundary region between ferroelectric order and dipolar glass disorder with small amount of sulphur (x=0.2-0.25) at low temperatures the nonergodic relaxor phase appears. The phase diagram was discussed in terms of random bonds and random fields.

Investigation of Ultrasonic and Acoustoelectric Properties of Ferroelectric-Semiconductor Crystals

The ultrasonic velocity and attenuation anomalies were observed near phase transitions (PT) in CuInP2(S,Se)6 semiconductor crystals, where S was partially substituted by Se. It was shown that, crystals became piezoelectric not only in low temperatures below PT in ferroelectric phase but also at room temperature, which is far above the phase transition and under DC bias electric field, the electrostriction induced piezoelectricity appears. The anomalies of ultrasonic velocity, attenuation, second harmonic, and acoustoelectric voltage were also observed near the phase transitions in Cu 6 PS 5 I single crystals. These crystals belong to the polar symmetry group 3m at room temperature and exhibit a ferroelastic phase transition at 270 K.

Highly efficient acousto-optic diffraction in Sn2P2S6 crystals.

We have studied the acousto-optic (AO) diffraction in Sn2P2S6 crystals and found that they manifest high values of an AO figure of merit. The above crystals may therefore be used as highly efficient materials in different AO applications.

Elastic and electromechanical properties of new ferroelectric-semiconductor materials of Sn2P2S6 family

Abstract The crystals of ferroelectric-semiconductor Sn2P2S6 exhibit a strong piezoelectric effect. We present measurements of ultrasonic and piezoelectric properties of other crystals of the Sn2P2S6 family: i.e., Sn2P2Se6 and CuInP2S6. The acoustoelectric method was applied to measure electromechanical coupling parameters in the ferroelectric phase of Sn2P2Se6 single crystals. The electromechanical coupling coefficients in lamellar CuInP2S6 crystalline plates were measured by the resonance method. The CuInP2S6 plate was employed as a piezoelectric transducer. The signal detected by this transducer was measured as a function of temperature and it was shown that the piezoelectric coupling disappears in the paraelectric phase. Anomalies at the phase transition have been observed at ultrasonic investigations of CuInP2S6 crystals.

Critical behavior near the Lifshitz point in Sn2P2(S1 − xSex)6 ferroelectric semiconductors from thermal diffusivity measurements

The thermal diffusivity of the ferroelectric family Sn(2)P(2)(Se(x)S(1 - x))(6) (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals, with the aim of studying the evolution of the ferroelectric transition with Se doping. Its change from second order character to first order while passing the Lifshitz point (x approximately 0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations. The critical behavior of the ferroelectric transition in terms of the different universality classes and their underlying physical dominant effects (tricriticality, long-range dipole interactions, Lifshitz point) has been discussed using thermal diffusivity measurements in the very close vicinity of the critical temperature. This study reveals that for Se concentrations around the Lifshitz point, long-range dipole interactions do not play a significant role and that the critical parameters are close to those predicted for the Lifshitz universality class.

Ultrasonic and Piezoelectric Investigation of Sn2P2S6 Type Photosensitive Ferroelectric-Semiconductor Crystals

The investigation results of ultrasonic and piezoelectric properties of Sn2P2(S,Se)6, CuInP2S6 and CuCrP2S6 ferroelectric-semiconductors single crystals are presented. In Sn2P2S6 type crystals ultrasonic attenuation caused by acoustoelectric interaction after illumination showed fast and long term relaxations. Anomalies of ultrasonic velocity and attenuation at phase transitions have been established in CuInP2S6 and CuCrP2S6 crystals. The piezoelectric detection of longitudinal ultrasonic signal by thin plate of CuCrP2S6 crystal was observed. In the high temperature phase pretransitional effects in ultrasonic and piezoelectric properties were clearly seen. Such effects can be determined by polar clusters, existing above phase transition temperature.

Phase transitions in CuInP2(SexS1−x)6 layered crystals

Abstract The low-frequency dielectric susceptibility of layered CuInP2(SexS1−x)6 mixed crystals is investigated. The morphotropic phase boundary is found between x = 0.75 and 0.8. Broadening of phase transition anomalies in CuInP2S6 is observed at substitution of sulfur atoms by selenium. At substitution of selenium by sulfur in CuInP2Se6 a second order phase transition from the paraelectric phase to an intermediate phase is clearly observed up to the morphotropic boundary, while dielectric anomalies at the first order transition from the intermediate phase to ferrielectric phase are completely extinguished.

Sound behavior near the Lifshitz point in proper ferroelectrics

The interaction between soft optic and acoustic phonons was investigated for

The Critical Behaviour of Ultrasonic Velocity at a Second‐Order Phase Transition in Sn2P2S6 Single Crystals

{\text{Sn}}_{2}{\text{P}}_{2}{({\text{Se}}_{0.28}{\text{S}}_{0.72})}_{6}