Ultrasonic Relaxation in Phase Transition Region in Ferroelectric Semiconductors of Sn₂P₂S₆ Family

Abstract:

The paper presents recent results of ultrasonic investigation of Sn₂P₂S₆ family ferroelectric crystals and their solid solutions in the temperature range 100-300 K. It was shown that in Sn₂P₂(S,Se)₆ crystals the critical ultrasonic velocity slowing down for longitudinal waves propagating along main crystallographic directions is quite sharp and large. The relative change of longitudinal ultrasonic velocity along z-axis at the phase transition gradually increased from 10 % in pure Sn₂P₂S₆ till 25 % for sample with 0.4 content of Se. Such large velocity change causes the large ultrasonic attenuation anomaly. The increase of relaxation time: τ=τ₀/(T_C-T) leads to the increase of attenuation. Prefactor τ₀ was shown to be very small and the critical attenuation anomaly arises in the narrow temperature range close to phase transition. In the 0.4 Se sample the phase transition is of the first order because small thermal hysteresis exists. The ultrasonic velocity behaviour in the ferroelectric phase was described using Landau theory and free energy expansion including sixth order terms. For (Sn,Pb)₂P₂S₆ system the critical ultrasonic anomalies were smaller and the phase transition temperature substantially decreased (for 0.45 Pb sample the phase transition point was at T_c =140 K). The ultrasonic anomalies at phase transition in (Pb_xSn_1-x)₂P₂S₆ have large hysteresis showing that transition is of the first order, far from the critical point.

• Periodical: Solid State Phenomena (Volume 184)
• Pages: 345-350
• DOI: https://doi.org/10.4028/www.scientific.net/SSP.184.345
• Online since: January 2012
• Authors: Vytautas Samulionis, Juras Banys, Yulian Vysochanskii
• Keywords: Ferroelectrics-Semiconductors, Phase Transition, Ultrasonic Relaxation

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