Ultrasonic Behavior Near Phase Transitions in (Pb_ySn_1−y)₂P₂S₆ Ferroelectric Materials

Abstract

The ultrasonic investigations were performed in Sn₂P₂S₆ family layered crystals, where tin was substituted by lead. We have shown that in mixed crystals the anomalies of ultrasonic velocity and attenuation appear in the vicinity of the phase transitions. The critical ultrasonic anomalies shifted to lower temperatures when Pb concentration increased from 0.1 to 0.3 in material. The relative change of longitudinal ultrasonic velocity along z-axis at the phase transition temperature gradually decreased from 30% in pure Sn₂P₂S₆ to 15% for sample with y = 0.3 content of Pb. Such large velocity change causes the large ultrasonic attenuation anomaly. The ultrasonic velocity and attenuation behaviour in the ferroelectric phase was described using Landau theory implying increase of order parameter relaxation time: τ = τ₀/(T_C–T) and free energy expansion including sixth order terms. It was shown that prefactor τ₀ increases with Pb content.

Keywords:

• Ferroelectrics
• phase transitions
• ultrasonic properties