We have investigated non-equilibrium properties of proper uniaxial Sn2P2(SexS1−x)6 ferroelectrics with the Type II incommensurate phase above Lifshitz point xLP ∼ 0.28. We measured dielectric susceptibility with cooling and heating rate ranging 0.002-0.1 K/min, and high-resolution ultrasound experiments and hypersound Brillouin scattering. For samples with x 0.28 clear anomalies were observed at incommensurate second order transition (Ti) and at first order lock-in transition (Tc) in the regime of very slow cooling rate, whereas the intermediate IC phase is not observed when the rate is faster then 0.1 K/min. In general, increasing the cooling rate leads to smearing the anomaly at Tc. We explain this effect in terms of Kibble-Zurek model for non-equilibrium second order phase transitions. In the ferroelectrics with strongly nonlinear local potential cooling rate defines concentration of domain walls and their size: domain width decreases when cooling rate increases. At certain conditions the size of domain is comparable to the incommensurate phase modulation period, which lies in micrometer scale in the vicinity of Lifshitz point and leads to pinning of the modulation period by domain wall.
In Sn2P2S6 ferroelectric under compression, the second‐order phase transition line is observed down to the tricritical point as the transition temperature decreases to 250 K. A tricriticality at a similar temperature is also revealed in the mixed crystals on S→Se substitution. For the mixed crystals with Sn→Pb substitution, ultrasound, hypersound, and low‐frequency dielectric studies also show heterophase features appearing on decrease of the ferroelectric transition temperature below a so‐called “waterline temperature” near 250 K. Such behavior agrees with the Blume–Emery–Griffiths (BEG) model appropriate for the ferroelectric system under investigation with a three‐well local potential for the order parameter fluctuations. The BEG model modified by the random‐field phase diagram in the vicinity of the tricritical point qualitatively explains the influence of the cationic substitution on the (PbySn1−y)2P2S6 ferroelectric properties.
Abstract:Elastic properties of (Pb Sn 1− ) 2 P 2 S 6 solid solutions were studied using Brillouin scattering technique. Different scattering geometries were used for sound velocities determination that make it possible to find all components of the stiffness tensor. The concentration dependencies of volume compressibility, the Grüneisen parameter and Debye temperature were investigated.
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