Ferroelectricity in (Pb_ySn_1−y)₂P₂S₆ mixed crystals and random field BEG model

Abstract

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.

Quantum paraelectricity and induced ferroelectricity by germanium doping of (PbySn1–y)2P2S(Se)6 single crystals

“…The experimental results of (Pb y Sn 1-y ) 2 P 2 S 6 showed that the phase diagram of (Pb y Sn 1-y ) 2 P 2 S 6 could be quite complicated with possible coexistence of ferroelectric-paraelectric states [7,8]. This is in agreement with the thermodynamical description based on the Blume-Emery-Griffith (BEG) model with random bond and random field defects, where the introduction of Pb is responsible for the randomness [9]. One more feature of the phase diagram predicted by the BEG model is the presence of tricritical point (TCP).…”

“…Also, in previous papers it was shown that for y > 0.3 and below 220 K the hysteresis appears [8]. It was related to the coexistence of paraelectric and ferroelectric phases, and evidenced a discontinuous character of the phase transition [9]. The phase diagram of these mixed crystals with substitution of tin for lead or sulfur for selenium is really an interesting point that was investigated in-depth in several studies [1,[7][8][12][13].…”

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

“…Indeed, below tricritical "waterline temperature" about 220-240 K and at compression above 0.6 GPa, the ferroelectric transition in Sn 2 P 2 S 6 evolves to first order [5,6]. Also, at tin by lead substitution in the (Pb y Sn 1−y ) 2 P 2 S 6 mixed crystals, for y> 0.3 and below 220 K, the hysteresis which is related to the paraelectric and ferroelectric phase coexistence appears that give evidence about a discontinuous character of the phase transition [7].…”

“…Taking into account the ideas about valence skipping [25] and charge disproportionation [26], the electronic origin of possible spontaneous polarization of Pb 2 P 2 S 6 crystal lattice is analyzed in the frame of EFK Figure 2: Temperature-pressure phase diagram for Sn 2 P 2 S 6 crystal and temperature-composition phase diagram for (Pb y Sn 1−y ) 2 P 2 S 6 mixed crystals. The tricritical points are located near 220 K for pressure P ≈ 0.6 GPa [6] or for concentration y ≈ 0.3 [7]. For Pb 2 P 2 S 6 crystal the calculated temperature-pressure phase diagram is shown.…”

“…Temperature dependence of dielectric susceptibility was investigated by Bridge scheme at frequencies range 10 4 − 10 8 Hz [7]. The Raman scattering was excited by He-Ne laser and analyzed by DFS-24 spectrometer with 1 cm −1 frequency resolution.…”