Neutron scattering study of ferroelectric Sn2P2S6under pressure

Abstract

Ferroelectric phase transition in the semiconductor Sn 2 P 2 S 6 single crystal has been studied by means of neutron scattering in the pressure-temperature range adjacent to the anticipated tricritical Lifshitz point (p ≈ 0.18 GPa, T ≈ 296 K). The observations reveal a direct ferroelectric-paraelectric phase transition in the whole investigated pressure range (0.18-0.6 GPa). These results are in a clear disagreement with phase diagrams assumed in numerous earlier works, according to which a hypothetical intermediate incommensurate phase extends over several or even tens of degrees in the 0.5 GPa pressure range. Temperature dependence of the anisotropic quasielastic diffuse scattering suggests that polarization fluctuations present above T C are strongly reduced in the ordered phase. Still, the temperature dependence of the (200) Bragg reflection intensity at p = 0.18 GPa can be remarkably well modeled assuming the order-parameter amplitude growth according to the power law with logarithmic corrections predicted for a uniaxial ferroelectric transition at the tricritical Lifshitz point.

Ferroelectricity in (PbySn1−y)2P2S6 mixed crystals and random field BEG model

“…The anomalous growth of the hypersound attenuation at 10 9 Hz for the y = 0.3 composition (Fig. 8) can be related to sufficiently small and fast heterophase fluctuations in the vicinity of the first-order ferroelectric phase transition.…”

“…In addition to the dilution effect, the ionicity of the Sn-S Figure 13 Open symbols (left) show the pressure dependence of the phase-transition temperature in Sn 2 P 2 S 6 crystal based on neutron diffraction [9] and X-ray diffraction [10] data. Solid symbols (center) denote the compositional dependence of the phase-transition temperature in Sn 2 P 2 (Se x S 1−x ) 6 crystals with an intermediate incommensurate phase at x > 0.28 [11,12].…”

“…Neutron diffraction data show [9] that under a pressure of 0.6 GPa, the tricritical point is reached in the temperature-pressure ferroelectric phase diagram. High-resolution X-ray diffraction temperature measurements at different pressures also clearly demonstrate the first-order character of the ferroelectric transition in Sn 2 P 2 S 6 crystals under a pressure of 1.2 GPa where the transition temperature decreases to 110 K [10].…”

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

“…The most important future of BEG model is a presence of tricritical point (TCP) on states diagram [3,4] -while the decrease of temperature of phase transition by pressure or by alloying in mixed crystals, the transition becomes of first order. 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].…”

Quantum paraelectric state and critical behavior in Sn(Pb)$_2$P$_2$S(Se)$_6$ ferroelectrics

“…The quantum paraelectric state is manifested in the Pb 2 P 2 S 6 crystal by the growth of the dielectric susceptibility while cooling down to 0 K. 5 A similar state also appears in the Sn 2 P 2 S 6 crystal at hydrostatic pressure p > 1.5 GPa. 13,14 The Pb 2+ cations have a smaller stereoactivity compared with Sn 2+ and this determines the suppression of ferroelectricity while substituting tin by lead. 5,15 In the case of Ge 2+ cations, the stereoactivity is bigger, which provokes a temperature rise in the ferroelectric phase transition in Sn 2 P 2 S 6 crystals with germanium impurity.…”