Sn2P2S6 Research Articles

Dynamic Stabilization of Metastable States in Triple-well Ferroelectric Sn2 P2 S6.

Polarization dynamics in ferroelectric materials is governed by the effective potential energy landscape of the order parameter. The unique aspect of ferroelectrics compared to many other transitions is the possibility of more than two potential wells; this prospect leads to complicated energy landscapes with new fundamental and functional properties. In this work, we reveal direct dynamic evidence of a triple-well potential in the metal thiophosphate Sn2 P2 S6 compound using multivariate scanning probe microscopy combined with theoretical simulations. Our key finding is that the metastable zero polarization state can be accessed through a gradual switching process and is surprisingly stabilized over a broad range of electric fields. Simulations confirm that the observed zero polarization state originates from a kinetic stabilization of the non-polar state of the triple well, as opposed to domain walls. Dynamically, the triple well of Sn2 P2 S6 becomes equivalent to antiferroelectric hysteresis loop. Therefore, this material combines the robust and well-defined domain structure of a proper ferroelectric with dynamic hysteresis loops present in an antiferroelectric state. Moreover, the triple-well also enhances mem-capacitive effects in Sn2 P2 S6 which are forbidden for ideal double-well ferroelectrics. These findings provide a path to tunable electronic elements for beyond binary high-density computing devices and neuromorphic circuits based on three thermodynamically stable ferroelectric states. This article is protected by copyright. All rights reserved.

Sn2P2S6: A lead-free reversible thermochromic ferroelectric with high near-infrared reflectance

Multifunctional thermochromic materials integrated optical, ferroelectric, and/or magnetic properties have been long envisioned and played an invaluable role in signal processing and non-contact optical information storage in smart windows. However, poor materials stability, together with high discoloration temperature, absence of ferroelectric, or magnetic behaviors, impedes most thermochromic materials for practical applications. Here, the thermochromic behavior of all-inorganic lead-free ferroelectric Sn2P2S6 (SPS) is extensively studied, which exhibits a reversible color change between light orange and red around the ferroelectric-paraelectric phase transition temperature (∼ 67 °C). In situ powder X-ray diffraction (PXRD) and Raman spectroscopy indicate the change of P-S and Sn-S bond distance, and thus result in the modification of (P2S6) octahedral geometry and the coordination environment of Sn2+ ions along with the evolution of thermochromism (TCM). Additionally, the first-principles calculations reveal that, the Sn 5s2 stereochemically active lone pair electrons evoke negative lattice expansion comparable to that of the known ZrW2O8 at warming and is responsible for the reversible TCM of SPS. SPS also possesses high near-infrared reflectance (R > 80 %, 1100-2500 nm). The integration of ferroelectricity, high near-infrared reflectance, and TCM below 100 °C renders potential applications of the titled compound in multifunctional optical smart windows.

Nanoparticle-Induced Property Changes in Nematic Liquid Crystals.

Doping liquid crystals with nanoparticles is a widely accepted method to enhance liquid crystal’s intrinsic properties. In this study, a quick and reliable method to characterise such colloidal suspensions using an optical multi-parameter analyser, a cross-polarised intensity measurement-based device, is presented. Suspensions characterised in this work are either plasmonic (azo-thiol gold AzoGNPs) or ferroelectric Sn2P2S6 (SPS) nanoparticles in nematic liquid crystals. The elastic constants and rotational viscosity showed nonlinear dependence on the concentration of AzoGNPs, initially increasing at lower concentrations and then decreasing at higher concentrations, indicating some degree of particle aggregation. For the SPS suspension, the elastic constant decreased with doping, while the rotational viscosity increased, in agreement with previous findings. Through viscosity measurements, the stability of SPS suspension over ten years is also highlighted.

Nature of thermoelectric properties occurring in defected Sn2P2S6chalcogenide crystals

Detailed first-principles calculations were performed to explain the physical nature of the thermoelectric properties occurring in Sn2P2S6(SPS) crystals with deviation from stoichiometry.

The temperature dependence of the photoinduced soft mode in Sn2P2S6 crystal

In this paper, we present the results of the temperature dependence studies of the photoinduced phonon mode which appears in a ferroelectric-semiconductor Sn2P2S6 (SPS) crystal under the influence of femtosecond laser pulses. We show that its behavior reminds the soft mode one near the phase transition. However, the observed decrease in the oscillation frequency in the vicinity of Curie temperature Tc is much weaker than previously reported. This can be explained by the shadowing of the pure soft mode by its interaction with other photoinduced modes.

Improved slow light performances using photorefractive two-wave mixing.

We experimentally observe an ultralow effective group velocity of 0.9cm/s of light pulses using the two-wave mixing process in an Sn2P2S6 (SPS):Te crystal at a visible wavelength. The time delay can be controlled through the nonlinear photorefractive gain and the input pulse duration. By optimizing the nonlinearity strength, we achieve a maximum fractional delay of 0.79 for a pulse duration of 100ms. Our photorefractive slow light system allows combining low group velocity with large delay-bandwidth product for pulse durations spanning three orders of magnitude.

Pseudocapacitive Sodium Storage by Ferroelectric Sn2 P2 S6 with Layered Nanostructure.

Sodium ion batteries (SIB) are considered promising alternative candidates for lithium ion batteries (LIB) because of the wide availability and low cost of sodium, therefore the development of alternative sodium storage materials with comparable performance to LIB is urgently desired. The sodium ions with larger sizes resist intercalation or alloying because of slow reaction kinetics. Most pseudocapacitive sodium storage materials are based on subtle nanomaterial engineering, which is difficult for large-scale production. Here, ferroelectric Sn2 P2 S6 with layered nanostructure is developed as sodium ion storage material. The ferroelectricity-enhanced pseudocapacitance of sodium ion in the interlayer spacing makes the electrochemical reaction easier and faster, endowing the Sn2 P2 S6 electrode with excellent rate capability and cycle stability. Furthermore, the facile solid state reaction synthesis and common electrode fabrication make the Sn2 P2 S6 that becomes a promising anode material of SIB.

Temporal dynamics of two-beam coupling and the origin of compensation photorefractive gratings in Sn_2P_2S_6:Sb

The photorefractive effect in Sn2P2S6 (SPS) is caused by a spatial redistribution of both positively charged and negatively charged carriers, which leads to the formation of two out-of-phase space charge gratings that partially compensate each other in the steady state. The unusual intensity dependence of the compensation grating buildup time is reported in this article for antimony doped SPS; it is explained by the thermal excitation of electrons into conduction band from the optically recharged Sb3+ → Sb2+ impurity ions.

Sulfur vacancies in photorefractive Sn2P2S6 crystals

A photoinduced electron paramagnetic resonance (EPR) spectrum in single crystals of Sn2P2S6 (SPS) is assigned to an electron trapped at a sulfur vacancy. These vacancies are unintentionally present in undoped SPS crystals and are expected to play an important role in the photorefractive behavior of the material. Nonparamagnetic sulfur vacancies are formed during the initial growth of the crystal. Subsequent illumination below 100 K with 442 nm laser light easily converts these vacancies to EPR-active defects. The resulting S = 1/2 spectrum shows well-resolved and nearly isotropic hyperfine interactions with two P ions and two Sn ions. Partially resolved interactions with four additional neighboring Sn ions are also observed. Principal values of the g matrix are 1.9700, 1.8946, and 1.9006, with the corresponding principal axes along the a, b, and c directions in the crystal. The isotropic parts of the two primary 31P hyperfine interactions are 19.5 and 32.6 MHz and the isotropic parts of the two primary Sn hyperfine interactions are 860 and 1320 MHz (the latter values are each an average for 117Sn and 119Sn). These hyperfine results suggest that singly ionized sulfur vacancies have a diffuse wave function in SPS crystals, and thus are shallow donors. Before illumination, sulfur vacancies are in the doubly ionized charge state because of compensation by unidentified acceptors. They then trap an electron during illumination. The EPR spectrum from the sulfur vacancy is destroyed when a crystal is heated above 120 K in the dark and reappears when the crystal is illuminated again at low temperature.

Ferroelectric and Semiconducting Properties of Sn2P2S6 Crystals with Intrinsic Vacancies

Thermoelectric properties of Sn2P2S6 (SPS) ferroelectric-semiconductor crystals confirm their hole-type conductivity at room temperature which inverses to the electron-type conductivity at heating to 500 K. Their dielectric susceptibility temperature anomaly near second order phase transition at 337 K and dielectric losses in paraelectric phase both strongly depend on the deviation from the stoichiometric content. Calculated in LDA approach donor energy level of the sulfur vacancies and acceptor level of the tin vacancies are placed near the top of valence band. Such position in the energy gap of donor states mostly determines the semiconductive properties of SPS crystals, and they is related to strong hybridization between sp2 hybrid orbitals of phosphorus atoms and p orbitals of sulfur atoms inside anions (P2S5)4− that contain sulfur vacancy.

INVESTIGATIONS OF TEMPERATURE DEPENDENCES OF ELECTRICAL RESISTANCE OF Sn2 P2 S6 FERROELECTRIC SEMICONDUCTOR CRYSTALS

Dependences of electrical resistance (R) and relative temperature coefficient of electrical resistance ( α ) versus temperature are investigated for Sn2 P2 S6 crystals. It is established that with increasing temperature, dependence R(T) exponencially decreases with different thermical coefficients (B). In this work coefficients (B) and energy of the doped levels are obtained for ferroelectric and paraelectric phases. It is established, that coefficient ( α ) has high values in both phases and also in the vicinity of the phase transition point the anomaly occurs. The energy of the doped levels is calculated: in ferroelectric phase 1, 0.12 E eV fe = , 2, 1.17 E eV fe = , and paraelectric phase 3, 0.87 − = E eV pa .

Ferroelectric nanoparticles in low refractive index liquid crystals for strong electro-optic response

Functional materials based on ferroelectric, inorganic nanoparticles, and low refractive index nematic liquid crystals show strong induced birefringence and dielectric anisotropy. Birefringence can increase by a factor of 2 and dielectric anisotropy by a factor of 3 as compared with nominally pure liquid crystals. The enhancement of the electro-optic performance is higher in liquid crystals with Sn2P2S6 (SPS) nanoparticles than with BaTiO3 nanoparticles. The shape and size distribution of both types of ferroelectric particles were characterized using atomic force microscopy. The average size of SPS nanoparticles was 45nm and of BaTiO3 nanoparticles was 20nm.

Ferroelectricity, Nonlinear Dynamics, and Relaxation Effects in MonoclinicSn2P2S6

An ab initio-based model of the temperature-induced ferroelectric phase transition in Sn2P2S6 (SPS) as a prototype of an unconventional ferroelectric is developed. The order parameter in SPS is found as the valley line on a total-energy surface of the zone-center fully symmetrical Ag and polar Bu distortions. Significant nonlinear coupling between order parameter and strain is observed. Monte Carlo simulations describe the additional low-temperature rearrangement in polar structure, which appears in domain boundaries, and describe the relaxation phenomena near the ferroelectric phase transition.

Domain Wall Orientations in Sn2P2S6 Type Ferroelectrics

We have investigated the ferroelectric domain structure a Sn2P2S6 (SPS) single crystal using a chemical etching technique. For this purpose new etching compound was elaborated. The results demonstrate that domain pattern of Sn2P2S6 is formed by the both charged and non-charged domain walls. The presence of the charged walls correlates well with previous.

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn2P2S6 (SPS) crystals is studied. A large gain factor of more then 15 cm-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis.

The influence of uniform pressure and electric field on phase transitions of the Sn 2P2S6 ferroelectric

The investigations of the dielectric and dilatometric properties of the Sn 2P2S6 proper ferroelectrics in the vicinity of the Lifshitz point under high pressure and external electric field was carried out. Based on the investigations on dielectric permeability anomalies the E, T-diagrams at different magnitudes of uniform pressure and p, T, E-diagram of this crystal have been constructed. The pressure behaviour of the coefficients of the thermodynamic potential expansion by degrees of the order parameter is derived.

Metal-Ferroelectric-Semiconductor structures based on thin Sn2P2S6 films

Abstract Hysteresis effects in the metal-ferroelectric-semiconductor (MFS) structures composed of thin Sn2 P2 S6 films on silicon substrates with resistivity of 10 and 70 0hm·cm have been studied using the method of C-V characteristics, By varying the amplitude of the bias voltage one may observe changes in the character of the hysteresis of C-V characteristics permitting the specific features to be traced of the interaction between a bound charge of the ferroelectric film and the subsurface bulk charge of the semiconductor. In the structures of either type, two mechanisms of the hysteresis phenomena are manifested, namely, the polarization and the injection mechanism. Temperature effect on the hysteresis in YIPS structures has also been studied.