Dynamic Stabilization of Metastable States in Triple‐Well Ferroelectric Sn₂P₂S₆

Abstract

photoconductive and photovoltaic properties. [3] Moreover, the material exhibits a substantial polarization of ≈15 µC cm −2 , [4,5] while being lead-free.A fundamentally intriguing question is that of the role of the polarization potential of Sn 2 P 2 S 6 in its known and prospective functionalities. The crystal structure of Sn 2 P 2 S 6 in its ferroelectric state is shown in Figure 1a. [6] As is typical for ferroelectric thiophosphates, the ferroelectricity originates from a relative displacement of the Sn 2+ cations with regard to the framework of (P 2 S 6 ) 4− anions. The driving force for the transition is the second-order Jan-Teller effect [7] due to electron lonepair stereoactivity of the Sn 2+ cations, together with possible valence fluctuations within the (P 2 S 6 ) 4− anions. [5,8,9] Unlike more ubiquitous double-well ferroelectrics (Figure 1b), Sn 2 P 2 S 6 has been reported to exhibit a uniaxial triple-well (Figure 1d) wherein a metastable nonpolar state coexists with polarized structures well below the phase transition to the ferroelectric state at ≈338 K. [4,8,10] Despite early reports of a possible triple-well potential in Sn 2 P 2 S 6 , direct experimental evidence for its existence remains scarce and controversial. Experimentally, Kiselev et al. have shown piezoresponse force microscopy (PFM) images of Sn 2 P 2 S 6 containing areas of suppressed piezoresponse around domain boundaries that appeared much broader than expected 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, leading to complicated energy landscapes with new fundamental and functional properties. Here, direct dynamic evidence is revealed of a triple-well potential in the metal thiophosphate Sn 2 P 2 S 6 compound using multivariate scanning probe microscopy combined with theoretical simulations. The key finding is that the metastable zero polarization state can be accessed through a gradual switching process and is stabilized over a broad range of electric fields. Simulations confirm that the observed zero polarization state originates from a kinetic stabilization of the nonpolar state of the triple-well, as opposed to domain walls. Dynamically, the triple-well of Sn 2 P 2 S 6 becomes equivalent to antiferroelectric hysteresis loops. Therefore, this material combines the robust and well-defined domain structure of a proper ferroelectric with dynamic hysteresis loops present in antiferroelectrics. Moreover, the triple-well enhances mem-capacitive effects in Sn 2 P 2 S 6 , 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 dynamic properties of the triple-well.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection

“…The atomic structures of the ferroelectric and paraelectric phases exhibit a monoclinic P n symmetry and orthorhombic P 21 /n symmetry, respectively. The displacement of Sn 2+ ionic units from the centrosymmetric positions would cause the off-center ordering of dipoles and thus spontaneous polarization with a polar axis ∼13° off the [100] crystallographic direction . Different from the layered ferroelectric materials with vdW stacking, , the strong covalent bonding of atoms in Sn 2 P 2 S 6 makes it rather difficult to isolate the bulk crystal into atomic layers by simple mechanical exfoliation.…”

Low-dimensional van der Waals materials for linear-polarization-sensitive photodetection: materials, polarizing strategies and applications

“…It is worth noting that a plethora of low-dimensional vdWMs, including AgBiP 2 Se 6 [209], CuInP 2 S 6 [210], Bi 2 TeO 5 [211], α-In 2 Se 3 [212,213], β * -In 2 Se 3 [214], GeS [215], GeSe [216], GaSe [217], SnSe [218], SnS [41], Bi 2 O 2 Se [219], Sn 2 P 2 S 6 [220], Co 2 NF 2 [221], Bi 1.8 Sm 0.2 O 3 [222], etc, have thus far proven to host intrinsic ferroelectricity. Furthermore, in addition to coupling with the extrinsic ferroelectric layer, previous studies have consolidated that the ferroelectricity of the photosensitive materials themselves can also bring about anisotropic photoresponse [223][224][225][226].…”

Nanoscale Probing of Electrical Memory Effects in van der Waals Layered PdSe2

“…Of particular interest are neuromorphic computing approaches that are underpinned by performing data storage and processing in the same physical space, in contrast to traditional computing architectures where data have to be constantly transferred between processors and memory, requiring additional time and energy. , It is therefore important to evaluate materials in terms of their suitability to be used in neuromorphic chips. One of the main desired characteristics is the possibility to encode memory states into the material structure, as well as the stability and accessibility of these states across a broad range of time scales specific to a desired circuit function. − In this regard, a wide range of functional mechanisms for changes in conductivity and capacitance have been studied, including electrochemical random access memory in which the resistivity can be gradually changed by varying the ion concentration, gradual formation of conductive filaments, ,− electrochemical metallization of cells, ferroelectric modulation due to polarization switching, , structural phase changes, metastable polarization states, and charge shielding . Emanating from their unique structural and electronic properties in addition to the ability to create ultrathin functional devices with low power consumption, two-dimensional materials are highly suitable candidates for neuromorphic hardware elements. ,, …”

“…This mesoscale technique bridges the gap between atomic scale behavior and macroscopic device performance. SMIM is an atomic force microscopy technique where a nanoscale tip scans across a sample surface. ,− In addition to providing information about the topography, SMIM measures local electrical properties by sending microwaves at ∼3 GHz through a nanoscale tip that is in contact with the sample, creating a near-field electromagnetic wave. The microwave signal that is reflected back into the probe depends on the tip–sample contact impedance, and demodulation provides information on nanoscale capacitive (SMIM-C) and resistive (SMIM-R) material properties.…”