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.
