The Multiwells—Switching Portraits of the Thiophosphate Family

Multiwell potentials make ferroelectrics fundamentally interesting, diverse and unique. In particular, multiwells can give rise to metastable states that control ferroelectric switching and the structure of domain walls. In this sense ferroelectric materials gain field-induced functionality by design, which is particularly promising for applications in microelectronic, electrocaloric and energy-storage functions. We have recently come across several distinct multiwell potentials in ferroelectrics comprising P₂S₆anion building blocks [1]. Although the materials in this family include both van der Waals and continuous solids with very different polarization properties [1], their multiwells share two common features – uniaxiality and substantial tunability with strain and chemical composition. On a fundamental level, the uniaxiality of the potential nearly guarantees new behaviors of domain walls and can give rise to polarization dynamics that may be unachievable in a double-well potential.<br/>In this talk, we will first show how nanoscale microscopy methods play central role in inferring the existence of multiwell potentials in thiophosphates [2-4], through direct and indirect observables. Indeed, despite almost two decade history of CuInP₂S₆, many functional properties such as giant negative electrostriction, multiplicity of the domain structure and strongly time-dependent switching, were first revealed with scanning probe techniques. These findings then led to computational predictions of the 4-well with first principles modeling, with subsequent quantitative verificaition by microscopy measurements [2]. Meanwhile unique features of switching hysteresis in Sn₂P₂S₆ consituted the first direct evidence of the proposed triple-well in this material, which enables coexistence of antiferroelectric-like switching loops with ferroelectric-like domain structure. Subsequently, we will focus on the interesting aspects of polarization dynamics that can be expected and observed in a multiwell potential, such as volatile and time-dependent polarization switching [3,4], field-induced order-disorder [4] and noise-induced restructuring of domain walls [5]. These properties collectively represent new prospects for ferroelectric applications, particularly in tunable and neuromorphic computing elements, while also posing a fresh frontier for revisiting and further understanding the emergence and dynamics of metastable polarization states. Finally, we will address the need for continued development of scanning probe methods capable of reproducible measurements of relatively more complex switching dynamics in multiwell systems, including combined piezoelectric and microwave microscopy, topological methods for data analysis and atomically-resolved imaging of materials with increased density of domain walls. The experiments were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, and conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Theoretical work at Vanderbilt University was supported by Department of Energy grant DE-FG02-09ER46554 and by the McMinn Endowment.<br/>1. M. A. Susner, et al., P.M, “Metal Thio- and Selenophosphates as Multifunctional van Der Waals Layered Materials.” <i>Advanced Materials</i> 29 (2017) 1602852.<br/>2. J. A. Brehm, et al., Tunable quadruple-well ferroelectric van-der-Waals crystals, <i>Nature Materials</i> 19 (2020) 43.<br/>3. S. M. Neumayer, et.al., P. M., “Polarization-controlled volatile ferroelectric and capacitive switching in Sn₂P₂S₆”, arxiv:2208.12734<br/>4. S. M. Neumayer, et al., “Ionic Control over Ferroelectricity in 2D Layered van der Waals Capacitors”, <i>ACS Applied Materials & Interfaces</i>14 (2022) 3018.<br/>5. N. Bauer, et al., “Structures and velocities of noisy ferroelectric domain walls”, arxiv:2208.02990, <i>Physical Review Materials in press</i> (2022).