Layered metal thiophosphates have recently been considered in the context of quasi-2D non-linear dielectric and ferroelectric materials, with properties distinct from a more common dichalcogenide family. A specific representative, CuInP2S6 (CIPS), exhibits order-disorder ferroelectric transition with Tc around room temperature. Although mobility of Cu+1 ions in CIPS has been inferred from previous measurements, the mechanism of the ionic transport and the accompanying change in material properties are presently unknown.
Here we employed scanning probe microscopy (SPM) to investigate ionic motion in paraelectric CIPS . Above critical electric field, we observe surface expansion well beyond what is expected of dielectric tunability. However, the expansion is almost completely reversible, indicating lack of permanent lattice damage. At 473K, the surface expansion can reach a giant 100 nm, nominally corresponding to over 30% strain, while still remaining reversible. All the measurements were carried out under strictly controlled environmental conditions, ruling out trivial artefacts.
We propose a hierarchical model where surface deformation in applied field proceeds through a sequence of piezoelectric deformation where Cu+1 maintains its intralayer position, Cu+1 hopping into the van-der-Waals gap and finally nucleation and growth of Cu on the surface. The reversibility of this process is tied to the resilience of the CuInP2S6 to cationic vacancies, which has been remotely hinted on by the prior studies of intentionally Cu-deficient phases, as well as the existence of a stable Cu-free In4/3P2S6 phase .
Within the model, we further developed the technique of electrochemical strain microscopy, an almost decade-old proposal to study ionic motion in solids via the associated strain deformation. Specifically, we directly measured the frequency and temperature dependence of surface expansion, and extracted both activation barrier and diffusivity coefficient for ionic motion.
Altogether, ionic mobility in CuInP2S6, when probed by local electric field, translates into giant shape-memory type of response, involving almost completely reversible extraction and reinsertion of Cu atoms in and out of the parent van-der-Waals crystal. CIPS has therefore provided the best yet model system for electrochemical strain microscopy, enabling further insight into how to utilize this technique for nanoscale studies of energy materials.
Research conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. Research partly supported by a research grant from Science Foundation (SFI) under the US-Ireland R&D Partnership Programme Grant Number SFI/14/US/I3113.
 Balke et al., submitted (2017)
 Susner et al., ACS Nano, 11, (2017), 7060