Single crystals of the van der Waals layered ferrielectric material CuInP2S6 spontaneously phase separate when synthesized with Cu deficiency. Here we identify a route to form and tune intralayer heterostructures between the corresponding ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6) phases through control of chemical phase separation. We conclusively demonstrate that Cu-deficient Cu1-xIn1+x/3P2S6 forms a single phase at high temperature. We also identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K both Cu+ and In3+ become mobile, while P2S64- anions maintain their structure. We therefore propose that this transition can be understood as eutectic melting on the cation sublattice. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size. At the fastest cooling rate, the dimensional confinement of the ferrielectric CuInP2S6 phase to nanoscale dimensions suppresses ferrielectric ordering due to the intrinsic ferroelectric size effect. Intralayer heterostructures can be formed, destroyed, and re-formed by thermal cycling, thus enabling the possibility of finely tuned ferroic structures that can potentially be optimized for specific device architectures.
Bibliographical noteFunding Information:
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 (P.M., P.G., M.C., A.B., Q.H., M.A.S., and M.A.M.). Experiments were partially conducted (AFM, Raman, and computational) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, United States Department of Energy. B.S.C. D.A.C., D.S., and J.W.M. acknowledge support from the United States Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Technology Division. Use of the Advanced Photon Source an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Manuscript preparation was funded by the Air Force Research Laboratory under an Air Force Office of Scientific Research grant (LRIR No. 14RQ08COR) and a grant from the National Research Council.
© 2017 American Chemical Society.
- 2D ferrielectric
- 2D heterostructures
- sublattice melting
- transition metal thiophosphate