| AVS 64th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Tuesday Sessions |
| Session 2D-TuA |
| Session: | Growth of 2D Materials |
| Presenter: | Petro Maksymovych, Oak Ridge National Laboratory |
| Authors: | M.A. Susner, Air Force Research Laboratory M. Chyasnavichyus, Oak Ridge National Laboratory Q. He, Oak Ridge National Laboratory B.S. Conner, Oak Ridge National Laboratory D.A. Cullen, Oak Ridge National Laboratory P. Ganesh, Oak Ridge National Laboratory D. Shin, Oak Ridge National Laboratory J.W. McMurray, Oak Ridge National Laboratory A. Borisevich, Oak Ridge National Laboratory M.A. McGuire, Oak Ridge National Laboratory Y. Ren, Argonne National Laboratory P. Maksymovych, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
Metal thiophosphate materials family offers a materials toolbox with broad functionality that includes magnetism, ferrielectricity and electron correlations. Here we report on heterostructure engineering of layered ferrielectric CuInP2S6, which controllably introduces 1D and 2D chemical boundaries into the crystal on bulk scale. Single crystals of mixed Cu1-xIn1+x/3P2S6 spontaneously phase separate into ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6) chemical domains, providing a new route to functional in-plane heterostructures in layered and 2D materials. We used high temperature x-ray diffraction and in-situ electron microscopy to conclusively demonstrate that this material forms a single phase at high temperature, and to identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K Cu1-xIn1+x/3P2S6 adopts a heavily disordered structure with respect to metal/vacant sites occupying the octahedral sites within a layer, thus indicating high Cu+ and In3+ mobilities. However, the framework of P2S6 anions remains invariant across this transition. Considering the results of our detailed measurements of the transition temperature as a function of Cu/In ratio, we propose that this transition can be understood as eutectic melting on the cation sublattice, conceptually similar to intermediate temperature behavior of halide superionic conductors. 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, forming an intricate mesh of in-layer heterostructures comprised of domains with distinct cation compositions. Heterostructures can be formed, destroyed, and reformed by thermal cycling. Using this mode of lattice manipulation, we demonstrate that the ferroelectric Tc can be both increased to a nearly record level (about 20K higher than the pure bulk CuInP2S6 of 305K) and completely suppressed well below room temperature, without changing the physical sample, chemical composition, or loss of reversibility. 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.
[1] Susner et al., “Metal Thio- and Selenophosphates as Multi-Functional van der Waals Layered Materials”, Advanced Materials, in press (2017)
[2] Susner et al., Cation-eutectic transition via sublattice melting in CuInP2S6/In4/3P2S6 van der Waals layered crystals, ACS Nano in review (2017).