2, Cardiff University, Cardiff, , United Kingdom
3, Argonne National Laboratory, Chicago, Illinois, United States
Metal thiophosphate materials family offers a materials toolbox with broad functionality that includes magnetism, ferrielectricity and electron correlations. One of their signature distinctions from transition metal dichalcogenides is the high mobility of certain ions, such as Cu+1 across the van-der-Waals gap. Here we report on heterostructure engineering of layered ferrielectric CuInP2S6, which controllably introduces 1D and 2D chemical boundaries into the crystal on bulk scale. The methodology relies on ionic mobility within the whole cation sublattice, both within and across the layers. We used high temperature x-ray diffraction, in-situ electron microscopy and atomic crorce microscopy to show that Cu-deficient Cu1-xIn1+x/3P2S6 material forms a single phase at high temperature, and spontaneously phase separates into ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6) phases coexisting within a single crystal. The high temperature (500 K) structure is heavily disordered , indicating mobility of both Cu+ and In3+ ions within the lattice. However, the framework of P2S6 anions remains invariant across this transition. 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. Therefore a combination of ionic conductivity and several partially incompatible lattice structures enables creating complex multifunctional materials. To this end, we will further demonstrate how this methodology applies to other combinations of thiophosphates, including the composites made of initially ferroelectric and antiferromagnetic phases. 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.
 Susner et al., “Metal Thio- and Selenophosphates as Multi-Functional van der Waals Layered Materials”, Advanced Materials, 10.1002/adma.201602852 (2017).
 Susner et al., Cation-eutectic transition via sublattice melting in CuInP2S6/In4/3P2S6 van der Waals layered crystals, ACS Nano, 11, (2017), 7060.