Cation-Eutectic Transition via Sublattice Melting in CuInP₂S₆/In_4/3P₂S₆ van der Waals Layered Crystals

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 (In_4/3P₂S₆) phases through control of chemical phase separation. We conclusively demonstrate that Cu-deficient Cu_1-xIn_1+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 P₂S₆^4- 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.