High-T_c Layered Ferrielectric Crystals by Coherent Spinodal Decomposition

Research in the rapidly developing field of 2D electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cation-deficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In_4/3P₂S₆ and ferrielectrically active CuInP₂S₆ are realized by controllable chemical phase separation in van der Waals bonded single crystals. CuInP₂S₆ by itself is a layered ferrielectric with a ferrielectric transition temperature (T_c) just over room temperature, which rapidly decreases with homogeneous doping. Surprisingly, in our composite materials, the ferrielectric T_c of the polar CuInP₂S₆ phase increases. This effect is enabled by unique spinodal decomposition that retains the overall van der Waals layered morphology of the crystal, but chemically separates CuInP₂S₆ and In_4/3P₂S₆ within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or synthesis conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D in-plane heterostructures with periodically alternating polar and nonpolar phases.