Joint Influence of Indium and Copper Cation Sublattices on the Origin of Ferrielectricity in 2D CuInP2S6

In the CuInP
2
S
6
crystal lattice the second order Jahn-Teller effects determine a double-well local potential for Cu
⁺
cations and a three-well local potential for In
³⁺
cations. At normal or positive pressure, the first order transition from the paraelectric to the ferrielectric phase occurs, which is determined by the higher energy of the side wells in respect to the central well of the In
³⁺
potential. With the rise of the negative compression, the side wells are stabilized and the paraelectric to ferrielectric structural rearrangement can evolve into the second order phase transition. Also, the paraelectric–ferrielectric transition becomes continuous at weakening of indium cations coupling under the influence of negative internal chemical pressure, which is induced at substitution of copper by silver or sulfur by selenium.