Anharmonic stabilization of ferrielectricity in CuInP$_2$Se$_6$

Using first-principles calculations and group-theory based models, we study the stabilization of ferrielectricity (FiE) in CuInP$_2$Se$_6$. We find that the FiE ground state is stabilized by a large anharmonic coupling between the polar mode and a fully symmetric Raman-active mode. Our results open possibilities for controlling the single-step switching barrier for polarization by tuning the Raman-active mode. We discuss the implications of our findings in the context of designing next-generation optoelectronic devices that can overcome the voltage-time dilemma.