Ferri-ionic Coupling in CuInP₂S₆ Nanoflakes: Polarization States and Controllable Negative Capacitance

We consider nanoflakes of van der Waals ferrielectric CuInP₂S₆ covered by an ionic surface charge and reveal the appearance of polar states with relatively high polarization ~5 microC/cm² and stored free charge ~10 microC/cm$%2$, which can mimic "mid-gap" states related with a surface field-induced transfer of Cu and/or In ions in the van der Waals gap. The change in the ionic screening degree and mismatch strains induce a broad range of the transitions between paraelectric phase, antiferroelectric, ferrielectric, and ferri-ionic states in CuInP₂S₆ nanoflakes. The states' stability and/or metastability is determined by the minimum of the system free energy consisting of electrostatic energy, elastic energy, and a Landau-type four-well potential of the ferrielectric dipole polarization. The possibility to govern the transitions by strain and ionic screening can be useful for controlling the tunneling barrier in thin film devices based on CuInP₂S₆ nanoflakes. Also, we predict that the CuInP₂S₆ nanoflakes reveal features of the controllable negative capacitance effect, which make them attractive for advanced electronic devices, such as nano-capacitors and gate oxide nanomaterials with reduced heat dissipation.