Designing layered two dimensional Van der Waals multiferroics and polar metals*

We have employed first-principles density functional theory calculations to investigate the electric, magnetic and electronic properties of ABP₂S₆ (A = Cu, Ni; B = Cr, Mn) two dimensional (2D) transition metal phosphorous chalcogenides (TMPC). We have investigated four (TMPCs) compounds namely CuCrP₂S₆ (CCPS), CuMnP₂S₆ (CMPS), NiCrP₂S₆ (NCPS) and NiMnP₂S₆ (NMPS) and reported unusual anti/ferroelectric (AFE/FE) and electronic properties. We have found a more stable ferroelectric state is in the Van der Waals (VdW) gap with higher polarization compared to the usual ferroelectric phase. We report CMPS and NMPS as polar half metals whereas NMPS can undergo metal-to-insulator transition driven by the polar distortion. At 300K, by performing molecular dynamic simulations, we have discovered a hybrid ’inter-intra’ antiferroelectric switching within the VdW gap for CCPS. The origin of such switching is found to be the relative displacement of the layers with respect to each other. The reported VdW FE/AFE 2D layered TMPC systems and thus can be considered as potential piezoelectric materials.