Designing multifunctional two-dimensional layered transition metal phosphorous chalcogenides

Layered two-dimensional (2D) transition metal phosphorous chalcogenides (TMPCs) are now in intense research focus due to their interesting ferroelectric and magnetic properties and compatibility with 2D electronic devices. Here, we have employed first-principles density functional theory calculations to investigate the electric and magnetic properties of ${\mathrm{ABP}}_{2}{\mathrm{S}}_{6}$ (A = Cu, Ni; B = Cr, Mn) TMPCs. We have systematically investigated four TMPCs compounds, namely, ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}, {\mathrm{CuMnP}}_{2}{\mathrm{S}}_{6}, {\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$, and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, and reported unusual antiferroelectric/ferroelectric (AFE/FE) and electronic properties. We have found a more stable ferroelectric state in van der Waals (vdW) gap with higher polarization compared to the usual ferroelectric phase in the insulating state. In case of ${\mathrm{CrMnP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, we have proposed them as polar half-metals as our analysis have revealed that ferroelectric distortions can persist in these systems in the metallic phase. Moreover, our analysis have shown that ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ can undergo metal-to-insulator transition driven by the polar distortion. We have identified two modes namely, ${\mathrm{\ensuremath{\Gamma}}}_{2}^{\ensuremath{-}}$ ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$, those are responsible for driving ferroelectricity and antiferroelectricity, respectively, into these systems. We could observe a rare and interesting phenomena, where the antipolar mode ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$ that is responsible for antiferroelectric distortion leads to a ferroelectric distortion when applied excessively on the system, which is unusual. Finally, we have performed molecular dynamics simulations at various finite temperatures. In case of ${\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ at 300 K, we have observed that the ferroelectric state within the vdW gap is stable. Interestingly, we have discovered a hybrid inter-intra layer antiferroelectric configuration within the vdW gap for ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}$. The layers start to move opposite to each other due to the temperature effect, which leads to the switching of the AFE state in the first layer. Further, increase of temperature results the switching in both the layers. The reported in-gap FE/AFE states in vdW gap can be tuned by uniaxial strain along the perpendicular direction of the 2D layer and thus the materials studied here can be considered as potential piezoelectric materials.