  Based on density functional theory (DFT), we performed first-principle
studies on the electronic structure, magnetic state and optical properties of
two-dimensional (2D) transition-metal phosphorous trichalcogenide MnPX$_3$ (X=S
and Se). The calculated interlayer cleavage energies of MnPX$_3$ monolayers
indicate the energetic possibility to be exfoliated from bulk phase, with good
dynamical stability confirmed by the absence of the imaginary contributions in
the phonon spectra. MnPX$_3$ monolayers are both N\'eel antiferromagnetic (AFM)
semiconductors with direct band gaps falling into the visible optical spectrum.
Magnetic interaction parameters were extracted within the Heisenberg model to
investigate the origin of the AFM state. Three in-plane magnetic exchange
parameters play important role in the robust AFM configuration of Mn ions. The
N\'eel temperatures ($T_\mathrm{N}$) were estimated by means of Monte Carlo
simulations, obtaining the theoretical $T_\mathrm{N}$ of $103$\,K and $80$\,K
for 2D MnPS$_3$ and MnPSe$_3$, respectively. With high spin state, Mn ions
arranged in honeycomb lattices, the spin-degenerated band structures exhibit
valley polarisation and was investigated in different biaxial in-plain strains,
considering the spin-orbital coupling (SOC). 2D MnPX$_3$ monolayers show
excellent performance on the optical properties, and the absorption spectra
were discussed in detail to find the transition mechanism. Different amount and
configuration of chalcogen vacancy were introduced into the MnPX$_3$
monolayers, and it is found that the electronic structures are heavily affected
depending on the vacancy geometric structure, leading to different magnetic
state and absorption spectra of defected MnPX$_3$ systems.
