Mott metal-insulator transitions in pressurized layered trichalcogenides

Transition metal phosphorous trichalcogenides, $M\mathrm{P}{X}_3$ ($M$ and $X$ being transition metal and chalcogen elements respectively), have been the focus of substantial interest recently because of their possible magnetism in the two-dimensional limit. Here we investigate material properties of the compounds with $M$ = Mn and Ni employing $\mathit{\text{ab-initio}}$ density functional and dynamical mean-field calculations, especially their electronic behavior under external pressure in the paramagnetic phase. Mott metal-insulator transitions (MIT) are found to be a common feature for both compounds, but their lattice structures show drastically different behaviors depending on the relevant orbital degrees of freedom, i.e. $t_{2\mathrm{g}}$ or $e_g$. MnPS${}_3$ undergoes an isosymmetric structural transition by forming Mn-Mn dimers due to the strong direct overlap between the neighboring $t_{\rm 2g}$ orbitals, accompanied by a significant volume collapse and a spin-state transition. In contrast, NiPS${}_3$ and NiPSe${}_3$, with their active $e_g$ orbital degrees of freedom, do not show a structural change at the MIT pressure or deep in the metallic phase. Hence NiPS${}_3$ and NiPSe${}_3$ become rare examples of materials hosting electronic bandwidth-controlled Mott MITs, thus showing promise for ultrafast resistivity switching behavior.