Chemical ordering and pressure-induced isostructural and electronic transitions in MoSSe crystal

Isostructural transitions in layered $M{X}_{2}$ compounds are governed by competing van der Waals (vdW) and Coulomb interactions. While an isostructural transition (at $P\ensuremath{\sim}$ 20 GPa) has been observed before metallization in ${\mathrm{MoS}}_{2}$ when subjected to pressure, it is surprisingly missing in layered ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoTe}}_{2}$. Using synchrotron x-ray diffraction and Raman spectroscopic measurements of structural and vibrational properties of layered MoSSe crystals subjected to pressures up to 30 GPa and first-principles density functional theoretical analysis, we demonstrate a layer sliding isostructural transition from its $2{H}_{c}^{\ensuremath{'}}$ structure (space group ${P6}_{3}mc$) to a mixed-phase of $2{H}_{a}^{\ensuremath{'}}+2{H}_{c}^{\ensuremath{'}}$ at $P\ensuremath{\sim}$ 10.8 GPa, marked by discontinuity in lattice parameters, pressure coefficients of Raman modes, and accompanying changes in electronic structure. The origin of the unusually lower transition pressure of MoSSe compared with ${\mathrm{MoS}}_{2}$ is shown to be linked to chemical ordering of S and Se atoms on the anionic sublattice, possible because of moderate lattice mismatch between the parent compounds ${\mathrm{MoS}}_{2}$ and ${\mathrm{MoSe}}_{2}$ and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at $P\ensuremath{\sim}3$ GPa and not reported earlier in the isostructural Mo-based chalcogenides, marked by a discontinuity in the pressure coefficient of the $c/a$ ratio and indirect band gap. The transition observed at $P\ensuremath{\sim}10.8$ GPa appears due to the change in the sign of the pressure coefficient of the direct band gap originating from inversion of the lowest-energy conduction bands. Our theoretical analysis shows that the phase transition at $P\ensuremath{\sim}18$ GPa marked by sharp changes in pressure coefficients of ${A}_{1}$ Raman modes is associated with the metallization of the $2{H}_{a}^{\ensuremath{'}}$ phase.