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

Isostructural transitions in layered MX2 compounds are governed by competing van der Waals (vdW) and Coulomb interactions. While an isostructural transition (at P similar to 20 GPa) has been observed before metallization in MoS2 when subjected to pressure, it is surprisingly missing in layered MoSe2 and MoTe2. 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 2H(c)' structure (space group P6(3)mc) to a mixed-phase of 2H(a)' + 2H(c)' at P similar to 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 MoS2 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 MoS2 and MoSe2 and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at P similar to 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 similar to 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 similar to 18 GPa marked by sharp changes in pressure coefficients of A(1) Raman modes is associated with the metallization of the 2H(a)' phase.