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

Abstract

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 ∼ 20 GPa) has been observed before metallization in MoS 2 when subjected to pressure, it is surprisingly missing in layered MoSe 2 and 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 structure (space group P 6 3 m c ) to a mixed-phase of 2 H ′ a + 2 H ′ c at P ∼ 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 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 MoS 2 and MoSe 2 and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at P ∼ 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 ∼ 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 ∼ 18 GPa marked by sharp changes in pressure coefficients of A 1 Raman modes is associated with the metallization of the 2 H ′ a phase.