New High-Pressure Phases of MoSe$_2$ and MoTe$_2$

Abstract: Three Mo-based transition metal dichalcogenides MoS$_2$, MoSe$_2$ and MoTe$_2$ share at ambient conditions the same structure 2H$_c$, consisting of layers where Mo atoms are surrounded by six chalcogen atoms in trigonal prism coordination. The knowledge of their high-pressure behaviour is, however, limited, particularly in case of MoSe$_2$ and MoTe$_2$. The latter materials do not undergo a layer-sliding transition 2H$_c$ $\rightarrow$ 2H$_a$ known in MoS$_2$ and currently no other stable phase besides 2H$_c$ is known in these systems at room temperature. Employing evolutionary crystal structure prediction in combination with \textit{ab initio} calculations we study the zero-temperature phase diagram of both materials up to Mbar pressures. We find a tetragonal phase with space group \textit{P4/mmm}, previously predicted in MoS$_2$, to become stable in MoSe$_2$ at 118 GPa. In MoTe$_2$ we predict at 50 GPa a transition to a new layered tetragonal structure with space group \textit{I4/mmm}, similar to CaC$_2$, where Mo atoms are surrounded by eight Te atoms. The phase is metallic already at the transition pressure and becomes a good metal beyond 1 Mbar. We discuss chemical trends in the family of Mo-based transition metal dichalcogenides and suggest that MoTe$_2$ likely offers the easiest route towards the post-2H phases.