New high-pressure phases of MoSe2 and MoTe2

Abstract

Three Mo-based transition-metal dichalcogenides ${\mathrm{MoS}}_{2},{\mathrm{MoSe}}_{2}$, and ${\mathrm{MoTe}}_{2}$ share at ambient conditions the same structure $2{H}_{c}$, consisting of layers where Mo atoms are surrounded by six chalcogen atoms in trigonal prism coordination. The knowledge of their high-pressure behavior is, however, limited, particularly in case of ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoTe}}_{2}$. The latter materials do not undergo a layer-sliding transition $2{H}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}$ $2{H}_{a}$ known in ${\mathrm{MoS}}_{2}$ and currently no other stable phase aside from $2{H}_{c}$ is known in these systems at room temperature. Employing evolutionary crystal structure prediction in combination with 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 P4/mmm, previously predicted in ${\mathrm{MoS}}_{2}$, to become stable in ${\mathrm{MoSe}}_{2}$ at 118 GPa. In ${\mathrm{MoTe}}_{2}$, we predict at 50 GPa a transition to a new layered tetragonal structure with space group I4/mmm, similar to ${\mathrm{CaC}}_{2}$, 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 ${\mathrm{MoTe}}_{2}$ likely offers the easiest route towards the post-$2H$ phases.