Identification of a Low Energy Metastable 1$T$-Type Phase for Monolayer VSe2.

Elucidating the polymorphism of transition metal dichalcogenide layers and the interplay between structure and properties is a key challenge for the application of these materials. We identify a novel low energy metastable phase of monolayer VSe$_2$ and elucidate its magnetic and electronic properties. This structure is distinct from the previously identified charge density wave (CDW) phase. However, while having rather distinct properties it is very close in energy to the CDW phase and is likely to be realized in experiments. Importantly, local bonding instabilities, as characterized by reconstruction of the electronic structure over a wide energy range, are important for this distortion, which includes both V off-centering in the octahedral coordination cages and a partial disproportionation into two distinct types of V. The phase does not have a ferromagnetic ground state. The results show that the physics of 1$T$ VSe$_2$ are richer than previously known with an interplay of Fermi surface instabilities and local bonding effects.