  The metastable 1$T'$ phase of layered transition metal dichalcogenides has
recently attracted considerable interest due to electronic properties, possible
topological electronic phases and catalytic activity. We report a comprehensive
theoretical investigation of intrinsic point defects in the 1$T'$ crystalline
phase of single-layer molybdenum disulfide (1$T'$-MoS$_2$), and provide
comparison to the well-studied semiconducting 2$H$ phase. Based on density
functional theory calculations, we explore a large number of configurations of
vacancy, adatom and antisite defects and analyse their atomic structure,
thermodynamic stability, electronic and magnetic properties. The emerging
picture suggests that, under thermodynamic equilibrium, 1$T'$-MoS$_2$ is more
prone to hosting lattice imperfections than the 2$H$ phase. More specifically,
our findings reveal that the S atoms that are closer to the Mo atomic plane are
the most reactive sites. Similarly to the 2$H$ phase, S vacancies and adatoms
in 1$T'$-MoS$_2$ are very likely to occur while Mo adatoms and antisites induce
local magnetic moments. Contrary to the 2$H$ phase, Mo vacancies in
1$T'$-MoS$_2$ are expected to be an abundant defect due to the structural
relaxation that plays a major role in lowering the defect formation energy.
Overall, our study predicts that the realization of high-quality flakes of
1$T'$-MoS$_2$ should be carried out under very careful laboratory conditions
but at the same time the facile defects introduction can be exploited to tailor
physical and chemical properties of this polymorph.
