Density Functional Theory insights into the structural, energetic, and mechanical properties of MoS 2 and MoSe 2 polytypes

The structural properties and phase transitions for the proposed polytypes of MoS2 and MoSe2 were theoretically investigated by means of density functional theory calculations. From our results obtained, the 2Hc structure has the smaller total energy per molecule for both MoS2 and MoSe2 despite the fact that the 3R polytype can coexist with the 2Hc phase for MoS2 compound, which are in agreement with experimental observations and with other theoretical results as well. However, our results for the Bulk Modulus of MoS2 are in disagreement with the published experimental data. By simulating the experimental environment from a theoretical point of view, our results indicated that, due to lubricant properties of MoS2 at low applied pressures, its response to applied pressure is not completely hydrostatic: there is also an uniaxial component, leading to low values for the experimental Bulk Modulus data compared with the theoretical one. Finally, our results indicate a phase transition from the 2Hc structure to the 2Ha one for MoS2 at pressures greater than 14.5 GPa, in agreement with the published experimental results. Moreover, a structural phase transition for MoSe2 is predicted, from the 2Hc structure to the 3R one, at pressures greater than 17 GPa.