  Single layer (SL) two-dimensional transition metal dichalcogenides (TMDs),
such as MoS2, ReS2, WSe2, and MoTe2 have now become the focus of intensive
fundamental and applied researches due to their intriguing and tunable physical
properties. These materials exhibit a broad range of structural phases that can
be induced via elastic strain, chemical doping, and electrostatic field effect.
These transitions in turn can open and close the band gap of SL-TMDs, leading
to metal-insulator transitions, and lead to emergence of more complex quantum
phenomena. These considerations necessitate detailed understanding of the
mesoscopic mechanisms of these structural phase transitions. Here we develop
the Landau-type thermodynamic description of SL-TMDs on example of
SL-(MoS2)1-x-(ReS2)x system and analyze the free energy surfaces, phase
diagrams, and order parameter behavior. Our results predict the existence of
multiple structural phases with 2-, 6- and 12-fold degenerated energy minima
for in-plane and out of plane order parameters. This analysis suggests that
out-of-plane ferroelectricity can exist in many of these phases, with the
switchable polarization being proportional to the out-of-plane order parameter.
We further predict that the domain walls in SL-(MoS2)1-x-(ReS2)x should become
conductive above a certain strain threshold.
