High-pressure behaviour of transition metal dichalcogenides MoS2, MoSe2 and MoTe2 from ab initio simulations

Abstract: We study the evolution of structural and electronic properties of layered semiconducting transition metal dichalcogenides, MoX2 (X = S, Se, Te) upon increasing pressure, employing ab initio DFT calculations in combination with metadynamics and evolutionary search. We found that in MoS2 a layer sliding structural transition occurs at 20 GPa, converting the 2Hc stacking stable at ambient pressure to the 2Ha stacking (known in 2H-NbSe2)[1]. This transition explains earlier X-ray and Raman observations and has been confirmed in two recent X-ray diffraction experiments [2,3]. In the same pressure region where the structural transition occurs the system undergoes also a semiconductor-semimetal transition due to band overlap and indirect gap closing [1].The layered 2Ha structure surprisingly remains stable over a broad pressure range and undergoes further structural transformation only at pressures 130-140 GPa [4]. At this ultrahigh pressure we found two competing transformation scenarios – chemical decomposition into MoS + S, where MoS adopts the CsCl-like structure, or transformation into a new metastable metallic MoS2 structure with space group P4/mmm [4]. The high-pressure structural behaviour of MoSe2 and MoTe2 was found to be different from that of MoS2 - the initial 2Hc stacking remains stable and no transition to 2Ha takes place [5]. Similarly to MoS2 we predict both materials to undergo pressure-induced semiconductor-semimetal transition at 28 GPa and 13 GPa, respectively [5]. Since the structure is stable in the metallization region, both MoSe2 and MoTe2 appear as candidates to search for the excitonic insulator phase. All three materials exhibit after metallization a low density of states at the Fermi level.

[1] Liliana Hromadová, Roman Martoňák, and Erio Tosatti, Phys. Rev. B 87, 144105 (2013).
[2] N. Bandaru et al., J. Phys. Chem. C 118, 3230 (2014).
[3] Z.-H. Chi et al., Phys. Rev. Lett. 113, 036802 (2014).
[4] Oto Kohulák, Roman Martoňák, and Erio Tosatti, Phys. Rev. B 91, 144113 (2015)
[5] Michaela Rifliková, Roman Martoňák, and Erio Tosatti, Phys. Rev. B 90, 035108 (2014).