Ab initio study of anisotropic properties in isomorphic TiX2 (X = S, Se, Te)

摘要

Isomorphic titanium dichalcogenides TiX2 (X = S, Se, Te) exhibit diverse and anisotropic chemical and physical properties. Bulk TiSe2 was found to exhibit anomalous behavior at low temperatures, leading to emergence of a charge density wave (CDW) due to superstructure formation, bulk TiS2 and TiTe2 do not exhibit CDW instability. The diversity is investigated through the electronic structure by using density functional theory-based full potential linearized augmented plane wave and projector augmented wave methods. The latest generalized gradient approximation-Perdew-Burke-Ernzerhof correction for solids-Trans-Blaha-modified Becke-Johnson potential confirm that TiS2 is semiconducting whereas TiSe2 and TiTe2 are metallic. The diverse properties are governed by two bands, S/Se/Te p-band near the Gamma-point and Ti 3d-band around M (L)-points. The phonon and energy band structures are correlated to explain the occurrence of CDW. Phonon mode with imaginary frequency indicates lattice instability and the displacements of Ti-Se atoms allows the mixing of states of the electron and hole pockets leading to lattice distortion, which lowers the energy of the TiSe2 system.