Evolution of crystal structures and electronic properties for TiS2 at high pressure

From Two-, One-, to Zero-Dimensional Vacancies: A Densification Pattern for a Typcial Transition-Metal Dichalcogenide of TiSe2

“…The variations of electronic structures in these three phases reflect the fact of the vanishing of lone pair electrons from 2D to 1D and 0D vacancies, corresponding with the transformation from layered to the spatial covalent network structure. Similar changes of electronic structures also occur in other TMDs such as TiTe 2 , 20 TiS 2 , 24 and TaS 2 . 21 Notably, a Dirac point emerges in the electronic band structure of Fe 2 P-type TiSe 2 along the Γ− A path, as shown in Figure 5c, which makes it a potential quantum material.…”

“…For example, P -3 m 1 TaS 2 also evolves into a 8-fold C 2/ m phase and then transforms into a tetragonal 10-fold I 4/ mmm phase under pressure, but the behaviors of vacancies under compression are the same with TiSe 2 . The same situation occurs in the case of TiS 2 . Although another 8-fold Immm phase instead of the C 2/ m phase is proposed for the second phase, the vacancy’s transition pattern from 2D, 1D, to 0D agrees with that of TiSe 2 , realizing the transfomation from a layered structure at ambient pressure to a spatial covalent network structure at high-pressure.…”

“…The same situation occurs in the case of TiS 2 . 24 Although another 8-fold Immm phase instead of the C2/m phase is proposed for the second phase, the vacancy's transition pattern from 2D, 1D, to 0D agrees with that of TiSe 2 , realizing the transfomation from a layered structure at ambient pressure to a spatial covalent network structure at high-pressure. Considering those TMDs MX 2 (M = Ti, Zr, Hf; X = S, Se, Te) with the same P-3m1 phase at ambient pressure, 44−46 they should also follow this phase transition pattern at high-pressure.…”

“…Finally, we should address that the unit cell size of the structure searching is a special key to determine the potential high-pressure phases. For example, within the unit cell containing the 1–4 formula, previous work proposed a Immm phase, instead of the C 2/ m phase, as the first high-pressure phase for TiS 2 . The present work constrains the searching unit cell within 6 formula, and thus more complex structures such as some possible CDW phases are excluded for our structure searching, due to the limited calculation sources but infinite possible phases.…”

“…18,19 As a basic thermodynamic variable, pressure would change the chemical bonding between the atoms, resulting in the occurrence of new structures with novel physical properties. For example, the ambient trigonal P-3m1 phase of TMDs undergoes a transformation to a monoclinic C2/m phase (TiTe 2 at 12 GPa, 20 TaS 2 at 20 GPa, 21 IrTe 2 at 5 GPa, 22 and VSe 2 at 12 GPa 23 ) and then to exhibit different phases at higher pressure such as the hexagonal Fe 2 P-type structure (TiTe 2 at 33 GPa 20 ) and the tetragonal I4/mmm phase (TaS 2 at 60 GPa, 21 TiS 2 at 52 GPa, 24 and HfSe 2 at 47.2 GPa 25 ). In spite of numerous studies on high-pressure phases of TMDs, a unified model to describe the structural transformations of TMDs is still absent, and thus, those high-pressure phases appear to be occasional and random.…”