Physical Insights on the Phonon Dispersion of TiS₂

Titanium disulfide is a quasi-2D transition-metal dichalcogenide relevant for various potential applications. To exploit its technological capabilities, it is important to determine its fundamental properties including the lattice dynamics. The TiS2 phonon dispersion curves available to date do not reproduce properly the experimental data for several reasons: i) all available experimental data are not taken into consideration, thus poor theory-experiment agreements have been obtained; ii) the (unknown) frequency of the infrared mode A(2u) is erroneously assumed; and iii) such incorrect assignment has propagated in the literature, particularly in phonon dispersion calculations. It is presented here a thorough density functional theory analysis to determine the phonon dispersion curves of TiS2, accounting for the frequencies of all experimental phonon data available to date. These include the frequencies of nine zone-edge Raman active modes of Ag-intercalated TiS2, in addition to infrared, Raman, and neutron scattering data. An incorrect frequency assignment of the A(2u) mode in the literature is thoroughly discussed. Moreover, results of attenuated total reflection terahertz spectroscopy applied to TiS2 are provided. A self-intercalation paradigm is presented to give a rationale for the temperature dependence of the poorly understood Raman features observed in pristine TiS2 at frequencies above the A(1g) mode.