Infrared spectroscopic measurements of structural transition and charge dynamics in 1T−TiTe2 under pressure

Layered semimetallic $1\mathit{T}\ensuremath{-}{\mathrm{TiTe}}_{2}$ has been reported to exhibit pressure-induced superconductivity, and some nontrivial topological states have also been suggested to appear under pressure. Here we investigate the optical properties of this compound by infrared reflectance measurements over a broad energy range under quasihydrostatic pressures up to 22 GPa. The observed conspicuous changes of infrared phonon spectra, in combination with x-ray diffraction data, provide clear evidence of an irreversible structural transition into a noncentrosymmetric space group ($P\overline{3}m1\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}P3m1$) at a low pressure ($\ensuremath{\sim}2.3$ GPa), before undergoing a subsequent transition to a monoclinic phase ($C2/m$) above 12 GPa. The pressure-induced irreversible gain of the low-frequency spectral weight of optical conductivity further supports the above crystal structure modification. The irreversible structural modification and associated change in electronic structure have been argued to be responsible for the emergence of superconductivity with enhanced ${T}_{c}$ and its persistence upon decompression. The low-pressure structural instability in $1\mathit{T}\text{\ensuremath{-}}{\mathrm{TiTe}}_{2}$ makes it necessary to revisit the theoretical investigation of its topological nature.