Pressure-induced topological crystalline insulating phase in TlBiSe2 : Experiments and theory

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We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_{2}$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_{2}$ show a systematic increase in frequencies under hydrostatic pressure up to $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, the linewidth of the ${A}_{1g}, N$, and ${E}_{g}$ phonon modes show clear anomalies at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\ensuremath{\lambda}$, anomalies in the Raman linewidth of ${E}_{g}$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number ${n}_{M}$ and ${\mathbb{Z}}_{2}$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number ${n}_{M}$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_{2}$ at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_{2}$ ternary chalcogenide $\mathrm{TlBi}{X}_{2}$ ($X=\mathrm{S}$, Se, Te) family.