Structural, vibrational, and electrical properties of 1T−TiTe2 under hydrostatic pressure: Experiments and theory

Abstract

We report the structural, vibrational and electrical transport properties up to 16 GPa of the 1T-TiTe2, a prominent layered 2D system, which is predicted to show a series of topologically trivial - nontrivial transitions under hydrostatic compression. We clearly show signatures of two iso-structural transition at 2 GPa and 4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes at around the same pressures, providing strong indication of unusual electron-phonon coupling associated to these transitions. Resistivity presents nonlinear behavior over similar pressure ranges providing a strong indication of the electronic origin of these pressure driven isostructural transitions. Our data thus provide clear evidences of topological changes at A and L point of the Brillouin zone predicted to be present in the compressed 1T-TiTe2. Between 4 GPa and 8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi 3D crystal network. First principles calculations suggest that the 2D to quasi 3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition to the pressure dependent isostructural phase transitions, our data also evidences the occurrence of a first order structural phase transition from the trigonal (P-3m1) phase at higher pressures. We estimate the start of this structural phase transition to be 8 GPa and the symmetric of the new high-pressure phase to be monoclinic (C2/m).