Pressure-induced anomalous enhancement of insulating state and isosymmetric structural transition in quasi-one-dimensional TiS3

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal TiS3 up to 29.9-39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive behavior of semiconductor TiS3 becomes increasingly insulating with pressure until P-C1 similar to 12 GPa, where extremes in all three axial ratios are observed. Upon further compression to P-C2 similar to 22 GPa, the XRD data evidence a structural phase transition. Based on our theoretical calculations, this structural transition is determined to be isosymmetric, i.e., without change of the structural symmetry (P2(1)/m), mainly resulting from rearrangement of the dangling S-2 pair along the a axis.