Pressure-induced anomalous enhancement of insulating state and isosymmetric structural transition in quasi-one-dimensional $\mathrm{Ti}{\mathrm{S}}_3$

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal $\mathrm{Ti}{\mathrm{S}}_3$ up to 29.9–39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive behavior of semiconductor $\mathrm{Ti}{\mathrm{S}}_3$ becomes increasingly insulating with pressure until $P_{C1}\sim 12\mathrm{GPa}$, where extremes in all three axial ratios are observed. Upon further compression to $P_{C2}\sim 22\mathrm{GPa}$, 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 ($P{2}_1/m$), mainly resulting from rearrangement of the dangling ${\mathrm{S}}_2$ pair along the $a$ axis.

Figure 1

(a) XRD pattern of $\mathrm{Ti}{\mathrm{S}}_3$ single crystal. The upper right panel shows the photo of $\mathrm{Ti}{\mathrm{S}}_3$ single crystal. (b) Energy-dispersive x-ray spectroscopy of $\mathrm{Ti}{\mathrm{S}}_3$ single crystal.

Figure 2

Resistance (R) versus temperature (T) curves in the pressure range of 1.5–12.5 GPa (a), 15.3–22.7 GPa (b), and 25.4–39.0 GPa (c). The current (I) is introduced along the $b$ axis of the single crystal. Note that upon compression the resistance rises increasingly at first until 12.5 GPa, then it decreases rapidly but still with insulating behavior (dR/dT < 0). And finally above 25.4 GPa, a metallic behavior [dR/dT > 0, inset of (c)] emerges and grows more and more evident up to 39.0 GPa, the highest pressure studied here.

Figure 3

(a) Synchrotron x-ray diffraction patterns of $\mathrm{Ti}{\mathrm{S}}_3$ with pressures up to 29.9 GPa ($\lambda =0.4246\AA$). Some new peaks at 22.4 GPa indicated by arrows evidence a structural transition. Standard Rietveld refinements with Le Bail method were used to fit the experimental data. Selected fitting results (red lines) at 0.5 GPa (Phase I) and 25.8 GPa (Phase II) are included. The XRD pattern upon decompression back to 0.2 GPa (denoted by D) is almost the same as the initial one at 0.5 GPa, manifesting a reversible structural transition. (b) Pressure dependence of the lattice parameters. (c) The unit-cell volume versus pressure was fitted by the third-order Birch-Murnaghan formula (solid lines, see text).

Figure 4

(a) Schematic representations of the ambient structure (Phase I) and the high-pressure structure (Phase II) of $\mathrm{Ti}{\mathrm{S}}_3$ along $b$ axis. (b) Calculated enthalpy difference between these two phases as a function of pressure. The inset shows the transition barrier from Phase I to Phase II at 15 GPa (black) and 20 GPa (red). (c) Unit-cell volume of these two optimized structures vs pressure. Theoretical calculations indicate an isosymmetric structural phase transition occurring in $\mathrm{Ti}{\mathrm{S}}_3$ at about 16 GPa, from ambient-pressure Phase I to high-pressure Phase II.

Figure 5

Calculated phonon-dispersion relations of Phase II at (a) 0 GPa and (b) 20 GPa.

Figure 6

Resistance of single-crystal $\mathrm{Ti}{\mathrm{S}}_3$ under P-T conditions and its relations to the axial ratios of $c$/$a$, $c$/$b$, and $a$/$b$. It is readily drawn that there are two critical pressure points when increasing the pressure: the first one is $P_{C1}\sim 12\mathrm{GPa}$, where the insulating behavior of semiconductor $\mathrm{Ti}{\mathrm{S}}_3$ reaches a maximum and all of the axial ratios show extreme points consistently; the second one is $P_{C2}\sim 22\mathrm{GPa}$, where metallization emerges accompanied by discontinuities of the axial ratios due to a structural transformation.