Pressure-induced electronic topological transition in Sb2S3.

We report the high-pressure vibrational properties and a pressure-induced electronic topological transition in the wide bandgap semiconductor Sb2S3 (E g = 1.7-1.8 eV) using Raman spectroscopy, resistivity and x-ray diffraction (XRD) studies. In this report, high-pressure Raman spectroscopy and resistivity studies of Sb2S3 have been carried out to 22 GPa and 11 GPa, respectively. We observed the softening of phonon modes [Formula: see text], [Formula: see text] and B 2g and a sharp anomaly in their line widths at 4 GPa. The resistivity studies corroborate this anomaly around similar pressures. The changes in resistivity as well as Raman line widths can be ascribed to the strong phonon-phonon coupling, indicating clear evidence of isostructural electronic topological transition in Sb2S3. The previously reported pressure dependence of a/c ratio plot obtained also showed a minimum at ~5 GPa consistent with our high-pressure Raman and resistivity results.

Pressure-induced structural changes and insulator-metal transition in layered bismuth triiodide, BiI3: a combined experimental and theoretical study.

Noting that BiI3 and the well-known topological insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, we determine here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. We report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure electrical resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. Interestingly, the metallic phase, at relatively low pressures, exhibits minimal resistivity at low temperatures, similar to that in Bi2Se3. We corroborate these findings with first-principles calculations and suggest that the drop in the resistivity of BiI3 in the 1-3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calculated Born effective charges reveal the presence of metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topology of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio below 2 GPa, are shown to give rise to changes in the slope of phonon frequencies near that pressure.

High pressure Raman spectroscopy of layered matlockite, PbFCl.

The pressure dependence of various inter- and intra-layer Raman modes has been studied on pristine matlockite compound, PbFCl, up to ~41 GPa. The low-frequency interlayer vibrational modes, A1g(1) and Eg(1), identified as rigid layer modes, exhibit non-monotonic behavior with increasing pressure. They exhibit points of inflexion at ~24 GPa and ~31 GPa respectively, indicating the onset of a subtle instability. The emergence of a new Raman mode (~181 cm(-1)) at ~24 GPa and a sudden large increase in the intensity of the A1g(1) mode signify the occurrence of a symmetry lowering structural transition of the parent tetragonal phase with enhanced interlayer coupling. Two more modes appear at higher pressures (~33 GPa) at frequencies below the A1g(1) mode and are ascribed to a monoclinically distorted phase (space group P21/m). High pressure x-ray diffraction studies performed up to ~47 GPa confirm the occurrence of the structural transitions with decreasing crystal symmetry. These observations are consistent with a picture in which the structural distortion involves destabilization of the tetragonal unit cell following a gradual change in the bonding nature from layer-like (2D) to non-layer like (3D) involving the Cl-bilayers along the c direction.