Bandgap closure and reopening in CsAuI3 at high pressure

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound CsAuI3, where Au adopts Au-I and Au-III valency. Raman spectroscopy shows softening with pressure of the vibration modes in the Au-III-I-4 square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds CsAuCl3 and CsAuBr3. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of CsAuI3, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.