"Bandgap closure and reopening in CsAuI3 at high pressure"

Shibing Wang: Alexander F. Kemper, Maria Baldini, M. C. Shapiro, Scott C. Riggs, Zhao Zhao, Zhenxian Liu, Thomas P. Devereaux, Ted H. Geballe, Ian R. Fisher, and Wendy L. Mao; Physical Review B, 06/09/14.

Additional Authors: Alexander F. Kemper, Maria Baldini, M. C. Shapiro, Scott C. Riggs, Zhao Zhao, Zhenxian Liu, Thomas P. Devereaux, Ted H. Geballe, Ian R. Fisher, and Wendy L. Mao

Abstract:

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound CsAuI3, where Au adopts AuI and AuIII valency. Raman spectroscopy shows softening with pressure of the vibration modes in the AuIII-I4 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.