Electronic structure of the gold complexes Cs 2 Au 2 X 6 ( X =I, Br, and Cl)

Electronic structures of halogen bridged mixed-valence (MV) complexes ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{X}_{6} (X$=I, Br, and Cl) have been investigated by means of reflectivity measurements. We have observed five polarization-dependent optical transitions in the energy range of 0.6\char21{}5.5 eV. The lower-lying three bands below \ensuremath{\sim}3 eV are due to the intermolecular transitions from Au${X}_{2}^{\ensuremath{-}}$ to Au${X}_{4}^{\ensuremath{-}}$ molecules, while the others are assigned to the intramolecular transitions. The optical gap ${E}_{\mathrm{gap}}$ increases from 1.31 eV for X=I to 2.04 eV for X=Cl, due to enhancement of the nearest-neighboring Coulomb repulsion V and the Jahn-Teller distortion of Au${X}_{4}^{\ensuremath{-}}(S)$. We further investigated external pressure effects on the electronic state, and found pressure-induced disappearance of the Raman-active Au-Br stretching modes, which indicates an electronic phase transition from the MV state to a single-valence (SV) state.