Electronic structure of the gold complexes ${\mathrm{Cs}}_2{\mathrm{Au}}_2{X}_6\hspace{0.5em}(X$=I, Br, and Cl)

Electronic structures of halogen bridged mixed-valence (MV) complexes ${\mathrm{Cs}}_2{\mathrm{Au}}_2{X}_6\hspace{0.5em}(X$=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 0.6–5.5 eV. The lower-lying three bands below ∼3 eV are due to the intermolecular transitions from Au$X_2^{-}$ to Au$X_4^{-}$ 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$X_4^{-}\mathrm{}\left(S\right)\mathrm{}$. 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.