OPTICAL INVESTIGATION OF THE INTERVALENCE CHARGE-TRANSFER INTERACTIONS IN THE 3-DIMENSIONAL GOLD MIXED-VALENCE COMPOUNDS CS(2)AU(2)X(6) (X=CL, BR OR I)

The intervalence charge-transfer (i.v.c.t.) transitions between the gold-(I) and -(III) ions in Cs(2)Au(2)X(6) (X = Cl Br or I) have been investigated. From the polarized reflectance spectra of a single crystal of Cs2Au2I6 and group-theory analysis; the i.v.c.t. bands have been assigned. The strongest reflectance peak was that of lowest energy, which was observed with the electric vector perpendicular to the c axis, and was assigned to the Au-I(5d(x2-y2))-->Au-III(5d(x2-y2)) transition in the ab plane. This indicates that a two-dimensional correlation between the gold ions is present in the charge-transfer interaction despite the three-dimensional perovskite structure of the compounds.