ANIONIC GOLD IN CS3AUO AND RB3AUO ESTABLISHED BY X-RAY-ABSORPTION SPECTROSCOPY

The novel ternary oxides Cs3AuO and Rb3AuO contain auride anions according to the ionic description (M(+))(3)Au-O2- (M = Cs, Rb). They exhibit various properties that signal the non-metallic features of gold, e.g. in the case of Cs3AuO the color and the transparency of the crystals and the semiconducting properties and for both compounds the decreased molar volumes. The anionic character of gold in these two compounds is confirmed by the X-ray absorption near edge structure (XANES) of the Au L(I) and L(III) spectra, using Au2O3, AuCl3, AuCN, AuCl, Au, and CsAu as reference compounds representing gold in various definite oxidation states. Comparison of the L(I) XANES spectra proves the anionic character of gold in the ternary oxides Cs3AuO and Rb3AuO because of the energy position of their absorption edges. In agreement to the empirically established law, the absorption edge shifts to higher energy with increasing valency of the absorbing atom.(1) This shift is not directly observable in the Au L(III) XANES spectra since a prominent peak is superposed onto the actual absorption edge for the Au(0), Au(1), and Au(3) compounds. Nevertheless, the expected shift to higher energy becomes apparent if the Au L(III) XANES spectra are deconvoluted. Thus the L(III) spectra, as well as the L(I) XANES spectra, are suitable for determining the valency if a deconvolution is carried out. The L(III) XANES spectra also confirm the presence of anionic gold in Cs3AuO and Rb3AuO. Finally, the areas of the pre-edge peaks in the L(III) XANES spectra give qualitative support to the interpretation given.