Valence instabilities of gold in perovskite structures The relativistic effects in gold stabilizes enormously the 6s level and destabilizes the 5d levels. As a result a variety of Au valencies from -1 to +5 are possible. Among these states the Au +2 is a special case. It is possible as an average over some gold complexes exhibiting Au-Au bonds, The formally 197 Au compounds like AuO or AuCl 2 are in fact Au + -Au +3 mixed valence species. The best examples of that group are Cs 2 Au 2 X 6 (X=Cl, Br, I) (Wells salts). Projection of the structure of Cs 2 Au 2 X 6

The crystal structure of Cs 2 Au 2 X 6 may be considered as distorted perovskite structures composed of corner sharing compressed Au + X 6 and elongated Au +3 X 6 octahedra With increasing pressure the Cs 2 Au 2 X 6 structures change generally to a less distorted perovskite type. In the case of Cs 2 Au 2 I 6 X-ray studies suggest an abrupt structural phase transition at 5.5 GPa to the tetragonal CsAuI 3 phase (at room temperature).

The 197 Au Mössbauer data show that at 4.2 K the structural transformation of Cs 2 Au 2 I 6 is gradual. At 12.5 GPa (at 4 K) Au appears in only one electronic state, Au +2. The small quadrupole splitting is an evidence of the distortion of the I 6 octahedra [ S.S. Hafner, N. Kojima, J. Stanek, Li Zhang. Phys. Lett. A 192(1994)385]

Conclusions The relativistic stabilization of gold 6s level and the destabilization of the 5d levels may explain not only the known unusual macroscopic properties of the metallic gold as yellow color, high density and chemical stability. This modification of the atomic level scheme leads to the interesting local electronic states of this element in compounds, which may be spectacularly investigated by 197 Au Mössbauer spectroscopy.