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 197Au compounds like AuO or AuCl2 are
in fact Au+-Au+3 mixed valence species. The best examples of that
group are Cs2Au2X6 (X=Cl, Br, I) (Wells salts).
Projection of the structure
of Cs2Au2X6
The crystal structure of Cs2Au2X6
may be considered as distorted
perovskite structures composed
of corner sharing compressed
Au+X6 and elongated Au+3X6
octahedra
With increasing pressure the Cs2Au2X6
structures change generally to a less
distorted perovskite type. In the case of
Cs2Au2I6 X-ray studies suggest an abrupt
structural phase transition at 5.5 GPa to the
tetragonal CsAuI3 phase (at room
temperature).
The 197Au Mössbauer data show that at 4.2 K the structural transformation of
Cs2Au2I6 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 I6 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 197Au Mössbauer
spectroscopy.