Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.
When molten, it is one of the very few liquids other than typical salts (composed of a metal and a nonmetal) in which mobile ions drive conductivity.
Gold has the highest electronegativity of all metals due to relativistic effects. It has been called a "pseudo-halogen". __TOC__
Preparation and reactions CsAu is obtained by heating a stoichiometric mixture of caesium and gold. Non-stoichiometric mixtures result in an alloy rather than an ionic compound. The two metallic-yellow liquids react to give a transparent yellow product.
Despite being a compound of two metals, CsAu lacks metallic properties since it is a salt with localized charges; it instead behaves as a semiconductor with band gap 2.6 eV. In this way it splits the difference between typical ionic solids, which are insulators, and typical metallic solids, which are conductors. This semiconductivity is a relativistic effect.
The compound hydrolyzes readily on contact with water, yielding caesium hydroxide, metallic gold, and hydrogen.
2 CsAu + 2 H2O → 2 CsOH + 2 Au + H2
The solution in liquid ammonia is brown, and the ammonia adduct is blue; the latter has ammonia molecules intercalated between layers of the CsAu crystal parallel to the (110) plane. Solutions undergo metathesis with tetramethylammonium loaded ion exchange resin to give tetramethylammonium auride.
The solid is transparent and red.
Crystal structure Caesium auride has a cubic lattice structure of the CsCl type. Each caesium atom is octahedrally coordinated with 8 gold atoms, and vice versa. The lattice constant at ambient conditions is approximately , close to that of CsCl but slightly larger due to the larger ionic radius compared to . The bonding is predominantly ionic, as found by X-ray photoelectron spectroscopy, because gold has a much higher electronegativity than caesium.
References
Further reading —includes photograph of the compound.