Caesium auride

Caesium auride is the inorganic compound with the formula CsAu. It is the Cs⁺ salt of the unusual Au⁻ anion.

Preparation and reactions

CsAu is obtained by heating a stoichiometric mixture of caesium and gold. 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.
The compound hydrolyzes readily, yielding caesium hydroxide, metallic gold, and hydrogen.
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 plane. Solutions undergo metathesis with tetramethylammonium loaded ion exchange resin to give tetramethylammonium auride.

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.