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Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.[2]
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.[3]
Gold has the highest electronegativity of all metals due to relativistic effects.[4] It has been called a "pseudo-halogen".[4]
CsAu is obtained by heating a stoichiometric mixture of caesium and gold.[5] Non-stoichiometric mixtures result in an alloy rather than an ionic compound.[5] The two metallic-yellow liquids react to give a transparent yellow product.[6]
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.[4] In this way it splits the difference between typical ionic solids, which are insulators, and typical metallic solids, which are conductors.[5] This semiconductivity is a relativistic effect.[4]
The compound hydrolyzes readily on contact with water, yielding caesium hydroxide, metallic gold, and hydrogen.[6]
The solution in liquid ammonia is brown, and the ammonia adductCsAu·NH3 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.[6]
The solid is transparent and red.[4]
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 4.24 Å, close to that of CsCl but slightly larger due to the larger Au− ionic radius compared to Cl−. The bonding is predominantly ionic,[5] as found by X-ray photoelectron spectroscopy, because gold has a much higher electronegativity than caesium.