Strange isn’t the word for cesium auride. Gold in liquid form? Indeed, yes! Discovered more than a half century ago, the properties of this cesium and gold compound are not yet fully understood. There are all sorts of curios in chemistry. Chemists love them dearly. I still remember one particularly strange molecule. It was affectionately named hexamethyl bathroom tile.
Recently I listened to some free online chemistry lectures. They were produced by MIT under its OpenCourseWareprogram. One of those lectures spoke of an alloy of cesium and gold that acts like a salt, cesium auride. Key to its forming is the difference in electronegativity between cesium and gold. It is roughly the same as the difference between sodium and iodine!
An atmospherically protected melt of cesium and gold on approaching stoichiometric amounts (50:50) quickly changes by electron transfer from the electropositive cesium atoms to the gold atoms, forming Cs+ and Au⁻ ions. The ions indicate it is indeed an ionic substance. But what is so special is that the gold does not take a positive ionic form, but a negative one. As a result, the melt changes from metallic yellow to clear yellow, solidifying into an orange crystalline sold.
Cesium auride was first discovered in 1943. It has a cesium chloride type structure. It melts at 863 degrees C. The compound’s electrical properties lie between those of a metal and those of an ionic solid. Cesium auride is thus a semiconductor. It is useful in the formation of specialty electronic devices.
The ionicity of cesium auride was confirmed from an analysis of the relativistic Hartree-Fock wavefunction. The catch is the compound reacts vigorously with water to form gold metal and cesium hydroxide. So moisture must be carefully avoided. A rubidium counterpart, RbAu, has also been described.
Cesium Auride Derivatives Undergoing Study
Cesium auride has been reacted in liquid ammonia solvent with a high cesium affinity macroreticular ion exchange resin producing tetramethylammonium auride, (CH3)4NAu.
Yet another modification derivative is cesium auride oxide, Cs₃AuO.
Other cesium and gold molecular combinations more recently discovered are undergoing study.