Copper, Silver and Gold Phosphides

The copper phosphides are crystalline compounds of metallic appearance and properties which are usually prepared by direct union of the elements. Phosphorus begins to combine with copper at about 400° C., and at 700° C. the copper was found to take up 20 per cent., some of which was expelled at higher temperatures. Slightly above the melting-point of the phosphide 14 per cent, was retained, which corresponds to tri-cuprous phosphide, Cu₃P. The velocity of the combination increases between 600° and 700° C. At ordinary pressures 15 per cent, of phosphorus is the limit of the amount which will remain dissolved in the fused mixture, and some of this is present as red phosphorus.

Cu₃P is a crystalline steel-grey or silvery-white substance which is harder than wrought iron. The melting-point is about 1018° C. and is lowered by additions of copper, as that of pure copper is by small additions of phosphorus; the Cu-Cu₃P freezing-point curves meet at a eutectic which corresponds to 8-2 per cent, of phosphorus with a freezing-point of 707° C.

The toughness and resistance to corrosion of the phosphor-bronzes are due to the presence of solid solutions of the phosphides in copper.

Cu₃P was also prepared by the action of phosphine on ammoniacal cuprous oxide and on the metal at about 200° C. and on cuprous chloride. Cu₅P₂ was made by the action of PF₃ or PCl₃ on copper at a red heat, also by the action of phosphine on cupric hydroxide or carbonate, and red phosphorus on cupric nitrate. When heated to a red heat it gave Cu₃P and Cu. Cu₃P₂ is said to be formed by the action of phosphine on cupric chloride. When yellow phosphorus was boiled with cupric sulphate and the precipitate washed and treated with acid dichromate the residue had this composition. The higher phosphides, of which CuP and CuP₂ have been reported, are powders of uncertain composition, easily oxidised by nitric acid or by heating in oxygen.

Other phosphides which have been prepared by direct combination, or by reaction in solution, are — Cu5P₂, Cu₂P, Cu₃P₂, CuP, CuP₂.

Silver phosphides, AgP, Ag₂P₃ and AgP₂, were said to be produced by heating the elements together, or by passing phosphorus vapour over heated silver. Molten silver absorbs phosphorus freely, but rejects most if not nearly all on solidification. Silver phosphides have also been prepared by other reactions, and it is noteworthy that the action of phosphine on silver nitrate gives a compound, Ag₃P.3AgNO₃, analogous to that which is first formed in the well-known test for arsine.

Gold, like silver, when in the molten state dissolves phosphorus and rejects it on solidification. A phosphide AuP has been prepared by passing a mixture of dry phosphine and ether vapour into an ether solution of gold chloride. The phosphorus is only loosely combined and is given off when the compound is warmed. Such phosphides behave like free phosphorus; they burn in the air, are oxidised by nitric acid, etc., and hydrolysed by water and alkalies.