Copper Arsenides

Investigation of the freezing point curve of the system Cu-As confirms the existence of the two arsenides, Cu₃As and Cu₅As₂, and there is evidence of the formation of the arsenide Cu₂As. An arsenide of composition Cu₃As2 is also known. The addition of arsenic to copper lowers the freezing point uniformly up to 14 per cent. As, when a steep fall occurs, reaching a minimum at 685° C. and yielding a eutectic mixture with 19.2 per cent. As corresponding approximately with Cu₅As. As the proportion of arsenic is increased, the curve rises until, at 747° C., the first compound, Cu₃As, containing 28.4 per cent. As, crystallises out. This is soon followed by the crystallisation at 807° C. of a second compound, Cu₅As₂, with 32.2 per cent. As. The freezing temperature then falls to a minimum at about 695° C., the eutectic mixture containing approximately 35 per cent, of As. With more arsenic the curve rises to 740° C. and, according to Hiorns, the arsenide, Cu₂As, with 37.34 per cent. As, separates; Friedrich, however, could not obtain this compound. Beyond this point the curve begins to fall, but with more than 43 to 44 per cent. As, alloys could not be obtained by fusion, the excess of arsenic rapidly burning off.

Many arseniferous minerals contain arsenides of copper in a more or less pure state. They are found in various localities, but especially in Michigan and Chile. The more important are domeykite, Cu₃As, and the allied minerals argento-domeykite, (Cu, Ag)₃As, mohawkite, (Cu, Ni, Co)₃As, and stibiodomeykite, Cu₃(As, Sb); also algodonite, Cu₆As, whitneyite and darwinite, Cu₉As.

The influence of small quantities of arsenic on copper has already been described. The thermal conductivity of Cu-As alloys has been investigated, as also has the electrical behaviour at temperatures as low as 1.26° Abs., obtained by means of liquid helium; whether or not the alloys are supraconductive at these temperatures has not been definitely determined. The structure of various Cu-As alloys has been investigated by means of the X-rays.

Tri-copper Arsenide, Cu₃As, has been prepared by melting a mixture of copper and arsenic under a layer of fused boric acid; by subjecting an intimate mixture of the elements in the required proportions to a pressure of 6500 atm.; by heating copper in arsenic vapour; by heating a mixture of copper and the hemi-arsenide, Cu₂As; by the action of arsine on an aqueous solution of potassium cuprochloride, excess of Cu⁺ ion being avoided, otherwise precipitation of copper occurs; and by the action of hydrogen at high temperature and pressure on copper arsenate. By heating copper or a suitable alloy at about 600° C. in arsenic vapour Koenig obtained brilliant crystals corresponding in composition with the various forms of domeykite enumerated above.

The density of Cu₃As is 6.7 to 7.7; the density calculated from crystallographic data is 8.22. The hardness is 3.0 to 3.5 on Mohs' scale. The specific heat is 0.0919. On heating, sublimation occurs at 345° to 370° C. The arsenide decomposes on strong heating. It is completely reduced when heated in hydrogen. It is stable towards hydrochloric acid, but is attacked by nitric acid.

The arsenide Cu₅As₂ has been prepared by passing a current of carbon dioxide and arsenic vapour over finely divided copper heated to the temperature of boiling sulphur; by the action of copper on arsenic trichloride or on arsenic dissolved in hydrochloric acid; and by the action of cuprous chloride on arsenic. Lustrous regular crystals of density 7.56 are obtained. These tarnish on exposure to air. When heated it loses arsenic and yields Cu₃As, which at a higher temperature also decomposes. Cu₅As₂ dissolves in nitric acid. It is readily attacked by chlorine or bromine.

The hemi-arsenide, Cu₂As, was described by Descamps as being formed when the black precipitate, resulting from the addition of arsenic to a solution of a copper salt, was fused under borax; according to Koenig, it is formed, with Cu₃As, when arsenic vapour is passed over heated copper. Hiorns described it as being formed by fusing the elements together (see above), but Friedrich did not observe this to be the case. Koenig described it as a grey crystalline mass of density 7.71 at 21° C.

Tri-copper Di-arsenide, Cu₃As₂, is formed when arsine acts on a solution of copper sulphate, or on dry copper sulphate or chloride; by subjecting a suitable mixture of the powdered elements to a pressure of 6500 atm.; by reducing cupric arsenite with fused potassium cyanide; or by the action at 100° C. of a solution of arsenic trichloride in hydrochloric acid on copper. It is obtained as a brittle bluish-grey crystalline mass of density 6.94.