Cobalt Arsenides

Cobalt alloys readily with arsenic and several compounds have been prepared. On the freezing point curve there is a eutectic point at 916° C. and 30 per cent. As, and maxima occur at 926° C., 959° C. and 1180° C., corresponding respectively with Co₅As₂ (33.7 per cent. As), Co₂As (38.9 per cent. As) and CoAs (about 55 per cent. As). The curve cannot be obtained for higher percentages of arsenic owing to volatilisation of the latter. The existence of a fourth arsenide, Co₃As₂, which dissociates without melting at 1014° C., is also indicated. Alloys containing from 12 to 46 per cent. As undergo a transformation on cooling, heat being evolved at temperatures ranging from 250° to 350° C., and a maximum heat development occurs at the composition Co₂As, when the transformation point is 352° C. The change is accompanied by a considerable increase in volume and the alloy is sometimes disrupted. The transformation temperatures for the definite arsenides Co₅As₂ and Co₃As₂ are respectively 828° and 915° C. Alloys containing up to 38 per cent, arsenic are magnetic; with more than this proportion they are non-magnetic. The addition of arsenic to cobalt causes increased hardness. Other arsenides of composition Co₂As₃, CoAs₂, Co₂As₅ and CoAs₃ have been described.

Tri-cobalt Di-arsenide, Co₃As₂, may be prepared by the action of arsenic trichloride on metallic cobalt at 800° to 1400° C.; or by heating mixtures of arsenic and cobalt in hydrogen or carbon monoxide at this temperature. It is also formed when powdered cobalt is heated in hydrogen containing arsenic vapour, and when cobalt arsenate or arsenite is reduced by hydrogen at 900° C. Its density is 7.82 at 0° C. When strongly heated it loses arsenic. It is only slowly attacked by fused alkali, or by hot concentrated hydrochloric or sulphuric acid. It dissolves readily in nitric acid or aqua regia, and reacts vigorously with oxygen, sulphur or chlorine.

Cobalt Monarsenide, CoAs, is obtained when the previous compound is heated at 600° to 800° C., when cobalt is heated to 275° to 335° C. in arsenic vapour, or when the elements in equal molecular proportions are heated for not less than a day at 730° C. It is a grey crystalline powder, of density 7.62 at 0° C., tarnishing slightly in the air. It melts at 1180° C. When strongly heated in hydrogen it loses arsenic. It resembles the arsenide Co₃As₂ in chemical properties. The lattice structure resembles that of the corresponding manganese and iron arsenides, the metal atoms forming zigzag chains, thus differing from nickel monarsenide, in which the metal atoms form straight rows.

Di-cobalt Tri-arsenide, Co₂As₃, is formed when cobalt monarsenide is heated at 400° to 600° C., or when cobalt is heated with arsenic trichloride at the same temperature. According to Beutell and Lorenz, it is formed when cobalt is heated in arsenic vapour at 345° to 365° C., but it slowly undergoes decomposition below 400° C. to form the di-arsenide, CoAs₂. It also decomposes when heated above 600° C. The density is 7.35 at 0° C.

Cobalt Di-arsenide, CoAs₂, is formed when cobalt is heated in arsenic vapour at 385° to 405° C. Its density is 6.97 at 0° C. It is grey in colour, slightly oxidised by air, and decomposes above 400° C. This arsenide occurs in two forms in Nature, smaltite (cubic), an important ore of cobalt, and safflorite (rhombic).

Cobalt Tri-arsenide, CoAs₃, is found in Nature as the mineral skutterudite, and is formed when cobalt is heated in arsenic vapour at 450° to 618° C.