Cobalt Disilicide, CoSi₂

Cobalt Disilicide, CoSi₂, results when cobalt is heated in the electric furnace with excess of silicon or with a mixture of copper silicide and silicon. It forms dark crystals, probably belonging to the cubic system; density 5.3; hardness 4.5. Sulphur has no action on it, and oxygen at 1200° C. only effects a superficial oxidation. It incandesces in fluorine if gently warmed; chlorine attacks it at 300° C., and bromine and iodine at dull red heat. Concentrated hydrogen fluoride readily attacks it, as also do the fused hydroxides of the alkali metals. Hot concentrated solutions of hydrogen chloride or of the alkali hydroxides attack it only slowly. It melts at 1277° C.

Two other silicides have been described, namely, CoSi₃ (m.pt. 1307° C.) and Co₃Si₂ (formed slowly at 1180-1215° C. in the solid state, from CoSi and Co₂Si).