Cobalt Monarsenide, CoAs

Cobalt Arsenide or Cobalt Monarsenide, CoAs, is obtained when the previous compound is heated from 600-800° C., and when cobalt is heated to 275-335° C. in arsenic vapour. It is a grey crystalline powder, of density 7.62, which tarnishes slightly in the air.

A substance having the composition required for a Cobalt Sesqui-arsenide, Co₂As₃, is formed at 400-600° C., and this, at temperatures below 400° C., is slowly converted into Cobalt Di-arsenide, CoAs₂. This substance occurs in nature as the minerals smaltite and safflorite, the former being an important ore of cobalt. It may also be formed by heating cobalt in arsenic vapour from 385-405° C. Its density is 6.97. It is grey in colour, slightly oxidised by air, and decomposes above 400° C.