Iron mon-arsenide, FeAs

Iron mon-arsenide, FeAs, is obtained by heating iron in excess of arsenic in the absence of air. This may be effected by heating iron in a current of arsenic vapour at 335° to 385° C., or with arsenic in a bomb tube at 680° C. It also results when the di-arsenide, FeAs₂, is reduced at 680° C. in a current of hydrogen.

As obtained by these methods, iron mon-arsenide is a silver-white crystalline substance, of density 7.83, and melting at 1020° C. It is non-magnetic.

Iron mon-arsenide has been found as dark, steel-grey rhombic crystals, of density 7.94, in the hearth of an old furnace in Cornwall.