Iron sesqui-phosphide, Fe₂P₃

Iron sesqui-phosphide, Fe₂P₃, may be prepared by heating ferric chloride to dull redness in phosphorus vapour, or by the action of phosphorus iodide upon reduced iron. It crystallises in brilliant grey needles, is not magnetic, and is insoluble in hydrochloric and in nitric acid, as also in aqua regia. It loses phosphorus when heated in air at bright red heat, and is slowly attacked by chlorine at dull red heat.

In addition to the foregoing, several other phosphides have been described, but the evidence in favour of these being separate chemical entities is not so convincing. Rose believed that he had prepared triferro tetraphosphide, Fe₃P₄, by the action of hydrogen phosphide upon pyrites: -

4PH₃ + 3FeS₂ = Fe₃P₄ + 6H₂S.

The product is bluish grey in appearance, permanent in air, but, when heated to a high temperature, it oxidises to phosphorus pentoxide and an iron phosphate, the former being evolved as fumes. Density 5.04. It is not magnetic.

By reduction of iron phosphate in hydrogen, or by passing vapours of phosphorus trichloride, tribromide, or tri-iodide over reduced iron at red heat, Granger obtained grey prismatic crystals of composition Fe₄P₃.Fe₅P₂ has also been described as forming glistening needles, almost insoluble in concentrated nitric and in dilute sulphuric acid, but readily soluble in aqua regia.