Iron Phosphides

Cast-iron which has been made from phosphatic ores contains phosphides which seriously affect the properties of the metal. Various compounds rich in phosphorus have been prepared by heating iron in phosphorus vapour, or iron with phosphoric acid, bone-ash, sand and carbon (Pelletier, Wohler, loc. cit., Berzelius; also Hatchett). The freezing-point diagram of the system iron- phosphorus shows several maxima and minima. The melting-point of iron was lowered from 1510° C. to about 1400° C. by the addition of 1.7 per cent, of phosphorus, but this was not an end-point of crystallisation. The first eutectic was found at 1003° C. with 10.2 per cent, of phosphorus, the solid phases being Fe and Fe₃P. The maximum freezing-point corresponding to Fe₃P was about 1100° C. There was a halt-point of crystallisation, or another eutectic, between this compound and Fe₂P, which melted at 1350° C. Solid solutions of these compounds, which may be recognised microscopically, increase the hardness of pure iron from 3.5 to 5.0 or 5.5, but above about 1 per cent, of phosphorus render it brittle. Other phosphides which have been reported are FeP, Fe₃P₄ and Fe₂P₃. The lower phosphides FeP and Fe₂P retain their phosphorus up to a red heat. The former has also been prepared at a red heat by the action of phosphine, thus:—

2FeS + 2PH₃ = 2FeP + 2H₂S + H₂
FeCl₃ + PH₃ = FeP + 3HCl

Most of the phosphides are insoluble or only slowly soluble in single acids, but are attacked by aqua regia and chlorine. Fe₃P dissolves in hydrochloric acid, thus:—

2Fe₃P + 12HCl + 8H₂O = 6FeCl₂ + 2H₃PO₄ + 11H₂

When iron containing phosphides is dissolved in acids one of the products is phosphine, which is formed in greater proportion (i.e. more of the phosphorus is present as phosphine) as the amount of phosphorus diminishes, i.e. between 0.1 and 0.03 per cent.