Iron Arsenides

The system Fe-As has been investigated by Friedrich, alloys containing up to 56 per cent. As having been examined. The following compounds are indicated on the freezing point curve: Fe₂As, Fe₃As₂, FeAs, and possibly Fe₅As₄. Thus the curve falls from the freezing point of iron (1535° C.) to a eutectic point at 30 per cent. As and 835° C., and rises to a maximum at 40.1 per cent. (Fe₂As) and 919° C. Less distinct maxima occur at 51.7 per cent. As (i.e. Fe₅As₄) and 964° C., and at 57.3 per cent. As (i.e. FeAs) and 1031° C., the latter point being obtained by extrapolation of the curve of solidification times. A reaction between the solid products occurs at 800° C. in all alloys containing 40 to 56 per cent. As, the maximum development of heat taking place with 47.2 per cent., corresponding with the formation of Fe₃As₂.

The above conclusions were confirmed by micrographic examination of the alloys, which were etched by means of a hot solution of iodine in potassium iodide. Alloys containing more than 40 per cent. As were non-magnetic. Those formed in the neighbourhood of a maximum were brittle. A study of the E.M.F. diagram of Fe-As alloys containing 6 to 56 per cent. As indicated the formation of Fe₂As and Fe₅As₄. The effect of adding small quantities of arsenic (up to 8 per cent.) to iron was examined by Oberhoffer and Gallaschik, who observed that on cooling the change point of the δ mixed crystals with liquid to y mixed crystals (which they recorded as 1440° C.) was depressed 80° by the presence of 0.5 per cent. As and remained constant with further addition. The change point on heating was not affected. With more than 3 per cent. As no change point could be detected. The maximum solubility of arsenic in δ-iron is 0.9 per cent, and in γ-iron 6.8 per cent. Micro-examination confirmed the thermal data and revealed homogeneous mixed crystals up to 6.67 per cent. As. The alloy with 7.29 per cent. As showed traces of eutectic.

A comprehensive X-ray investigation has been made of Fe-As alloys containing up to 56.9 per cent. As, the highest content obtainable. The alloys were prepared by dropping pellets of arsenic into molten pure iron contained in a magnesia crucible in an atmosphere of nitrogen. The displacement of Fe lines indicated that α-iron will hold approximately 5 per cent, of arsenic in solution at room temperature. With increasing arsenic the first compound indicated, Fe₂As, has a simple tetragonal lattice with a = 3.627 A. and c = 5.973 A., two molecules forming the unit cell. The As atom is surrounded by 4Fe at 2.40 A., 4Fe at 2.60 A. and 1Fe at 2.41 A. The Fe-As distances are less than those calculated from the normal atomic radii. The arsenide Fe₃As₂ could not be found in slowly cooled alloys, but quenched alloys of the proper composition, when examined microscopically, presented an appearance suggesting high-temperature stability (above 795° C.) for Fe₃As₂, the compound breaking down at lower temperatures into Fe₂As and FeAs. The arsenide FeAs has a simple orthorhombic lattice with a = 3.366 A., b = 6.016 A. and c = 5.428 A., each unit cell containing four molecules. The lattice structure resembles that of the corresponding cobalt arsenide. The crystal structure of various minerals containing iron arsenide, for example, lollingite, FeAs₂, safflorite (Co, Fe)As₂, rammelsbergite (Ni, Co, Fe)As₂, and certain arseno-sulphides, including arsenopyrite, has been investigated. The conclusions as regards lollingite are not in agreement, and further study is desirable. Buerger gives the following structure: space group, V_h¹²; two molecules in unit cell, with dimensions a = 2.85, b = 5.25 and c = 5.92 A.; the effective As radius 1.23 A.; Fe-As = 2.35 and Fe radius 1.12 A., as in marcasite, with which mineral lollingite is isomorphous.

A metallographic and analytical examination of the ternary system Ni-Fe-As shows the formation of the crystalline double arsenides 2Fe₂As.Ni₅As₂ and 4Fe₂As.Ni₅As₂.

Iron Subarsenide, Fe₂As, is formed by melting a mixture of the two elements in the requisite proportions. It melts at 919° C. A product of the same composition is obtained when a mixture of borax and arsenopyrite is heated in a carbon crucible and the product digested with hydrochloric acid.

The conditions under which the formation of the arsenides Fe₃As₂ and Fe₅As₄ may occur. The former was obtained by Brukl as a black precipitate by the action of arsine on an alcoholic solution of ferrous ammonium sulphate. The product was only slightly attacked by concentrated hydrochloric or sulphuric acid but was soluble in nitric acid, aqua regia and bromine water.

Iron Monarsenide, FeAs, may be obtained by heating iron in a current of arsenic vapour at 835° to 380° C.; by heating a mixture of the elements in a bomb tube at 680° C., or a mixture of iron, arsenious oxide and carbon in an electric arc furnace; by the action of fused potassium cyanide on iron arsenate; by reduction of the di-arsenide at 680° C. in a current of hydrogen; or by dropping a solution of a ferrous salt into an atmosphere of arsine.

It forms silver-white, rhombic crystals, of density 7.83, and melting point 1020° C. according to Hilpert and Dieckmann or 1031° C. according to Friedrich. It is non-magnetic. Steel-grey crystals of the arsenide of density 7.94 have been found associated with tin sulphide in the hearth of an old tin smelting furnace in Cornwall.

The product, of density 7.22, which results when iron is heated in arsenic vapour at 395° to 415° C. agrees in composition with the formula Fe₂As₃. The existence of such an arsenide has not been confirmed, however, although some forms of lollingite approach this composition.

Iron Di-arsenide, FeAs₂, occurs as the minerals lollingite and arsenoferrite, and may be made artificially by heating iron in arsenic vapour at 480° to 618° C., or by heating a finely powdered mixture of the elements in a bomb tube at 700° to 750° C. After treatment with dilute hydrochloric acid the pure di-arsenide is obtained as a silver-grey microcrystalline powder of density 7.38. It melts at 980° to 1040° C. It is insoluble in hydrochloric acid, whether dilute or concentrated, but is slowly oxidised by nitric acid, yielding arsenic acid. Heated with concentrated sulphuric acid, sulphur dioxide is evolved. When heated in air it burns, yielding arsenious oxide and ferric oxide. It is non-magnetic.

When the mineral lollingite is heated in vacuo it loses 23.8 per cent, of arsenic, the residue containing two unidentified constituents. Arsenopyrite, treated similarly, decomposes at 675° to 685° C.