Magnesium Arsenide, Mg₃As₂

Magnesium Arsenide, Mg₃As₂, may be prepared by heating magnesium powder with a slight excess of arsenic in the absence of air; it is obtained as a grey, metallic, microcrystalline powder, of density 3.165 and melting point 800° C. It has also been obtained by heating arsenic and magnesium filings in hydrogen - this product was described as a brittle, chocolate-brown mass with a metallic lustre; by heating a mixture of arsenic, magnesium and sand; and by igniting the mixed elements - the reaction in this case being very violent.

Magnesium arsenide decomposes rapidly on exposure to air. It reacts with water and acids to yield arsine; with ethyl alcohol at the boiling point it also gives arsine, and at 280° to 295° C. ethyl- and diethyl-arsines, with a trace of triethylarsine, are formed. The crystal structure of the arsenide has been investigated by the X-ray powder method. It resembles the corresponding zinc arsenide and has a cubic lattice containing two molecules of Mg₃As₂ in the unit cell, the side of which is 6.10 A., the calculated density being 3.26. The arrangement of the atoms in the lattice represents an unusual type of structure which is defined by the co-ordinates As = (¼, ¼, ¼), (¾, ¾, ¾), (¼, ¾, ¾), (¾, ¼, ¾); and Mg = (½, 0, 0), (0, ½, 0), (0, 0, ½), (0, ½, ½), (½, 0, ½), (½, ½, 0). The distance Mg-Mg is 3.05 A., that of As-Mg, 2.64 A., while that of As-As is 4.31 A. From these data the radius of the Mg atom is calculated to be 1.525 A., and the atomic diameter of As 1.02 A.; these values correspond roughly to the values for the neutral atoms (1.62 and 1.16 A., respectively) calculated by Goldschmidt. The structure of magnesium arsenide is not ionic.