Zinc Arsenides

Arsenic readily alloys with zinc, and arsenides of composition Zn₃As, Zn₂As, Zn₃As₂, ZnAs and ZnAs₂ have been described. A thermal study of the system Zn-As has shown, however, that only Zn₃As₂ and ZnAs₂ exist. Both are brittle and they melt at 1015° and 771° C., respectively. Trizinc diarsenide may be prepared by melting zinc in arsenic vapour and hydrogen, by subjecting zinc and arsenic in appropriate proportions to a pressure of 6500 atmospheres, or by heating the mixture of elements at 800° C. out of contact with air. A compact but fragile crystalline mass, of density 5.578, is obtained. The crystal structure has been investigated by the X-ray powder method, and is found to resemble that of magnesium arsenide. According to Natta and Passerini, it has a cubic lattice containing two molecules of Zn₃As₂ in the elementary cell, with a = 5.81 A. and a calculated density of 5.854. The arrangement of the atoms in the lattice is the same as in the case of the magnesium compound. The distance

Zn-Zn is 2.90 A., that of Zn-As 2.52 A. and of As-As 4.11 A. The atomic radius of zinc is calculated to be 1.45 A. and the atomic diameter of arsenic 1.19 A., values which correspond roughly with those for the neutral atoms, it being concluded that the structure of the arsenide is non-ionic, a conclusion which is confirmed by the value of the heat of formation, which is 30.3 calories per mole. This is low in comparison with that of zinc blende which, though containing only one atom of zinc in the molecule, has a heat of formation of 43 calories per mole. According to von Stackelberg and Paulus, the arsenide crystallises in the tetragonal system, with eight molecules in the unit cell, and space group D_4h¹⁵.

The hardness of Zn₃As₂ is approximately 3 (Mohs' scale). At 672° C. it undergoes a reversible transformation, α-Zn₃As₂ ⇔ β-Zn₃As₂. With dilute sulphuric acid arsine is evolved.

Zinc Di-arsenide, ZnAs₂, is prepared by heating zinc and arsenic together in suitable proportions. It is grey in colour and of hardness approximately 3. It dissolves in acids with evolution of arsine.