Cadmium Arsenides

The freezing point, density and atomic volume curves indicate the existence of two arsenides, Cd₃As₂ and CdAs₂. The freezing point curve was obtained by cooling mixtures of the elements melted under fused alkali chloride; no mixed crystals were observed. Two other arsenides have been described: Cd₆As, said to be formed when cadmium and arsenic were melted together, and Cd₃As, obtained by reducing cadmium arsenate with fused potassium cyanide, the product being white, with a slight reddish tinge and a metallic lustre, and of density 6.26. The reddish tinge is common to alloys containing 22 to 50 atoms As per cent.; those with more arsenic exhibit a bluish tinge.

Cd₃As₂ has been prepared by subjecting a mixture of powdered cadmium and arsenic, in appropriate proportions, to a pressure of 6500 atmospheres, by passing arsenic vapour mixed with hydrogen or an inert gas over heated cadmium, by fusing a mixture of the elements under a layer of fused lithium and potassium chlorides, and by slowly dropping an ammoniacal solution of cadmium sulphate into a globe filled with arsine. It forms reddish octahedra and cubes of density 6.25 at 20° C. and of hardness nearly 3.5 (Mohs' scale). From an X-ray analysis by the powder method Passerini concluded that the crystals belong to the cubic system, with a non-ionic structure, but von Stackelberg and Paulus state that the arsenide crystallises in the tetragonal system, with 8 molecules in the unit cell, and space group D_4h¹⁵. In the co-ordinated lattice the arsenic atoms constitute a slightly deformed closest cube packing in the tetrahedral interstices of which are the cadmium atoms. According to Schemtschuschny, the arsenide undergoes some form of reversible transformation at 578° C. The melting point is 721° C. It dissolves slowly in cold dilute acids yielding arsine. Halogens and oxidising agents such as concentrated nitric acid or aqua-regia vigorously attack it, sometimes with incandescence.

CdAs₂, obtained by fusing the elements in the requisite proportions under fused alkali chloride, is bluish-grey in colour, of density 5.86 at 20° C., and of hardness 3.5 to 4.0 on Mohs' scale. It melts at 621° C.