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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 D4h15. 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.

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