Silver Arsenides

A study of the system Ag-As gives no evidence of the existence of any definite compound of the two elements. When the mixtures were heated in sealed tubes it was observed that the addition of silver lowered the melting point of arsenic to a eutectic point at 540° C. and 25.1 atoms per cent, of arsenic. Solid solutions only formed with from 0 to 6 atoms per cent, of arsenic. Alloys between this composition and the eutectic undergo a transformation at 595° C. due to a reaction between the solid and liquid phases, forming a new solid solution with a limiting concentration of 10.5 atoms per cent.; a second transformation occurs at 374° C. over almost the whole range of composition and is due to a eutectoid decomposition of the solid solution. There is a marked tendency to undercooling, which may suppress these changes. The alloys obtained are grey and brittle.

The arsenide Ag₃As occurs in an impure form in Nature as huntilite. It may be prepared artificially by dropping an aqueous solution of silver nitrate or acetate into an atmosphere of arsine. Spring obtained products of composition Ag₆As and Ag₃As by compressing mixtures of the two elements in powder form at 6500 atmospheres. Descamps gently fused silver arsenate with potassium cyanide and obtained hard, brittle, white crystals of density 8.51 and of composition AgAs, which lost arsenic on heating. When fused under a layer of boric acid at a high temperature this substance was converted into Ag₃As, the density of which was 9.51. Products of composition AgAs₃ and AgAs₄ have also been described, but it is doubtful whether any arsenide other than Ag₃As exists as a chemical individual.

An X-ray study of the structure of Ag-As alloys has been made, and the results support those obtained by thermal examinations as stated above.