Antimony Arsenides

Equilibrium diagram of the system Sb-As

A crystalline arsenide of composition Sb₂As was described by Descamps, who heated a mixture of the elements under fused boric oxide. A thermal investigation of the system antimony-arsenic shows, however, that the two elements alloy in all proportions, forming a continuous series of solid solutions, no chemical compound being formed. The alloys freeze over a range of temperature and do not have a single sharp melting point; this is shown by the freezing point curves (fig.) obtained by Mansuri, who found that the alloy of lowest freezing point (605° C.) contained 13 per cent, of arsenic. Parravano and de Cesaris found the minimum on the liquidus curve to be at 612° C. corresponding with 17.5 per cent, of arsenic. Padoa examined the electrical conductivity of alloys at various temperatures and observed a pronounced maximum shown by an alloy of composition Sb₂As at -10° C. At high temperatures alloys of antimony and arsenic have a gel-like structure which is retained after rapid cooling.

Antimony and arsenic frequently occur associated in Nature, the most important mineral rich in arsenic being allemontite, the composition of which approximates to SbAs₃.

The ternary system Pb-Sb-As has been investigated by thermal and metallographic methods; the antimony and arsenic form a continuous series of solid solutions which are insoluble in the lead but form eutectics with it. For the alloys rich in arsenic the eutectic melts at 294° C., while for those rich in antimony the temperature is 252° C.