Phase diagrams of arsenic, antimony, and bismuth at pressures up to 70 kbars

Abstract

The meltings under pressure of arsenic, antimony, and bismuth have been determined up to nearly 70 kbar by means of differential thermal analysis (DTA). Arsenic initially melts with a positive dT/dP, the fusion curve flattening out near 940°C at the highest pressures attained. The fusion curve of antimony exhibits practically no change with pressure initially, but takes on a slightly negative slope at higher pressures until a triple point is encountered near 57 kbar and 567°C; the high-pressure polymorph melts with a positive slope of 4.3°C/kbar. For bismuth, a triple point between the liquid and polymorphs IV and V occurs near 38.0 kbar and 296°C, in addition to the well-studied liquid-I-II and liquid-II-IV triple points at lower pressures. DTA was also used to determine the solid-solid phase boundaries in bismuth, triple points occurring between II, III, and IV near 24.0 kbar and 180°C and between III, IV, and V near 53 kbar and 174°C. The III-IV transition occurs at different temperatures upon heating and cooling and this hysteresis interval increases with pressure. No hystereses were observed for the IV-V and III-V transitions. At room temperature and at pressures beyond the well-known I-II and II-III transitions, the III-V transition is next encountered near ~78-82 kbar (as estimated by extrapolation). Suggestions are made for the structures of the high-pressure Bi polymorphs and analogies are drawn between Bi, Sb, and As for the melting and distortion under pressure of the normal rhombohedral structure.