Alternating sequence of ring and chain structures in sulphur at high pressure and temperature.

Sulphur is the subject of a renewed intensive research effort aiming at characterizing its complex phase diagram. Nonetheless, on reading recent literature on its high-pressure phases, a confused picture emerges where different techniques obtain different results. We have investigated the phase diagram of sulphur by X-ray diffraction at high pressures and temperatures (6-11 GPa and 300-1,000 K), and have observed a new occurrence of a molecular S6 phase, in addition to the previously described polymeric helical trigonal and tetragonal phases. The stability regime of the S6 phase has been defined through the observation of coexistence points with each of the polymeric phases and the trigonal-S6-tetragonal triple point. The existence of these high-pressure phases allows us to define a framework that reconciles the apparently contradictory results existing in the literature. Moreover, the classic idea that pressure induces polymerization in sulphur is refined by the direct observation of an alternating sequence of molecular and polymeric phases that we report here.