Properties of the Group VI B Elements Under Pressure. III. Phase-Diagram Studies of Various Forms of Sulfur

The phase diagram of sulfur has been investigated up to 60 kbar by differential thermal conductivity analysis in piston-cylinder and tetrahedral devices. The data indicate that the melting curve of orthorhombic sulfur rises to a triple point between orthorhombic, fibrous, and liquid sulfur at 37 kbar and 410°C. The solid-solid transformation appears to be irreversible, and the fibrous phase is identical to that produced at high pressure by Geller using annealing techniques. The apparent triple point can be changed to 29 kbar and 380°C by utilizing samples containing an initial nucleation of fibrous sulfur, indicating that the true equilibrium triple point is lower than that determined by ordinary "static" measurements. The pressure dependence of the melting points of fibrous and white (insoluble $\omega$) sulfur produced metastably by annealing at high pressure and temperature was also determined. Since melting is found to be reversible above the triple point it is postulated that there exists a transition from the high-viscosity polymerized liquid present between 0.8 and 37 kbar to another liquid form. A possible mechanism for explaining the melting-curve maxima of Group $\mathrm{VI}B$ elements is discussed.