Properties of Group VI B Elements Under Pressure. I. Melting Curves of S, Se, and Te

The melting curves of S, Se, and Te have been determined to 45 kbar by differential thermal-conductivity analysis in a tetrahedral anvil apparatus. The data for sulfur show that the melting point increases approximately linearly from 114°C at atmospheric pressure to about 700°C at 45 kbar. Rhombic sulfur melted under pressure is found to crystallize upon solidification in the amorphous plastic form, which can be retained metastably at atmospheric pressure. The selenium melting curve is slightly concave toward the pressure axis and rises from 220°C at atmospheric pressure to 650°C at 40 kbar. A maximum in the melting curve of tellurium is observed at about 10 kbar and 480°C. These experimental results are discussed in relation to the current ideas on the general nature of melting curves. The maximum in the melting curve of tellurium is explained in terms of coordination in the liquid state and the existence of a liquid-state structural change of a statistical nature, which appears to divide the liquid into a metallic and a semiconducting phase. It is predicted that selenium, and, perhaps, sulfur also have maxima in their melting curves. The normalized change in resistivity upon melting ($\frac{\Delta \rho }{\rho }$) is correlated with the atmospheric-pressure melting-curve slopes $m_0$ for a number of elements. A plot of $m_0$ versus ($\frac{\Delta \rho }{\rho }$) shows that the data for the elements considered, except the alkali metals and the Group VI B elements, very roughly define a linear relationship. The sign of $m_0$ and ($\frac{\Delta \rho }{\rho }$) is the same for all elements considered except S, Se, and Te. A general relation between the change upon melting in the short-range order of the atomic distribution and the sign of $m_0$ is also discussed. The semimetals, for which $m_0<0\mathrm{}$, undergo striking changes in short-range order, whereas the metals, for which $m_0>0\mathrm{}$, exhibit a continuity of short-range order.