The tellurium delirium or a crystallographer's paradise

The group VI elements Se and Te have been the subject of a wide variety of experimental studies at high pressure, and their complex phase diagrams contain a number of phases in both the solid and liquid states. At ambient conditions, both Se and Te are semiconductors and have a trigonal crystal structure comprising infinite helices. On compression at room temperature, both elements undergo a number of structural phase transitions to low-symmetry structures that are known to be metallic and superconducting. In Te there are four known phase transitions at 4, 7, 10 and 27 GPa, while in Se there are five reported transitions at 14, 23, 28, 60, and 140 GPa. There are also reported structural transitions in the liquid forms. However, while the structures of the numerous high-pressure forms of Te and Se are generally agreed to be known, a recent high-resolution powder diffraction study of tellurium phase III revealed numerous additional reflections not accounted for by any of the previously reported structures. We have therefore made detailed diffraction studies of both Te and Se with the aim of resolving and understanding their high-pressure structural behaviour. The results of these diffraction studies will be presented, including a surprising and entirely new type of elemental structure that accounts for all the observed reflections in Te-III.