A collaborative research team from multiple institutions, including Harbin Institute of Technology, Argonne National Laboratory, Harvard University, and Geophysical Laboratory has discovered unexpected dynamics and volume expansion associated with pressure-induced crystallization of amorphous selenium. The development and application of the state-of-the-art high pressure research facilities at multiple beamlines at the Advanced Photon Source during the last two years ensure have made the work possible. The initial appearance of the crystal phase and the transformation between monoclinic and trigonal phases was monitored temporally and spatially, revealing the structural origin of the long-standing enigma of time-dependent electrical properties of the material on compression. The unusual volume expansion phenomenon under pressure associated with the crystallization of an "over-pressurized" metastable phase found in this study may be more common than previously believed.

The research involved the application of several cutting-edge methods: in situ high pressure synchrotron x-ray diffraction (XRD), novel diamond anvil cell microtomography techniques, as well as first-principles electronic structure calculations. These investigations have revealed the anomalous pressure-induced behavior of amorphous selenium, in particular the direct experimental observation of a volume expansion of a material under high pressure. These results demonstrate the importance of using new time- and spatially- resolved high-pressure XRD and imaging techniques to understand the kinetics of structural transformations in materials under extreme conditions. The new microtomographic technique developed in this project could find widespread use in determinations of the equations of state of glasses and melts up to megabar pressures, which is crucial to many problems in earth, planetary, and materials sciences [Liu, et al., Proc. Nat. Acad. Sci., 105, 13229-13234 (2008)].