Paper No. 12
Presentation Time: 1:30 PM-5:30 PM
HIGH PRESSURE AND TEMPERATURE BEHAVIOUR OF COVELLITE, CU(II)S
The high pressure behaviour of covellite, Cu(II)S and its Se-analog, klockmannite have been relatively less studied compared to other metal-sulfide systems. Nonetheless, a survey of experimental and theoretical literature reveals rich phase behaviour for both phases; including decomposition to umangite (Cu3Se2) + kruatite (CuSe2-II), pressure-induced amorphization, pressure-induced ordering and intriguing bond compressibilities - at high and low temperatures - as well as - under pressure. The aims of this study were to investigate, with both x-ray absorption spectroscopy (XAS) and diffraction measurements (XRD) some of these aspects of the phase behaviour of these materials. We have used XAS measurements to determine, at the onset of amorphization in covellite, if there is any related electronic, or, more pertinent - structural transition. If amorphization does happen, we would require a technique that is sensitive to bond lengths on a local scale and is thus not impeded by the lack of long-range order that is required by XRD techniques. Several reversible transitions were noted under compression, at room and elevated temperatures, to 30 GPa, which were later confirmed by XRD. Furthermore, in the XRD dataset, no amorphous-like state was reached, although patterns are heavily strain-broadened and we observe a structural transition at 18 GPa, the previously noted onset pressure for amorphization. These patterns sharpen with increasing T to reveal diffraction patterns similar too, but unlike, the hexagonal covellite phase. Analysis is underway to resolve the structures of these phases through the coupling of both XRD and XAS measurements. From our preliminary analysis of both datasets we find that there are two structural transitions in covellite to 30 GPa, the transitions are reversible and the onset pressure decreases with increasing temperature. Broadly speaking, the observations of previous groups are confirmed. However, we differ in interpretation and this is probably due to the resolution limits of various methods involved. We expect that most anomalous behaviour can be explained through our observation of these transitions - particularly those related to bond softening. Further analysis will be carried out to confirm this.