Extremes of pressure: a diamond anvil cell for studying high pressure glass
20 December 2009
An emerging field in condensed matter physics is the study of materials under extreme conditions. This includes the study of materials under high pressure and one of the most useful devices for this type of study is the diamond anvil cell. Following a successful application to the Royal Society a diamond anvil cell has been purchased. This cell is modified for the studies of amorphous materials at pressures of up to 25 GPa. The diamond anvil cell comprises two gem quality diamonds that are arranged in an opposed geometry that squeeze on a metal gasket; a hole in the gasket contains a pressure transmitting fluid and the sample. Pressure is generated at the tips of the anvil by slowly moving the anvils together.
The cell that has been purchase uses a highly controllable gas membrane device which allows constant pressure to be maintained if the cell is heated. A resistive heater is also provided to allow studies at temperatures of up to 1000oC. Diamonds are transparent to visible light and X-rays and accordingly X-ray diffraction patterns, Raman and Brillouin spectra can be obtained for materials at high pressure. Diamond anvil cells have not previously been used extensively for high pressure, because diamond anvil cells contribute a large background to the amorphous scattering. Recently, colleagues at Argonne National Laboratory have shown that that the background contribution can be reduced if the diamond is perforated and quantitative diffraction data has been obtained for pressures of up to 10GPa have been achieved for diamond anvils which had been mechanically perforated to minimise the amount of diamond in the beam path and more recently amorphous diffraction data has been collected to pressures of 32GPa with diamonds perforated by laser drilling.
The Diamond Anvil cell will be used initially to study three network-forming glasses SiO2, GeO2 and B2O3 all of which are candidate polyamorphic systems. High energy X-ray diffraction measurements will be performed at Argonne National Laboratory on these three glasses and the aim of this research is to understand the structural changes that accompany the widely reported changes in density that can occur over narrow pressure intervals. Changes in network glass structure with pressure are analogous to the polymorphic changes that occur in crystalline phases and represent the phenomenon of polyamorphism; formation of different amorphous forms of the same substance with different structures and thermodynamic properties. This high pressure research will provide not only a better understanding of the changing structure of the amorphous materials under pressure but will provide insight into to the nature of polyamorphism and the inferred possibility of first-order transitions in liquids at high pressure.
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