A Paris-Edinburgh Cell for Liquid Silicate Structural Studies Using Monochromatic Diffraction and Soller Slits
Abstract:A Paris-Edinburgh Press (PEP) has been commissioned at the GSECARS beamline 13-ID-C, with a Si (311) high-energy monochromator and a multi-channel collimator (MC) assembly, which consists of two arrays of fine slits (WC blades) arranged in two concentric circular arcs. Both arrays consist of 75 slits with 0.8° separation and are located 50 and 200 mm, respectively, from the center. Slit widths of the inner and outer arrays are 0.05 and 0.20 mm, respectively. By oscillating the silts during data collection, background scattering can be effectively removed. Similar MC assemblies have been used extensively with PEP at ESRF for studying metallic liquids and low-Z materials . The PEP is mounted on a general purpose diffractometer , with an area detector (MAR CCD) mounted on the two-theta arm. With unfocused incident monochromatic beam (65keV) collimated at 0.2 mm, 30 min is sufficient to collect weak signals of a 2 mm diameter amorphous silicate sample, with minimal background scatter from the surrounding solid pressure medium. An analysis shows that with a fine incident beam of 0.05 mm, a collimation depth of 0.5 mm can be achieved at two-theta angles above 10°. To increase pressure and temperature range, we have developed a cupped-toroidal Drickamer (CTD) anvil . The anvil design, with a central depression, a toroidal groove and a small tapered angle, combines features of modified Drickamer anvil and the traditional PE anvil. By optimizing the parameters for the CTD anvil design, pressures corresponding to the mantle transition zone can be generated. Cell assemblies with thermally insulating materials have been developed and temperatures up to 2000°C have been maintained steadily over hours. In the future, the large horizontally focusing Kirkpatrick-Baez mirror will be applied to focus the incident beam, thus allowing samples with diameters below 0.5 mm in diameter to be studied. The excellent spatial selectivity provides an exciting opportunity for liquid structure studies in the PEP. (We thank Mohamed Mezouar of ESRF for the design of the MC and advice during the setup)
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