HR: 08:15h
AN: V41B-02    [PDF]
TI: X-Ray Microdiffraction at Megabar Pressures
AU: * Mao, H
EM: mao@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AB: High-pressure x-ray diffraction (XRD) provides unique, important sources of structural information of minerals in the Earth's deep interior, but encounters major limitations. The restriction to forward diffraction geometry (2$\theta$ less than $90\deg$) severely limits the accuracy. With the 50-5 $\mu$m size x-ray beam typically used to probe samples at 30-200 GPa, the number of crystals covered by the x-ray beam is often too few for good polycrystalline XRD, but too numerous for single-crystal XRD. Single-crystal XRD method with monochromatic x-ray source and 2-d detector works satisfactorily for crystal size larger than 20 $\mu$m, but when the crystal is significantly less than 5 $\mu$m, the sample signals are often overwhelmed by the background. Energy dispersive XRD with polychromatic x-radiation has been used successfully to determine unit-cell parameters of smaller single crystals, but the intensity information is unusable for structural refinement because this method requires rotation of the small crystal relative to the small x-ray beam. Recent integration of panoramic diamond anvil cell$^{1}$ (PDAC) with synchrotron x-ray microdiffraction$^{2}$ (XRMD) method has finally overcome these limitations and can potentially revolutionize the high-pressure XRD field. This XRMD method focuses polychromatic x-radiation to submicrometer size to resolve very small single crystals, and collects Laue spots with a 2-d CCD detector. The PDAC allows complete forward, $90\deg$, and back scatterings, while the background signal is minimized by directing the incident x-ray beam through single-crystal diamonds (i.e., avoiding the beryllium seats and gasket). The incident beam can be changed to monochromatic, tuned through the full energy (wavelength) range, and focused to the identical submicrometer spot for d-spacing determination of each Laue spot. All polychromatic Laue spots are collected simultaneously from the same x-ray sampled volume, thus reliable for structure determination. The development provide long-sought solutions to important technical and scientific issues, including the fundamental changes of silicates, oxides, ices, and condensed gases that are the main components of deep planetary interiors. $^{1}$ H. K. Mao, J. Xu, V. V. Struzhkin, et al., Science 292, 914 (2001). $^{2}$ N. Tamura, R. S. Celestre, A. A. MacDowell, et al., Rev. Sci. Instrum. 73, 1369 (2002).
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 3994 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting