Three-dimensional X-ray diffraction in the diamond anvil cell: application to stishovite
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AbstractThree-dimensional X-ray diffraction can be used for characterizing the orientation, position, and strain tensor of single grains in a polycrystalline aggregate. Here, we show how the method is well suited for diamond anvil cell data with heterogeneous grain sizes, with an application to two samples of stishovite at 15 and 26 GPa. For each grain, we obtain a well-defined orientation matrix and cell parameters. Center of mass position can also be adjusted to the experimental data, with errors in the present experiment. Finally, strain tensors are adjusted for the individual grains. The stress distribution obtained is in agreement with expectations from the diamond anvil cell geometry and previous measurements of stishovite strength. Advantages and potential for improvement of the method are then discussed.Keywords: high pressurediamond anvil cellthree-dimensional X-ray diffractionmicrostructurestishovitetexture AcknowledgementsThe author wish to thank Masaki Akaogi for providing the stishovite sample, Patrick Cordier for fruitful discussions, Michael Hanfland for sharing his laser heating system, the ESRF high pressure sample environment support for providing diamond anvil cells, and the ESRF for the allocation of beamtime. This work was supported by the ANR "Dislocations Under Pressure (DIUP)" program (ANR-07-5CJC-0136-01), and the Hungarian National Innovation Office, the French Ministry of Research and Education, and the French Ministry of Foreign Affairs through the Partenariat Hubert Curien (PHC) Balaton (projects 19479 PB and 27861 QJ).Notes†This paper was presented at the LIth European High Pressure Research Group (EHPRG 51) Meeting in London (UK), 1–6 September 2013.Keywords:
Stishovite
Diamond anvil cell
Abstract The diffraction by diamonds of a diamond-anvil cell has been observed to cause sharp dips in the intensity transmitted by the cell. As a result, the intensities of sample reflections measured in X-ray single crystal studies at high-pressure are significantly reduced when the diamond diffraction attenuates either the beam incident on the sample, or the one diffracted from it. This effect should be taken into account in the data collection, by avoiding, for each reflection, the diffractometer settings at which diffraction from the diamonds occurs.
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Reflection
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Stishovite
Diamond anvil cell
Noble gas
Triple point
Coesite
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In this work, the high pressure behavior of four major SiO2 polymorphs were investigated by means of dynamic compression: α-quartz, fused silica, stishovite and α-cristobalite. Here, laser shock compression and dynamic diamond anvil cell (dDAC) techniques were applied and the concomitant use of hard X-ray radiation at synchrotron- and X-ray free electron laser (XFEL) facilities made a time-resolved investigation of the lattice response at high pressures possible.
Stishovite
Diamond anvil cell
Free-electron laser
Free electron model
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Thermomechanical modeling of a sample assembly (sample plus pressure transmitting medium) in a laser-heated diamond–anvil cell (LHDAC) is presented. Finite elements numerical calculation afforded to obtain the temperature distribution and the induced thermal pressure field, showing that a non-negligible pressure increase (called thermal pressure) occurs in the laser-heated zone. When argon is used as a pressure transmitting medium, thermal pressure can reach 20%–30% of the normal pressure measured in the cold zone. This modeling is supported by experimental studies. It is shown that discrepancies between diamond–anvil cell and large volume press experiments on the coesite to stishovite transition are quantitatively explained by the thermal pressure effect. Moreover, thermal pressure also explains the anomalous low thermal expansion coefficient obtained by x-ray diffraction studies in LHDAC.
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Diffraction studies at extreme pressure-temperature conditions encounter intrinsic difficulties due to the small access angle of the diamond anvil cell and the high background of the diffraction peaks. Energy-dispersive x-ray diffraction is ideal for overcoming these difficulties and allows the collection and display of diffracted signals on the order of seconds, but is limited to one-dimensional information. Materials at high pressures in diamond anvil cells, particularly during simultaneous laser heating to temperatures greater than 3000 K often form coarse crystals and develop preferred orientation, and thus require information in a second dimension for complete analysis. We have developed and applied a diamond cell rotation method for in situ energy-dispersive x-ray diffraction at high pressures and temperatures in solving this problem. With this method, we can record the x-ray diffraction as a function of χ angle over 360°, and we can acquire sufficient information for the determination of high P–T phase diagrams, structural properties, and equations of state. Technical details are presented along with experimental results for iron and boron.
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Pressure media are one of the most effective deterrents of pressure gradients in diamond-anvil cell (DAC) experiments. The media, however, become less effective with increasing pressure, particularly for solid pressure media. One of the most popular ways of alleviating the increase in pressure gradients in DAC samples is through laser annealing of the sample. We explore the effectiveness of this technique for six common solid pressure media that include: alkali metal halides LiF, NaCl, KCl, CsCl, KBr, as well as amorphous SiO2. Pressure gradients are determined through the analysis of the first-order diamond Raman band across the sample before and after annealing the sample with a near-infrared laser to temperatures between ∼2000 and 3000 K. As expected, we find that in the absence of sample chamber geometrical changes and diamond anvil damage, laser annealing reduces pressure gradients, albeit to varying amounts. We find that under ideal conditions, NaCl provides the best deterrent to pressure gradients before and after laser annealing, at least up to pressures of 60 GPa and temperatures between ∼2000 and 3000 K. Amorphous SiO2, on the other hand, transforms in to harder crystalline stishovite upon laser annealing at high pressures resulting in increased pressure gradients upon further compression without laser annealing.
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