In situ Synchrotron SC-XRD

Pressures exceeding 50 GPa are attainable in DACs with culet size of 250 μm or less, which means that the initial sample chamber size should be 100-125 μm before loading of the pressure-transmitting medium. After gas loading and upon compression, the chamber size will rapidly shrink until it stabilizes at the point where the pressure medium solidifies, which is at ~5 GPa in the case of neon.

At this pressure the size of the sample chamber is around half of the initial size. Above this pressure, the chamber will not shrink drastically, instead of that, however, the gasket will become much thinner during compression. The ~50 μm in diameter chamber is limiting the linear size of the sample to a

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maximum of ~30 μm (Fig. 2.9), in order to prevent the risk of touching the edge of the gasket, which could further cause damage or stress on the crystal. In order to rich high pressures (above approximately 30 GPa), the initial thickness of the gasket indentation should not exceed 30 μm, which limits the thickness of the sample to <15 μm. The size of the X-ray beam of a conventional in-house diffractometer is ~500 μm FWHM, therefore also the gasket is irradiated by the beam. A metal gasket is diffracting much more strongly than the sample. Although many experiments can be carried out in-house, the effect of the diffraction from the gasket is a particularly big drawback in case of the experiments with very weakly scattering materials, such as SiO2. Even testing of coesite and cristobalite crystals of the required size in air prior putting them into a DAC was shown to be problematic in terms of very low diffracting intensities. Once put in the DAC, absorption of MoKα radiation by the DAC significantly reduces the beam intensity, resulting in low intensities from the sample, combined with strong diamond’s and gasket’s signals. Therefore the HP-DAC experiments could only be carried out at synchrotron radiation facilities.

FIGURE 2.9.View on a sample chamber in a diamond-anvil cell loaded with three different cristobalite crystals at room pressure before (left) and after neon gas-loading at 9 GPa (right).

Diamonds are almost transparent to X-rays with energies above 20 keV, even more so in the case of a short-wavelength synchrotron source (λ~0.3Å) so there is no need for absorption corrections when using the beam which is perfectly centered on the sample. However, the typical beam size of about 5-8 μm (FWHM) limits the single crystals size to be either significantly larger or smaller than the beam

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itself. Another problem of in situ data collection is intense diamond diffraction which contributes to the very intense diffraction spots which need to be eliminated in order to find the sample (Fig. 2.10).

The major problem encountered for the in situ SC-XRD using diamond-anvil cells is the limited access to the reciprocal space. To assure the sufficient 2θ coverage and therefore the proper coverage of the reciprocal space one needs to choose the diamonds, the support seats and the DAC with high opening angles. The success of an experiment depends strongly on the choice of the DAC and diamond geometry, but also on the crystal quality. The total number of reflections collected in a DAC is far smaller than collected for the same crystal in the air due to the restricted coverage of the reciprocal space. This leads to greater uncertainties in the structural parameters (positional or displacement parameters, reliability values, etc…). Under compression, the unit cell volume will decrease in real space, causing the expansion of the reciprocal lattice and therefore the decrease in the total number of accessible reflections. All these hindrances are particularly limiting in the case of low-symmetry structures, such as monoclinic coesite, due to the requirement of accessing many more reflections of the reciprocal space in order to gain accurate structural information. Notably, pressure-induced phase transitions are often driven by symmetry-lowering mechanisms, so the increase in pressure commonly leads to a decrease in data quality.

Pressure regions of interest (e.g. where a structural change occurs) were first determined by much faster in situ Raman spectroscopy and then investigated by collecting SC-XRD data sets. The parameters for the XRD data collection were first determined based on a wide scan image taken of the crystal while the DAC is rotating at a constant speed from the zero position up to the angle of its maximal opening in both directions around the ω-axis of the goniometer. This image contains enough reflections of various intensities to allow a proper estimate of exposure time and step size for the full data collection performed as step scans of in 0.5° or 1.0° in ω for a determined amount of time until the entire accessible opening angle of ±40° is covered.

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FIGURE 2.10. Representative step-scan data set of cristobalite X-I (Chapter 6) collected at high pressure (20 GPa) presented in the Ewald Explorer window of the CrysAlis software. On the left, all collected reflections contain low intensity sample reflections along with strong reflections of diamond, neon and rhenium. On the right, reciprocal lattice of the sample is extracted from the entire data set and the hkl reflections are indexed based on a unit cell choice. Intensities of the reflections can be readily reduced and integrated.