The merits and challenges of single-crystal X-ray diffraction in the diamond anvil cell

Abstract:

Single-crystal X-ray diffraction (SC-XRD) data have traditionally allowed us to obtain explicit solutions of complex structures, detect small structural distortions, retrieve accurate displacement parameters as well as provide chemical characterization of new materials. Recent advances in the diamond anvil cell designs, combined with the cutting-edge equipment available at high-pressure X-ray diffraction beamlines, such as those at GSECARS (sector 13, APS, USA) as well as the established protocols of performing such experiments at extreme conditions, make the SC-XRD method more and more appealing to the high-pressure community nowadays. It is now possible to study in great details the crystal structure, physical and chemical properties of minerals in the megabar pressure range and after annealing or new phase synthesis at temperatures above 5000 K in the laser-heated DAC. Even at such extreme conditions, where the coverage of the reciprocal space is restricted by the DAC design, analysis of the SC-XRD data can provide unique lattice information than the one-dimensional diffraction patterns collected from powdered samples. Here we review the sample and DAC preparations that are necessary prior to a SC-XRD experiment, we describe the data collection procedures at GSECARS, and we discuss the data processing including the analysis of mixed-phase poly-crystalline samples (i.e. multigrain analysis) with various software. A few examples, including carbonate minerals and metal oxides, coupled with relatively weakly scattering compounds, such as CO₂ and H₂O, will be presented in order to demonstrate not only the challenges but also the merits of using the single-crystal's approach for solving the structures of complex high-pressure polymorphs or novel compounds.