![]() This size can be reduced by adding sets of secondary slits and pinholes. The final beam size on the sample is approximately 0.08 mm × 0.08 mm. The beam is focused on the sample by a toroidal Pt-coated mirror at 33 m from the source and 12 m from the sample. The fixed operating energy for the endstation is 25 keV ( λ ≃ 0.5 Å). A primary beam splitter allows 0.5 mrad of radiation in the horizontal plane to intercept the monochromator, a liquid-nitrogen-cooled Si(111) single crystal. Primary carbon filters act as high-bandpass filters, cutting the energy below 4 keV. The high-pressure Xpress beamline endstation makes use of the X-ray beam produced by a multipole superconducting wiggler, operating with a magnetic field of 3.5 T. These examples concern the structure refinement of low- and high-pressure polymorphs of clinoenstatite, the structure determination of the high-pressure polymorphs of CaCO 3, and the equation of state of the mineral leucophenicite. Three examples will illustrate the feasibility of single-crystal diffraction at the beamline, with data of a suitable quality for performing structure refinements, structure solution and equation-of-state determination. We here report the first single-crystal measurements at the Xpress beamline of the Elettra synchrotron, the Italian national synchrotron facility. The requirements for successful single-crystal diffraction experiments concern the stability of the X-ray source, low divergence of the beam, as well as the mechanical stability of the goniometer system. This may be crucial, for example, when only natural or synthetic samples of very small size (∼5–20 µm) are available or when the solution of complex crystal structures requires the simultaneous collection of diffraction data from at least two crystals with different orientation within the same DAC. Synchrotron radiation allows single-crystal X-ray diffraction to be performed on very small samples, allowing these experiments in a wide range of pressure and temperature ( T) (McMahon et al., 2013 Merlini & Hanfland, 2013 Dera et al., 2013 Dubrovinsky et al., 2010 ), possibly even using several samples loaded in a single DAC (Merlini et al., 2015 Yuan & Zhang, 2017 ). Miletich et al., 2000 ), the absorption of primary and diffracted X-ray radiation by the diamond anvils, and the shadowing of significant portions of the reciprocal lattice by the metallic components of the DAC may pose severe limitations on the results achievable by in situ high-pressure single-crystal X-ray diffraction experiments. However, the limited size of the pressure chambers of diamond anvil cells (DACs) ( e.g. Single-crystal diffraction at high pressure is one of the most relevant techniques for understanding the atomic-scale behavior of matter at non-ambient conditions. Most of the problems addressed by high-pressure experiments require, to be properly understood, a structural knowledge at the atomic scale. In materials science, high-pressure diffraction allows to disclose the relationships between the crystal structure and the physical–chemical properties of crystalline compounds and induce phase transitions to polymorphs of industrial or technological interest ( e.g. In Earth sciences, in situ high-pressure (H P) diffraction experiments are needed to determine accurate elastic constants of minerals, by fitting the experimental volume–pressure ( V– P) data by equations of state (Duffy & Wang, 1998 Angel, 2000 ), as well as to explore phase transitions, shedding light on the potential mineralogy of the Earth's interior (Boffa Ballaran et al., 2013 Dubrovinsky et al., 2010 Dera, 2010 ). High-pressure diffraction grew, during the last decades, from being a niche pioneering method to a widely used multidisciplinary technique.
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