Abstract
Single-crystal x-ray diffraction (SCXRD) is an important tool to study the crystal structure and phase transitions of crystalline materials at elevated pressures. The Partnership for eXtreme Xtallography (PX2) program at the GSECARS 13-BM-C beamline of the Advanced Photon Source aims to provide state-of-the-art experimental capabilities to determine the crystal structures of materials under extreme conditions using SCXRD. PX2 provides a focused x-ray beam (12 × 18 µm2) at a monochromatic energy of 28.6 keV. High-pressure SCXRD experiments are performed with a six-circle diffractometer and a Pilatus3 photon-counting detector, facilitated by a membrane system for remote pressure control and an online ruby fluorescence system for pressure determination. The efficient, high-quality crystal structure determination at PX2 is exemplified by a study of pressure-induced phase transitions in natural ilvaite [CaFe2+2Fe3+Si2O7O(OH), P21/a space group]. Two phase transitions are observed at high pressure. The SCXRD data confirm the already-known ilvaite-I (P21/a) → ilvaite-II (Pnam) transformation at 0.4(1) GPa, and, a further phase transition is found to occur at 22.8(2) GPa where ilvaite-II transforms into ilvaite-III (P21/a). The crystal structure of the ilvaite-III is solved and refined in the P21/a space group. In addition to the ilvaite-I → ilvaite-II → ilvaite-III phase transitions, two minor structural modifications are observed as discontinuities in the evolution of the FeO6 polyhedral geometries with pressure, which are likely associated with magnetic transitions.
Highlights
TO THE PX2 PROGRAMHigh-pressure single-crystal x-ray diffraction (SCXRD) is a unique way to determine crystal structure at elevated pressures.1–3 Compared with powder XRD, in which peak indexing can be challenging, high-pressure Single-crystal x-ray diffraction (SCXRD) has the advantage of unambiguous peak indexing
In addition to the ilvaite-I → ilvaite-II → ilvaite-III phase transitions, two minor structural modifications are observed as discontinuities in the evolution of the FeO6 polyhedral geometries with pressure, which are likely associated with magnetic transitions
The use of diamond anvil cell (DAC) for highpressure SCXRD has its limitations: first, a DAC usually has a limited opening angle, which effectively limits the region of reciprocal space that can be explored; second, the diamond anvils absorb x-rays and reduce the signal-to-noise ratio of the diffraction data, which can be significant for low-energy laboratory x-ray sources
Summary
High-pressure single-crystal x-ray diffraction (SCXRD) is a unique way to determine crystal structure at elevated pressures. Compared with powder XRD, in which peak indexing can be challenging, high-pressure SCXRD has the advantage of unambiguous peak indexing. High-pressure single-crystal x-ray diffraction (SCXRD) is a unique way to determine crystal structure at elevated pressures.. It is planned to add a Si(400) crystal that works at 34.5 keV to the existing Rowland-circle monochromator, which will increase the reciprocal space probing range by 76%, given the current DAC opening angle. The increased reciprocal space probing range will significantly improve the data completeness for SCXRD, and will significantly improve the data quality and the ability to determine complex structures discovered at high pressure. It is planned to upgrade the x-ray focusing mirrors to take advantage of the small x-ray source size after the APS upgrade and to generate a tight x-ray focus, which will help reduce sample sizes and reach higher pressures
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