A modular, multi-diagnostic, automated shock tube for gas-phase chemistry Review of Scientific Instruments 90, 064104 (2019);

We experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low‐spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo‐driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X‐ray diffraction images were continuously collected every 5–50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single‐crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle.

Studying the structural behavior of metal–organic frameworks (MOFs) under hydrostatic pressures is a steadily growing area. The goal of active research is identification of overarching composition–structure principles including both properties of technological relevance and material behavior of academic interest such as material stability criteria, mechanical properties, pressure-induced phase transitions, negative compressibilities, and guest-dependent high-pressure structural responses. Here the current literature on the high-pressure structural responses of MOFs is reviewed with focus on bulk moduli, pressure-transmitting medium dependent properties, pressure-induced phase transitions, and amorphization processesproperties that are derived from high-pressure X-ray diffraction studies. After highlighting topical examples, aspects of high-pressure diffraction studies on MOFs are summarized that are important to advance the field such as the rigorous reporting of experimental and analytical procedures, opportunities that come with custom-made high-pressure diffraction setups, the nature and impact of the pressure-transmitting medium, and the role of forging even closer ties between computation and experiment.

Introduction(Mg,Fe)O ferropericlase is the second most abundant mineral in Earth's lower mantle (e.g., Irifune et al., 2015). At room pressure, ferrous iron (Fe 2+ ) in ferropericlase adopts the high-spin electronic configuration where the six 3d-electrons partially fill the t 2g and e g orbitals, maximizing the number of unpaired electrons. At a pressure of about 40 GPa, a distortion of the iron-containing octahedral sites in the crystal structure triggers a change in iron electronic configuration to the low-spin state, where electron pairing in the t 2g orbitals is favored (Badro et al., 2003). Several studies have shown that the properties of ferropericlase in mixed-spin state are markedly different from those in either high-or low-spin state. In particular, the spin crossover is accompanied by a reduction in the Fe 2+ ionic radius (Shannon, 1976) enhancing the compressibility of ferropericlase over the pressure range where the spin crossover occurs (