Magnesium-based alloys attract significant interest as cost-efficient hydrogen storage materials allowing the combination of high gravimetric storage capacity of hydrogen with fast rates of hydrogen uptake and release and pronounced destabilization of the metal–hydrogen bonding in comparison with binary Mg–H systems. In this review, various groups of magnesium compounds are considered, including (1) RE–Mg–Ni hydrides (RE = La, Pr, Nd); (2) Mg alloys with p-elements (X = Si, Ge, Sn, and Al); and (3) magnesium alloys with d-elements (Ti, Fe, Co, Ni, Cu, Zn, Pd). The hydrogenation–disproportionation–desorption–recombination process in the Mg-based alloys (LaMg12, LaMg11Ni) and unusually high-pressure hydrides synthesized at pressures exceeding 100 MPa (MgNi2H3) and stabilized by Ni–H bonding are also discussed. The paper reviews interrelations between the properties of the Mg-based hydrides and p–T conditions of the metal–hydrogen interactions, chemical composition of the initial alloys, their crystal structures, and microstructural state.

/pdf/mg-based-compounds-for-hydrogen-and-energy-storage-4d8irvw5n9.pdf

Mg-based compounds for hydrogen and energy storage

The first fully automated small-molecule robotic X-ray diffractometer is described. After demonstrating the utility of the instrument using multiple samples of ammonium bitartrate, we investigated the conformational chirality of diphenyl dichalcogenide (E2Ph2, where E = S, Se, or Te). Structural and computational studies suggest that the two enantiomers are energetically indistinguishable. Therefore, it was unsurprising that we found (in 35 suitable data collections) the proportion 0.51:0.49 of M-S2Ph2 to P-S2Ph2 in the bulk sample. Interestingly, after 65 data collections of Te2Ph2, (46 provided suitable data sets), we found the proportion 0.72 ± 0.13 of M-Te2Ph2, suggesting there could be a statistically significant preference for the M-enantiomer in the sample examined here. We found that Se2Ph2 underwent homochiral crystallization, with all 24 crystals being M. Our experiments may represent a salutary lesson in statistical analysis.

/pdf/automated-chemical-crystallography-3a78wbgmcu.pdf

Automated chemical crystallography.

Large differences are found in the quality of crystal structures obtained from different diffraction methods. The most accurate studies identify all atomic sites, including hydrogens, while others lack even the resolution needed to locate individual atoms. The gauge including projector augmented wave method (GIPAW) provides a technique to further refine any of these structures under lattice constraints. Here, the sensitivity of solid-state NMR 13C shift tensor principal value data to GIPAW refinement is investigated. The refinement is shown to improve X-ray powder, X-ray single crystal and even neutron single crystal diffraction data. Convergence to a single structure is observed in most cases. Surprisingly, the final refined structures usually diverge from the original neutron diffraction coordinates – data typically viewed as the most accurate. To ensure that the structural changes represent improvements, three metrics are monitored comprising fit to 13C shift tensors, forces upon the atoms and changes in atomic positions relative to a reference structure. In all cases these parameters improve upon refinement suggesting that GIPAW creates structures surpassing the accuracy of single crystal neutron diffraction data. However, the influence of thermal motions remains unknown. Improvements are seen most strongly in forces and NMR fits and least in atom positions. This study evaluates reasonably accurate model structures to quantify improvements. However, structures obtained from lower resolution methods (e.g. electron diffraction) will benefit most from GIPAW refinement. In such structures the refinement has the potential to convert structures with questionable atom positions into coordinates rivaling neutron diffraction single crystal data.

Refining crystal structures with experimental 13C NMR shift tensors and lattice-including electronic structure methods

How many different intermetallic compounds are known so far, and in how many different structure types do they crystallize? What are their chemical compositions, the most abundant ones and the rarest ones? These are some of the questions we are trying to find answers for in our statistical analysis of the structures of the 20 829 intermetallic phases included in the database Pearson's Crystal Data, with the goal of gaining insight into some of their ordering principles. In the present paper, we focus on the subset of 13 026 ternary intermetallics, which crystallize in 1391 different structure types; remarkably, 667 of them have just one representative. What makes these 667 structures so unique that they are not adopted by any other of the known intermetallic compounds? Notably, ternary compounds are known in only 5109 of the 85 320 theoretically possible ternary intermetallic systems so far. In order to get an overview of their chemical compositions we use structure maps with Mendeleev numbers as ordering parameters.

/pdf/more-statistics-on-intermetallic-compounds-ternary-phases-47uwzdtkv5.pdf

More statistics on intermetallic compounds - ternary phases.

This review is intended to be a chemist-friendly introduction to what quasicrystals (QCs) and approximant crystals (ACs) are and what chemists may be able to contribute to the field Readers will first be exposed to a must-know history of QC/ACs, then warmed up with the somewhat distant and prior concepts of metal clusters in halides, oxides etc, and then to polyanionic clusters in Zintl phases and intermetallic systems Information on these last two has originated over about the last 50 years We will draw on some more chemical insights and information on how these might be related and applicable to new and expanded QC and AC systems Then follow our experiences on electronic and chemical tuning of five QC and AC systems and the structural regularities within ACs, from which important clues for quasicrystal structure modeling are evident

A Chemical Approach to the Discovery of Quasicrystals and Their Approximant Crystals

/pdf/report-on-a-project-on-3d-imaging-of-the-biological-cell-by-4zpe9eguj5.pdf

Report on a project on 3D imaging of the biological cell by single-particle X-ray diffraction

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Report on a project on 3D imaging of the biological cell by single-particle X-ray diffraction