X‐Ray Diffraction as a Local Probe Tool

For the structural characterization of nanoscale objects, X‐ray diffraction is widely used as a technique complementing local probe analysis methods such as scanning electron microscopy and transmission electron microscopy. Details on strain distributions, chemical composition, or size and shape of nanostructures are addressed. X‐ray diffraction traditionally obtains very good statistically averaged properties over large ensembles—provided this averaging is meaningful for ensembles with sufficiently small dispersion of properties. In many cases, however, it is desirable to combine different analysis techniques on exactly the same nano‐object, for example, to gain a more detailed insight into the interdependence of properties. X‐ray beams focused to diameters in the sub‐micron range, which are available at third‐generation synchrotron sources, allow for such X‐ray diffraction studies of individual nano‐objects.     

In situ X‐ray Diffraction Studies of Cation and Anion Inter­calation into Graphitic Carbons for Electrochemical Energy Storage Applications

Graphite is a redox‐amphoteric intercalation host and thus capable to incorporate various types of cations and anions between its planar graphene sheets to form so‐called donor‐type or acceptor‐type graphite intercalation compounds (GICs) by electrochemical intercalation at specific potentials. While the LiC_x/C_x donor‐type redox couple is the major active compound for state‐of‐the‐art negative electrodes in lithium‐ion batteries, acceptor‐type GICs were proposed for positive electrodes in the “dual‐ion” and “dual‐graphite” cell, another type of electrochemical energy storage system. In this contribution, we analyze the electrochemical intercalation of different anions, such as bis(trifluoromethanesulfonyl) imide or hexafluorophosphate, into graphitic carbons by means of in situ X‐ray diffraction (XRD). In general, the characterization of battery electrode materials by in situ XRD is an important technique to study structural and compositional changes upon insertion and de‐insertion processes during charge/discharge cycling. We discuss anion (X^–) and cation (M⁺) intercalation/de‐intercalation into graphites on a comparative basis with respect to the M_x⁺C_n^– and C_n⁺X_n^– stoichiometry, discharge capacity, the intercalant gallery height/gallery expansion and the M–M or X–X in‐plane distances.     

X‐Ray and Electron Diffraction Study of Poly(p‐dioxanone)

Summary: Solution‐grown lamellar crystals of poly(p‐dioxanone) (PPDX) have been crystallized isothermally from butane‐1,4‐diol at 100 °C. The crystal structure of PPDX has been determined by interpretation of X‐ray fiber diagrams of PPDX fibers and electron diffraction diagrams of lozenge‐shaped chain‐folder lamellar crystals. The unit cell of PPDX is orthorhombic with space group P2₁2₁2₁ and parameters: a = 0.970 nm, b = 0.742 nm, and c (chain axis) = 0.682 nm. There are two chains per unit cell, which exist in an antiparallel arrangement.

Transmission electron micrograph of PPDX chain‐folded lamellar crystals obtained by isothermal crystallization and its electron diffraction diagram.     

Spatiotemporal Kinetics in Solution Studied by Time‐Resolved X‐Ray Liquidography (Solution Scattering)

Information about temporally varying molecular structure during chemical processes is crucial for understanding the mechanism and function of a chemical reaction. Using ultrashort optical pulses to trigger a reaction in solution and using time‐resolved X‐ray diffraction (scattering) to interrogate the structural changes in the molecules, time‐resolved X‐ray liquidography (TRXL) is a direct tool for probing structural dynamics for chemical reactions in solution. TRXL can provide direct structural information that is difficult to extract from ultrafast optical spectroscopy, such as the time dependence of bond lengths and angles of all molecular species including short‐lived intermediates over a wide range of times, from picoseconds to milliseconds. TRXL elegantly complements ultrafast optical spectroscopy because the diffraction signals are sensitive to all chemical species simultaneously and the diffraction signal from each chemical species can be quantitatively calculated from its three‐dimensional atomic coordinates and compared with experimental TRXL data. Since X‐rays scatter from all the atoms in the solution sample, solutes as well as the solvent, the analysis of TRXL data can provide the temporal behavior of the solvent as well as the structural progression of all the solute molecules in all the reaction pathways, thus providing a global picture of the reactions and accurate branching ratios between multiple reaction pathways. The arrangement of the solvent around the solute molecule can also be extracted. This review summarizes recent developments in TRXL, including technical innovations in synchrotron beamlines and theoretical analysis of TRXL data, as well as several examples from simple molecules to an organometallic complex, nanoparticles, and proteins in solution. Future potential applications of TRXL in femtosecond studies and biologically relevant molecules are also briefly mentioned.     

In Situ Three‐Dimensional Synchrotron X‐Ray Nanotomography of the (De)lithiation Processes in Tin Anodes

The three‐dimensional quantitative analysis and nanometer‐scale visualization of the microstructural evolutions of a tin electrode in a lithium‐ion battery during cycling is described. Newly developed synchrotron X‐ray nanotomography provided an invaluable tool. Severe microstructural changes occur during the first delithiation and the subsequent second lithiation, after which the particles reach a structural equilibrium with no further significant morphological changes. This reveals that initial delithiation and subsequent lithiation play a dominant role in the structural instability that yields mechanical degradation. This in situ 3D quantitative analysis and visualization of the microstructural evolution on the nanometer scale by synchrotron X‐ray nanotomography should contribute to our understanding of energy materials and improve their synthetic processing.     

The Reactivity of Cr‐Te Superlattice Reactants and of Co‐deposited Cr‐Te Films: Studies with in‐situ X‐ray Diffractometry

The reactivity of a Cr‐Te multilayer film consisting of 326 Cr/Te double‐layers with about 88(3) at% Te was investigated with in‐situ X‐ray diffraction. The occurrence of a superstructure reflection caused by the multilayer system is observed. During the annealing procedure between 80 and 90 °C the layer‐by‐layer arrangement is destroyed and elemental Te crystallizes. After 6 h isothermal annealing at 140 °C CrTe₃ starts to crystallize. The amount and the crystal size of CrTe₃ increase with raising temperature and Te is consumed during the growth process. At 220 °C the formation of crystalline Cr₂Te₃ is observed and between 220 and 260 °C CrTe₃ is completely decomposed into Cr₂Te₃. The two Cr tellurides grow as highly textured materials. The size of CrTe₃ crystals is significantly larger than that of Cr₂Te₃. For a co‐deposited Cr‐Te film with composition 77(3) at% Te the tri‐telluride and elemental Te crystallize simultaneously. The texture of CrTe₃ is less pronounced and different reflections of the tri‐telluride occur besides the (h00)‐reflections. Up to 160 °C a pronounced increase of the crystallite size is observed and the crystals are significantly larger than those obtained with the multifilm sample. At 130 °C the crystallization kinetics seem to be controlled by nucleation.     

The Crystal Structure of Trimethyl Borate by Neutron and X‐ray Powder Diffraction

The crystal structure of one of the simplest organoboron compounds, trimethyl borate does not appear to have been determined hitherto. The compound is of interest for the study of π‐donor ligands and their interaction with the π‐acceptor behavior of trigonal boron and the consequences of such interactions on molecular structure. We used powder neutron (with isotopically labeled material) and X‐ray diffraction to determine the crystal structure of trimethyl borate at 15 K and 200 K (neutron) and 200 K (X‐ray). The material is hexagonal (Z = 2) with a = b = 6.950(8) Å and c = 6.501(3) Å at 15 K. The unit cell volume is 272.00(1) ų. The space group is P6₃/m (SG 176) at 15 K and 200 K. This is the first crystal structure solved on the Neutron Powder Diffractometer (NPDF) at the Lujan Center.     

Aspherical‐Atom Modeling of Coordination Compounds by Single‐Crystal X‐ray Diffraction Allows the Correct Metal Atom To Be Identified

Single‐crystal X‐ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid‐state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical‐atom least‐squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld‐atom refinement (Acta Crystallogr. Sect. A­ 2008 , 64, 383–393; IUCrJ. 2014 , 1,61–79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B­ 2013 , 69, 91–104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear‐coordinate 3d metal complexes, for which the wrong element is found if standard independent‐atom model scattering factors are relied upon, are studied, and it is shown that only aspherical‐atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed.     

Characterization of a Multicomponent Lithium Lithiate from a Combined X‐Ray Diffraction,NMR Spectroscopy,and Computational Approach

An unusual lithium lithiate [Li(diglyme)₂][(diglyme)Li₂(C₄H₃S)₃], made up from three carbanions, two lithium cations, and a single donor base molecule in the anion and a single lithium cation, coordinated by two donor base molecules, is investigated in a combined study including X‐ray diffraction, NMR spectroscopy and computational approaches in solution and the solid state. While the multicomponent lithiate is the only species present in the solid state, solution NMR spectroscopy and computational methods were employed to identify a second species in solution. The dimer [(diglyme)Li(C₄H₃S)]₂ coexists with the lithiate in solution in a 1:1 ratio, the more the higher the polarity of the solvent is. Only the combination of this multitude of methods provides a firm picture of the whole.