Ab Initio Structure Solution Research Articles

New Findings on the Crystal Polymorphism of Imepitoin.

Scientific and industrial reasons dictate the study of the solid state of imepitoin, a highly safe and tolerable anticonvulsant drug used in the therapy of epileptic dogs that was approved in the Europe Union in 2013. Our investigations allowed us to discover the existence of a new polymorph of imepitoin, which finds itself in a monotropic relationship with the crystalline form (polymorph I) already known and present on the market. This form (polymorph II), obtained by crystallization from xylene, remains metastable under ambient conditions for at least 1 year. Both solid forms were characterized by thermal (DSC and TGA), spectroscopic (FT-IR and Raman), microscopic (SEM and HSM), and diffractometric techniques. The thermodynamic relationship between the two polymorphs (monotropic) is such that it is not possible to study the melting of polymorph II, not even by adopting appropriate experimental strategies. Our measurements highlighted that the melting peak of imepitoin actually also includes an onset of melt decomposition. The ab initio structure solution, obtained from synchrotron X-ray powder diffraction data collected at room temperature, allowed us to determine the crystal structure of the new polymorph (II). It crystallizes in the monoclinic crystal structure, P21/c space group (#14), with a = 14.8687(6) Å, b = 7.2434(2) Å, c = 12.5592(4) Å, β = 107.5586(8)°, V = 1289.61(8) Å3, and Z = 4.

TiO2 II: The high‐pressure Zr‐free srilankite endmember in impact rocks

AbstractTiO2II, a high‐pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro‐to‐nanometer scale, there are no ab initio structure solutions of natural TiO2II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti0.67,Zr0.33)O2. Nanoscale electron diffraction investigation of TiO2II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α‐PbO2. The linear a and c decrease, and b increase with Ti content indicate TiO2II as Zr‐free srilankite endmember in the binary system ZrO2‐TiO2. Thereby the name srilankite should be used referring to TiO2II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile‐to‐TiO2II transition, founding their finely intermixing nanocrystals in the same TiO2 crystal, where TiO2II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso‐orientation, with TiO2II [100] parallel to rutile [100], TiO2II [010] parallel to rutile [011], and TiO2II [001] parallel to rutile (0–11). The rutile‐TiO2II iso‐orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO2II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO2II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post‐shock temperatures below 500°C followed by rapid quenching.

Scanning Precession Electron Tomography (SPET) for Structural Analysis of Thin Films along Their Thickness

Accurate structure analysis of epitaxial perovskite thin films is a fundamental step towards the ability to tune their physical properties as desired. Precession-assisted electron diffraction tomography (PEDT) has proven to be an effective technique for performing ab initio structure solutions and refinements for this class of materials. As the film thickness or the region of interest (ROI) decrease in size, the capacity to collect PEDT data with smaller electron beams is a key parameter and ROI tracking becomes a major issue. To circumvent this problem, we considered here an alternative approach to acquiring data by combining PEDT with a scan over an area, extracting the intensities collected at different positions and using them to perform accurate structure refinements. As a proof of concept, a Scanning Precession Electron Tomography (SPET) experiment is performed on a 35 nm thick perovskite PrVO3(PVO) film deposited on a SrTiO3 (STO) substrate. This way, it was possible to detect small changes in the PVO structure along the film thickness, from the variation in unit cell parameters to atomic positions. We believe that SPET has the potential to become the standard procedure for the accurate structure analysis of ROIs as small as 10 nm.

True molecular conformation and structure determination by three-dimensional electron diffraction of PAH by-products potentially useful for electronic applications.

The true molecular conformation and the crystal structure of benzo[e]dinaphtho[2,3-a;1',2',3',4'-ghi]fluoranthene, 7,14-diphenylnaphtho[1,2,3,4-cde]bisanthene and 7,16-diphenylnaphtho[1,2,3,4-cde]helianthrene were determined ab initio by 3D electron diffraction. All three molecules are remarkable polycyclic aromatic hydrocarbons. The molecular conformation of two of these compounds could not be determined via classical spectroscopic methods due to the large size of the molecule and the occurrence of multiple and reciprocally connected aromatic rings. The molecular structure of the third molecule was previously considered provisional. These compounds were isolated as by-products in the synthesis of similar products and were at the same time nanocrystalline and available only in very limited amounts. 3D electron diffraction data, taken from submicrometric single crystals, allowed for direct ab initio structure solution and the unbiased determination of the internal molecular conformation. Detailed synthetic routes and spectroscopic analyses are also discussed. Based on many-body perturbation theory simulations, benzo[e]dinaphtho[2,3-a;1',2',3',4'-ghi]fluoranthene may be a promising candidate for triplet-triplet annihilation and 7,14-diphenylnaphtho[1,2,3,4-cde]bisanthene may be a promising candidate for intermolecular singlet fission in the solid state.

Structure Determination Feasibility of Three-Dimensional Electron Diffraction in Case of Limited Data

During the last two decades, three-dimensional electron diffraction (3D ED) has undergone a renaissance, starting with the introduction of precession (Precession Electron Diffraction Tomography, PEDT) that led to variations on the idea of collecting as much of the diffraction space as possible in order to solve crystal structures from sub-micron sized crystals. The most popular of these acquisition methods is based on the continuous tilting/rotation of the crystal (so-called Microcrystal Electron Diffraction, MicroED) akin to the oscillating crystal method in X-ray crystallography, which was enabled by the increase of sensitivity and acquisition speed in electron detectors. While 3D ED data is more complex than the equivalent X-ray data due to the higher proportion of dynamical scattering, the same basic principles of what is required in terms of data quality and quantity in order to solve a crystal structure apply; high completeness, high data resolution and good signal-to-noise statistics on measured reflection intensities. However, it may not always be possible to collect data in these optimum conditions, the most common limitations being the tilt range of the goniometer stage, often due to a small pole piece gap or the use of a non-tomography holder, or the position of the sample on the TEM grid, which may be too close to a grid bar and then the specimen of interest becomes occluded during tilting. Other factors that can limit the quality of the acquired data include the limited dynamic range of the detector, which can result on truncated intensities, or the sensitivity of the crystal to the electron beam, whereby the crystallinity of the particle is changing under the illumination of the beam. This limits the quality and quantity of the measured intensities and makes structure analysis of such data challenging. Under these circumstances, traditional approaches may fail to elucidate crystal structures, and global optimization methods may be used here as an alternative powerful tool. In this context, this work presents a systematic study on the application of a global optimization method to crystal structure determination from 3D ED data. The results are compared with known structure models and crystal phases obtained from traditional ab initio structure solution methods demonstrating how this strategy can be reliably applied to the analysis of partially complete 3D ED data.

Wodegongjieite, ideally KCa3(Al7Si9)O32, a new sheet silicate isostructural with the feldspar polymorph kokchetavite, KAlSi3O8

AbstractWodegongjieite occurs in the Cr-11 chromitite orebody of the Luobusa ophiolite in the Kangjinla district, Tibet, China. It is found in two inclusions in corundum: (1) as a partial overgrowth (holotype) up to 1.5 μm thick around a spheroid 20 μm across of wenjiite (Ti10(Si,P,□)7), kangjinlaite (Ti11(Si,P)10), zhiqinite (TiSi2) and badengzhuite (TiP), and (2) as pools up to 0.25 μm wide filling interstices between wenjiite, jingsuiite (TiB2), osbornite–khamrabaevite (Ti[N,C]) and corundum. Energy dispersive analyses gave Al2O3 34.09, SiO2 49.11, K2O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K0.58Sr0.26Ca2.25Al7.20Si8.80O31.20, ideally KCa3(Al7Si9)O32, for Si + Al = 16 cations.Single-crystal studies were carried out with three-dimensional electron diffraction providing data for an ab initio structure solution in the hexagonal space group P6/mcc (#192) with a = 10.2(2) Å, c = 14.9(3) Å, V = 1340(50) Å3 and Z = 2. Density (calc.) = 2.694 g⋅cm–3. The refinement, which assumes complete Si–Al disorder, gives average T1–O and T2–O bond lengths both as 1.65 Å. It was not practical to use unconstrained refinement for the occupancies of the large cation sites 6f and 2a. The ab initio model shows clearly that the two cation sites have different sizes and coordination. Consequently, we imposed the condition (1) that all the K occupies the 2a site as the average K–O bond length of 3.07 Å is close to the average K–O bond lengths reported in kokchetavite and (2) that all the Ca occupies the 6f site as the average Ca–O bond length of 2.60 Å (2.36 Å and 2.84 Å for Ca–O1 and Ca–O3, respectively) is reasonable for Ca–O. Assuming that all K and all Ca are located at the 2a site and 6f site, respectively, Sr occupancies of these sites could be refined. Thermal parameters are positive and in a reasonable range. The structure is a sheet silicate isostructural with the K-feldspar polymorph kokchetavite, with two crystallographically distinct sites for K, but not with the topologically identical anorthite polymorph dmisteinbergite (CaAl2Si2O8) with only a single site for Ca. Substitution of K by Ca at the 6f site is associated with marked rotation of the Si,Al tetrahedra and a collapse of the structure to accommodate the smaller Ca ion.The spheroid of intermetallic phases is believed to have formed from the interaction of mantle-derived CH4 + H2 fluids with basaltic magmas at depths of ~30–100 km, resulting in precipitation of corundum that entrapped intermetallic melts. Associated immiscible silicate melt of granodioritic composition crystallised metastably to wodegongjieite instead of a mixture of anorthite and K-feldspar.

Structure determination of intermetallic phases in bulk Al alloys by 3D electron diffraction tomography

Secondary phases in bulk metallic samples are difficult to characterize by conventional X-ray diffraction because the diffracted intensities are dominated by the contribution of the bulk material. Electron diffraction provides a better spatial resolution and one can obtain single crystal diffraction data from submicron sized precipitates. In this work we show that 3D electron diffraction tomography can yield diffraction data sets suitable for ab initio structure solution. We apply the recently proposed low-dose electron diffraction tomography method to 3 different phases in Al-based alloys: θ’-Al 2 Cu, Al 6 Fe and Al 4.5 FeSi. The structures of these phases were unambiguously determined even though there are other phases with similar chemical compositions and EDS analysis is not sufficient to distinguish them. • Intermetallics’ chemical composition can be too similar to be distinguished by EDS. • Intermetallics’ unit cell parameters can be too similar to be distinguished by SAED. • Precipitate phases can be unambiguously identified by 3D electron diffraction. • 3 phases of increasing complexity are identified.

Wenjiite, Ti10(Si,P,☐)7, and kangjinlaite, Ti11(Si,P)10, new minerals in the ternary Ti-P-Si system from the Luobusa ophiolite, Tibet, China

Abstract The new minerals wenjiite, Ti10(Si,P,☐)7 (IMA2019-107c) and kangjinlaite, Ti11(Si,P)10 (IMA2019-112b) occur with badengzhuite, zhiqinite, and a K-bearing dmisteinbergite-like mineral in a spheroid 20 μm across enclosed in corundum from the Cr-11 podiform chromitite orebody near the Kangjinla, Luobusa ophiolite, Tibet, China. In addition, wenjiite occurs with deltalumite, jingsuiite, osbornitekhambaraevite, and the K-bearing dmisteinbergite-like mineral in a lamellar intergrowth 100 μm long, also enclosed in corundum from the same locality. The new minerals were characterized by energy-dispersive spectroscopy and three-dimensional electron diffraction, which enabled us to obtain an ab initio structure solution and dynamical refinement from grains a few micrometers across hosted in a FIB lamella. Four analyses of wenjiite from the spheroid gave in wt% Si 21.67, P 6.24, Ti 66.39, V 1.37, Cr 2.20, Mn 0.97, and Fe 1.17 (normalized to 100), which corresponds to (Ti0.93Cr0.03Mn0.01Fe0.01V0.02)10 (Si0.79P0.21)6.51 on the basis of 10 cations excluding Si and P. The simplified formula is Ti10(Si,P)6.5, or more generally Ti10SixPy, where x > y and 6 ≤ (x + y) ≤ 7, i.e., Ti10(Si,P,☐)7. Wenjiite has hexagonal symmetry, space group: P63/mcm (no. 193), with a = 7.30(10) Å, c = 5.09(10) Å, V = 235(6) Å3, Z = 1, and is isostructural with xifengite, mavlyanovite, synthetic Ti5Si3, and synthetic Ti5P3.15. Four analyses of kangjinlaite gave in wt% Si 25.56, P 9.68, Ti 62.35, V 0.21, Cr 0.83, Mn 0.42, and Fe 0.95 (normalized to 100), which corresponds to (Ti10.65V0.03Cr0.13Mn0.06Fe0.14)Σ11.01(Si7.43P2.55)Σ9.99. The simplified formula is Ti11(Si,P)10. Kangjinlaite is tetragonal, with space group: I4/mmm (no. 139), a = 9.4(2) Å, c = 13.5(3) Å, V = 1210(50) Å3, Z = 4, and is isostructural with synthetic compounds of the Ho11Ge10 type, being the most compact of these phases. Despite there now being over 70 compounds containing 38 elements isostructural with Ho11Ge10, synthesis of an analog of kangjinlaite has not been previously reported in either the Ti-P or Ti-Si binary systems or in a multicomponent system. The previously deduced crystallization sequence with decreasing temperature of the four minerals in the spheroid is wenjiite → kangjinlaite → zhiqinite + badengzhuite. This sequence is consistent with relationships reported in 9 binary systems containing intermetallic compounds of Ge and Sn isostructural with Mn5Si3 and Ho11Ge10. In eight of these systems the Mn5Si3 analog melts congruently, whereas the Ho11Ge10 analog never does. Instead, the Ho11Ge10 analog melts peritectically, generally to an Mn5Si3 analog and less commonly to compounds with 5:4 stoichiometry. Final crystallization of the spheroid to zhiqinite + badengzhuite is expected to be well below the temperature of 1500 °C for the congruent melting of zhiqinite in the Ti-Si system, i.e., in the range of ~1100–1300 °C.

Chemical crystallography by serial femtosecond X-ray diffraction

Inorganic–organic hybrid materials represent a large share of newly reported structures, owing to their simple synthetic routes and customizable properties1. This proliferation has led to a characterization bottleneck: many hybrid materials are obligate microcrystals with low symmetry and severe radiation sensitivity, interfering with the standard techniques of single-crystal X-ray diffraction2,3 and electron microdiffraction4–11. Here we demonstrate small-molecule serial femtosecond X-ray crystallography (smSFX) for the determination of material crystal structures from microcrystals. We subjected microcrystalline suspensions to X-ray free-electron laser radiation12,13 and obtained thousands of randomly oriented diffraction patterns. We determined unit cells by aggregating spot-finding results into high-resolution powder diffractograms. After indexing the sparse serial patterns by a graph theory approach14, the resulting datasets can be solved and refined using standard tools for single-crystal diffraction data15–17. We describe the ab initio structure solutions of mithrene (AgSePh)18–20, thiorene (AgSPh) and tethrene (AgTePh), of which the latter two were previously unknown structures. In thiorene, we identify a geometric change in the silver–silver bonding network that is linked to its divergent optoelectronic properties20. We demonstrate that smSFX can be applied as a general technique for structure determination of beam-sensitive microcrystalline materials at near-ambient temperature and pressure.

The Elusive Structure of Magadiite, Solved by 3D Electron Diffraction and Model Building

In addition to a great swelling ability, layered silicates also allow the functionalization of their interlayer region to form various robust green materials that are used as CO2 adsorbents, drug carriers, or catalysts. Here, the unique magadiite structure, which has resisted elucidation despite many attempts and applications the material offers, is finally described. A material-specific strategy allowed the use of 3D electron diffraction which led to the success of deciphering the atomic structure. In order to enable an ab initio structure solution of the electron beam sensitive material, a sodium-free dehydrated form of magadiite was synthetically isolated, and, from that, it was subsequently possible to derive a structure model for the sodium form of magadiite, later successfully refined against powder X-ray diffraction data. Furthermore, a geometry optimization and simulations of spectroscopic data with DFT methods confirm the obtained crystal structure of sodium magadiite. These results finally prompted a detailed description of the layers and of the chemically active interlayer region and provide a huge impact toward the design of new and more efficient materials based on functionalized magadiite and related structures.

The structure solution of the F-type i-ZnMgHf from the X-ray diffraction

ABSTRACTWe report a successful growth of the ZnMgHf F-type icosahedral quasicrystal in the form of faceted single grains. By varying the heat treatment parameters we were able to obtain single crystals with a quality suitable for a single crystal X-ray diffraction which was conducted in a synchrotron facility. Ab initio structure solution by a charge flipping algorithm manifests the existence of two types of Bergman clusters. Clusters are differentiated by the preferential occupation of the high-symmetry positions by hafnium in a rhombic triacontahedron, being the outer shell of the atomic cluster.This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.

Structural elucidation of microcrystalline MOFs from powder X-ray diffraction.

Metal organic frameworks (MOFs), also known as coordination polymers/networks, have experienced a rapid upsurge in the last 20 years in part due to the precise visualization of their atomic arrangement in the solid state. Structure-function relationship properties in MOFs is a key step to understand the potential of a material for its applications in advanced technologies, which can be only understood if full structure determination is carried out. Single crystal X-ray diffraction is the most reliable technique for the 3D description of the atomic arrangement of a crystalline material, but it needs a suitable single crystal both in size and in quality. Sadly, it often occurs that it is not possible to grow crystals of enough quality for single crystal XRD analysis. In MOF synthesis, rather often the products of certain synthetic approaches such as fast crystallization or mechanochemical reactions are obtained as powders. Also, gas, temperature, and solvent induced reactions render single crystals unsuitable for single crystal XRD. In such cases, structure elucidation of MOFs must be carried out using ab initio powder XRD analysis. Unfortunately, structure solution from powder XRD data is more complicated than that from single crystal XRD data. In this article, a short overview of crystal structure solution from powder XRD and how this technique has been applied in the structure solution of MOFs using direct-space strategy is given. Examples of microcrystalline MOFs obtained by solvothermal methods, synthesized by instant synthesis and mechanochemical methods and products obtained after solid-state reactivity are highlighted. The reported cases are challenging structure solution examples carried out by direct-space strategy using powder XRD data, and show that ab initio powder XRD structure solution is a powerful technique that allows many chemical reactions whose products cannot be carried out by single crystal X-ray analysis to be understood. Hopefully, the strength of ab initio powder XRD structure elucidation with the few cases shown in this Perspective will encourage members of this field of research to exploit this technique to make further progress in MOF chemistry.

Ab initio structure solution of a proteolytic fragment using ARCIMBOLDO.

Crystal structure determination requires solving the phase problem. This can be accomplished using ab initio direct methods for small molecules and macromolecules at resolutions higher than 1.2 Å, whereas macromolecular structure determination at lower resolution requires either molecular replacement using a homologous structure or experimental phases using a derivative such as covalent labeling (for example selenomethionine or mercury derivatization) or heavy-atom soaking (for example iodide ions). Here, a case is presented in which crystals were obtained from a 30.8 kDa protein sample and yielded a 1.6 Å resolution data set with a unit cell that could accommodate approximately 8 kDa of protein. Thus, it was unclear what had been crystallized. Molecular replacement with pieces of homologous proteins and attempts at iodide ion soaking failed to yield a solution. The crystals could not be reproduced. Sequence-independent molecular replacement using the structures available in the Protein Data Bank also failed to yield a solution. Ultimately, ab initio structure solution proved successful using the program ARCIMBOLDO, which identified two α-helical elements and yielded interpretable maps. The structure was the C-terminal dimerization domain of the intended target from Mycobacterium smegmatis. This structure is presented as a user-friendly test case in which an unknown protein fragment could be determined using ARCIMBOLDO.

Professional

During the plenary session of the 66th Annual Denver X-Ray Conference, the following awards were presented. The 2017 Jenkins Award, recognizing lifetime achievement in the advancement of the use of X-rays in materials analysis, was presented to James Kaduk, Polycrystallography, Inc. and Illinois Institute of Technology, for his broad achievements in discovery, research, and education in the use of X-rays for materials analysis. The 2017 Barrett Award, in recognition of outstanding contributions to powder diffraction, was presented to Daniel Louer, France, for his involvement with powder diffraction in indexing, line profile analysis, and ab initio structure solution, as well as his service to the community through ICDD and EPDIC. Dr. Louer was unable to attend DXC and will accept the award at the ICDD Annual Meetings in 2018. The 2017 Jerome B. Cohen Student Award, recognizing outstanding achievements of student research in X-ray analysis, was won by Howard Joress, Cornell University, for his work, A Polycapillary-Based Method of Monochromatic Time-Resolved X-ray Reflectivity. Howard presented his work at the conference during the General XRD session. For further information, visit the Web page, http://www.dxcicdd.com/ The recipient of the 12th Asada Award, which is presented by the Discussion Group of X-ray analysis, Japan, in memory of the late Professor Ei-ichi Asada (1924–2005) to promising young scientists in X-ray analysis fields in Japan, is Dr. Koji Nakanishi (Ritsumeikan University, Kusatsu, Shiga), “Advanced instrumentation in soft-X-ray absorption spectroscopy for innovative battery research.” The ceremony was held during the 53rd Annual Conference on X-Ray Chemical Analysis, at Tokushima University, Tokushima. For additional news about X-ray analysis and other spectroscopy sciences, please browse the Wiley website. http://www.SpectroscopyNow.com

Cu(II) bifunctional (N,O,O′) coordination polymer: A case study for complex ab-initio crystal structure determination from PXRD data

The synthesis of a novel Cu(II) bifunctional (N,O,O′) coordination polymer, the Cu(PzHP) compound, has been carried out by coupling Cu(II) salts and the newely synthesized H2(PzHP) organic linker (H2(PzHP) = 1-[4-(1H) -pyrazolyl]-2-methyl-3-hydroxy-4(1H)-pyridinone). Despite to the numerous attempts done for the growth of suitable single crystals and to the synthetic trials carried out to ameliorate the crystallinity of the Cu(PzHP) coordination polymer, its diffraction pattern was always characterized by a discouraging, less-than-ideal, crystallinity. Notwithstanding the extended disorder, leading to a higher than required space group symmetry, we succeeded in the description of its relevant structural features by following an unconventional route to ab-initio structure solution. In this process consideration on the coordination ability of the ligand and on the space group symmetry have been taken into account to ‘manually’ achieve the initial structural model, highlighting that, in some cases, it is still possible to ascertain by PXRD the crystal structure of a material that shows challenging, broad, PXRD pattern.

Polymorphism and Solid–Gas/Solid–Solid Reactions of Isonicotinic Acid, Isonicotinamide, and Nicotinamide Copper Chloride Compounds

The crystal structures of eight new copper chloride coordination complexes with substituted pyridines have been determined through a combination of single crystal X-ray diffraction and ab initio structure solution from X-ray powder diffraction data. The polymeric compounds [CuCl2(INAc)2]n (1), [CuCl2(INAm)2]n (2), and [CuCl2(NAm)2]n (3) (INAc = isonicotinic acid; INAm = isonicotinamide; NAm = nicotinamide) consist of chains of CuCl4 edge-sharing rhombi decorated by the pyridine-based ligands. All three compounds display similar hydrogen bonding interactions but differ in the arrangement of the polymeric chains. 1, 2, and 3 react reversibly with hydrochloric acid in solution/vapor form to produce the corresponding tetrachlorocuprate salts [CuCl4](H-INAc)2(H2O) (4a), [CuCl4](H-INAc)2 (4b), [CuCl4](H-INAm)2 (form I/form II 5a/5b), and [CuCl4](H-NAm)2 (form I/form II 6a/6b). 4a exhibits discrete square planar [CuCl4]2– ions, whereas the Cu in 4b, 5a, and 5b is in a 4 + 2 coordination. All four compounds displ...

Temperature- and moisture-dependent studies on alunogen and the crystal structure of meta-alunogen determined from laboratory powder diffraction data

Starting from a synthetic sample with composition Al2(SO4)3·16.6H2O, the high-temperature- and moisture-dependent behavior of alunogen has been unraveled by TGA measurements, in situ powder X-ray diffraction as well as by gravimetric moisture sorption/desorption studies. Heating experiments using the different techniques show that alunogen undergoes a first dehydration process already starting at temperatures slightly above 40 °C. The crystalline product of the temperature-induced dehydration corresponds to the synthetic equivalent of meta-alunogen and has the following chemical composition: Al2(SO4)3·13.8H2O or Al2(SO4)3(H2O)12·1.8H2O. At 90 °C a further reaction can be monitored resulting in the formation of an X-ray amorphous material. The sequence of “amorphous humps” in the patterns persists up to 250 °C, where a re-crystallization process is indicated by a sudden appearance of a larger number of sharp Bragg peaks. Phase analysis confirmed this compound to be anhydrous Al2(SO4)3. Furthermore, meta-alunogen can be also obtained from alunogen at room temperature when stored at relative humidities (RH) lower than 20 %. The transformation is reversible, however, water sorption of meta-alunogen to alunogen and the corresponding desorption reaction show considerable hysteresis. For RH values above 80 %, deliquescence of the material was observed. Structural investigations on meta-alunogen were performed using a sample that has been stored at dry conditions (0 % RH) over phosphorus pentoxide. Powder diffraction data were acquired on an in-house high-resolution diffractometer in transmission mode using a sealed glass capillary as sample holder. Indexing resulted in a triclinic unit cell with the following lattice parameters: a = 14.353(6) Å, b = 12.490(6) Å, c = 6.092(3) Å, α = 92.656(1)°, β = 96.654(1)°, γ = 100.831(1)°, V = 1062.8(8) Å3 and Z = 2. These data correct earlier findings suggesting an orthorhombic cell. Ab-initio structure solution in space group Poverline{1}, using simulated annealing, provided a chemically meaningful structure model. The asymmetric unit of meta-alunogen contains three symmetry independent SO4-tetrahedra and two Al(H2O)6 octahedra. The polyhedra are isolated, however, linkage between them is provided by Coulomb interactions and hydrogen bonding. In addition to the water molecules which directly belong to the coordination environment of the aluminum cations there are two additional zeolitic water sites (Ow1 and Ow2). If both positions are fully occupied meta-alunogen corresponds to a 14-hydrate. Structural similarities and differences between the previously unknown structure of meta-alunogen and alunogen are discussed in detail. Since hydrous aluminum sulfates have been postulated to occur in Martian soils, our results may help identifying meta-alunogen by X-ray diffraction not only on the surface of the Earth but also using the Curiosity Rover’s ChemMin instrument.

Advanced Transmission Electron Microscopy for Li-Ion Battery Cathodes

The design and improvement of the cathode materials for Li-ion batteries requires the detailed knowledge on the crystal structure at different charge/discharge states and comprehensive understanding of the processes occurring at the nanoscale or even atomic scale level, as many electrode materials demonstrate highly inhomogeneous non-equilibrium behavior. Advanced transmission electron microscopy (TEM) is by far the most suitable and direct tool to view materials down to atomic scale. Recent progress in the electron diffraction methods, related to implication of quantitative electron diffraction tomography, and in the aberration-corrected scanning TEM imaging will be illustrated here with the examples of atomic structure investigation of various cathode materials. Precession electron diffraction and electron diffraction tomography provide quantitative diffraction data with substantially suppressed dynamical effects, enabling reliable structure solution and refinement. In contrast to X-ray and neutron diffraction, the electron diffraction experiments require extremely small quantity of material, which is typically less than 1 mm3, making this method applicable to virtually all samples extracted from electrochemical cells. Electron diffraction patterns can be taken at a very low incoming electron dose, enabling investigation of the materials sensitive to the electron beam irradiation damage, such as polyanion and mixed-anion Li-ion battery cathodes, particularly in their charged state. The capability of quantitative electron diffraction in ab initio structure solution, locating Li atoms and refining the occupancy of the Li positions will be demonstrated using the Li2CoPO4F, Li2FePO4F and LiMn0.5Fe0.5PO4 cathode materials [1]. Aberration-corrected scanning transmission electron microscopy (STEM) techniques deliver the information on the local structural state with sub-angstrom resolution. High angle annular dark field STEM (HAADF-STEM) imaging provides clear visualization of the cation positions, whereas annular bright field STEM (ABF-STEM) shows the location of the “light” elements, such as O and Li. HAADF-STEM method has been applied to investigate the capacity and voltage fading in the layered rock-salt type oxides, which are determined to large degree by the cumulative local structure changes upon continuous electrochemical cycling. A comparative HAADF-STEM study of the layered oxides in the Li-Ru-Ti-O and Li-Ru-Sn-O systems at different stages (pristine, fully charged, discharged and cycled over different number of times) allowed establishing the cation migration pathways during charge-discharge processes and identifying the cation traps responsible for the degradation of the electrochemical performance [2]. Clear visualization of the structure modifications in the oxygen sublattice upon Li extraction is delivered by the ABF-STEM method enabling direct observation of O-O peroxo dimers in Li0.5IrO3 and O vacancy formation in LixFe0.56TeO6 [3,4] and helping to establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides. The figure shows [001] HAADF-STEM and ABF-STEM images of Li0.5IrO3 and the enlarged ABF-STEM image demonstrating the alternating O-O pairs with long and short (marked with dumbbells) projected distances. These O-O dimers arise from twisting the opposite triangular faces of the IrO6 octahedra (shown in yellow). 1. J.Hadermann, A.M.Abakumov, S.Turner, Z.Hafideddine, N.R.Khasanova, E.V.Antipov, G.Van Tendeloo, Chem. Mater., 23, 3540 (2011). 2. M.Sathiya, A.M.Abakumov, D.Foix, G.Rousse, K.Ramesha, M.Saubanère, M.L.Doublet, H.Vezin, C.P.Laisa, A.S.Prakash, D.Gonbeau, G.Van Tendeloo, J-M. Tarascon, Nature Mater., 14, 230 (2015). 3. E.McCalla, A.S.Prakash, E.Berg, M.Saubanere, A.M.Abakumov, D.Foix, B.Klobes, M.-T.Sougrati, G.Rousse, F.Lepoivre, S.Mariyappan, M.-L.Doublet, D.Gonbeau, P.Novak, G.Van Tendeloo, R.P.Hermann, J.-M.Tarascon, J. Electrochem. Soc., 162, A1341 (2015). 4. E.McCalla, A.M.Abakumov, M.Saubanère, D.Foix, E.J.Berg, G.Rousse, M.-L.Doublet, D.Gonbeau, P.Novák, G.Van Tendeloo, R.Dominko, J.-M.Tarascon, Science, 350, 1516 (2015). Figure 1

Structural insights into M2O-Al2O3-WO3 (M = Na, K) system by electron diffraction tomography.

The M2O-Al2O3-WO3 (M = alkaline metals) system has attracted the attention of the scientific community because some of its members showed potential applications as single crystalline media for tunable solid-state lasers. These materials behave as promising laser host materials due to their high and continuous transparency in the wide range of the near-IR region. A systematic investigation of these phases is nonetheless hampered because it is impossible to produce large crystals and only in a few cases a pure synthetic product can be achieved. Despite substantial advances in X-ray powder diffraction methods, structure investigation on nanoscale is still challenging, especially when the sample is polycrystalline and the structures are affected by pseudo-symmetry. Electron diffraction has the advantage of collecting data from single nanoscopic crystals, but it is frequently limited by incompleteness and dynamical effects. Automated diffraction tomography (ADT) recently emerged as an alternative approach able to collect more complete three-dimensional electron diffraction data and at the same time to significantly reduce dynamical scattering. ADT data have been shown to be suitable for ab initio structure solution of phases with large cell parameters, and for detecting pseudo-symmetry that was undetected in X-ray powder data. In this work we present the structure investigation of two hitherto undetermined compounds, K5Al(W3O11)2 and NaAl(WO4)2, by a combination of electron diffraction tomography and precession electron diffraction. We also stress how electron diffraction tomography can be used to obtain direct information about symmetry and pseudo-symmetry for nanocrystalline phases, even when available only in polyphasic mixtures.

Put Your Trust in Powder: Negative and Zero Thermal Expansion Materials

Powder diffraction is one of the most powerful structural probes available to the materials chemist. It invariably plays a critical role in the preparation of new phases and is increasingly exploited via in-situ studies to understand and control complex synthetic routes to fleetingly stable materials. Ab initio structure solution followed by Rietveld refinement frequently provides the first structural information on new functional materials, and powder diffraction is often the method of choice for probing structure-property relationships under non-ambient or in-operando conditions, or for following the structures of samples undergoing structural phase transitions. In this presentation I'll show how powder diffraction has been crucial in understanding the properties of so-called negative thermal expansion (NTE) materials – here inorganic oxides which contract on heating. Powder methods have provided key information on the thermodynamic stability of these materials, the low temperature synthetic routes required to prepare them, the average and local distortions that lead to NTE, and on the often complex phase transitions they undergo. I'll also discuss how powder diffraction can probe glass-like relaxation processes which occur over remarkably long timescales in some of these materials and how the synthetic control enabled by in-situ studies has led to the preparation of single-phase isotropic materials whose expansion properties can be systematically tuned from negative to zero to positive values (alpha –8 to +6 * 10–6 K–1) [1]. I'll emphasise how new techniques such as Parametric- [2] and Symmetry-Mode-Rietveld [3] refinement has been crucial in the study of these materials.