Global Rietveld Refinement

Global optimisation methods of structure determination from powder diffraction data have risen to prominence in a relatively short space of time and they now constitute a key approach in the examination of polycrystalline molecular organic materials. A correctly formulated global optimisation approach may be regarded as a “global Rietveld refinement” that is capable of delivering accurate crystal structures from high-quality powder diffraction data. This paper focuses on how accuracy at all stages of a powder diffraction experiment impacts upon the overall structure solution process and particular attention is paid to assessing the degree of accuracy with which structures are returned from the global optimisation process.     

Crystal structure solution and prediction via global and local optimization

Simulations machinery broadly available today allows direct-space solutions of crystal structures based on high quality powder diffraction data for systems with up to some 15–20 degrees of freedom. Analogous machinery for predicting new structures is rarely unsuccessful, but has had limited systematic application. The prime challenges for structure solution are for degrees of freedom beyond some 15–20, and for powder diffraction data which is of insufficient quality to allow an unambiguous indexing. In structure prediction, the corresponding issues are (i) enumeration strategies, (ii) collated access to the results of prior predictions, (iii) relating results in some fashion to likelihood of occurrence, encompassing finite temperature issues, and (iv) rational approaches to the synthesis of designed structures. Recent progress in the field has been steady and, while solutions to these key problems are not immediately obvious, the pace of progress over the past 5 years suggests major headway is imminent in a number of these areas.     

Anisotropic lattice distortions in biogenic aragonite

Composite biogenic materials produced by organisms have a complicated design on a nanometre scale. An outstanding example of organic-inorganic composites is provided by mollusc seashells, whose superior mechanical properties are due to their multi-level crystalline hierarchy and the presence of a small amount (0.1-5 wt%) of organic molecules. The presence of organic molecules, among other characteristics, can influence the coherence length for X-ray scattering in biogenic crystals. Here we show the results of synchrotron high-resolution X-ray powder diffraction measurements in biogenic and non-biogenic (geological) aragonite crystals. On applying the Rietveld refinement procedure to the high-resolution diffraction spectra, we were able to extract the aragonite lattice parameters with an accuracy of 10 p.p.m. As a result, we found anisotropic lattice distortions in biogenic aragonite relative to the geological sample, maximum distortion being 0.1% along the c axis of the orthorhombic unit cell. The organic molecules could be a source of these structural distortions in biogenic crystals. This finding may be important to the general understanding of the biomineralization process and the development of bio-inspired 'smart' materials.     

Structural characterization of luminescent Ca0.95Cd0.05S

Cadmium-doped calcium sulphide has great potential as a broadband light source in both powder and thin film electroluminescent devices. We now report the first structural investigation of polycrystalline cadmium-doped calcium sulphide using a combination of transmission electron microscopy (TEM) with microanalysis, and X-ray diffraction. We show that the crystals formed by direct reaction of CaS and CdS under excess sulphur do not yield an even distribution of cadmium. We also show that the crystals are heavily dislocated with defect structures typical of rock-salt structures. We show that TEM can be successfully applied to the study of these moisture-sensitive materials without the formation of artefacts and that TEM should be a valuable tool for studying their thin film structures.     

Three-dimensional distribution of polymorphs and magnesium in a calcified underwater attachment system by diffraction tomography

Biological materials display complicated three-dimensional hierarchical structures. Determining these structures is essential in understanding the link between material design and properties. Herein, we show how diffraction tomography can be used to determine the relative placement of the calcium carbonate polymorphs calcite and aragonite in the highly mineralized holdfast system of the bivalve Anomia simplex. In addition to high fidelity and non-destructive mapping of polymorphs, we use detailed analysis of X-ray diffraction peak positions in reconstructed powder diffraction data to determine the local degree of Mg substitution in the calcite phase. These data show how diffraction tomography can provide detailed multi-length scale information on complex materials in general and of biomineralized tissues in particular.     

Probing the structure of heterogeneous diluted materials by diffraction tomography

The advent of nanosciences calls for the development of local structural probes, in particular to characterize ill-ordered or heterogeneous materials. Furthermore, because materials properties are often related to their heterogeneity and the hierarchical arrangement of their structure, different structural probes covering a wide range of scales are required. X-ray diffraction is one of the prime structural methods but suffers from a relatively poor detection limit, whereas transmission electron analysis involves destructive sample preparation. Here we show the potential of coupling pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The demonstration is carried out on a high-pressure pellet containing several carbon phases and on a heterogeneous powder containing chalcedony and iron pigments. The present method enables a non-invasive structural refinement with a weight sensitivity of one part per thousand. It enables the extraction of the scattering patterns of amorphous and crystalline compounds with similar atomic densities and compositions. Furthermore, such a diffraction-tomography experiment can be carried out simultaneously with X-ray fluorescence, Compton and absorption tomographies, enabling a multimodal analysis of prime importance in materials science, chemistry, geology, environmental science, medical science, palaeontology and cultural heritage.     

Methodologies for structural investigations of organic lead halide perovskites

Fundamentally, organic lead halide perovskites (OLHPs), which have attracted tremendous attention in the field of photovoltaic technology, are the materials organized in the platform of a perovskite structure by hybridizing the organic component with the inorganic lead halide. In understanding the highly efficient photovoltaic performance of perovskite solar cells (PSCs), crystallographic investigation of OLHPs has been an accompanying field of study because opto-electronic properties are believed to be closely related to their crystal structure. This article reviews the progress of the structural investigations of OLHPs. Macro-scale structural analyses, including X-ray diffraction, are discussed to introduce the investigation methods for the overall properties of OLHPs. Subsequently, the progresses of the microstructural investigations, such as transmission electron microscopy, are described to understand the situation of the micro-scale crystallographic approaches that were achieved recently. In addition, electron damages, which are the obstacles for the structural investigation of OLHPs in electron microscopy, are discussed, and the methodologies alleviating the electron damages are introduced. Based on this review, we believe that the advances in structural investigations are crucial for further advancements in the research on the OLHPs for PSCs and related applications.     

硅氧烷化合物的合成与应用进展

基于硅氧键特点以及不同条件的化学反应是构建结构迥异、性能独特的新型有机/无机硅氧功能材料的重要方法,近年来,引起了学术界的普遍关注。新型硅氧功能材料兼具有机/无机化合物性质,以其良好的生物相容性、耐高低温性以及电绝缘性能被广泛应用于众多领域。本文综述了硅氧烷化合物设计、合成与应用的研究领域及发展现状,重点介绍线性结构(一维结构)、非线性结构(二维结构)、多面体低聚倍半硅氧烷化合物(三维结构)以及有机/无机杂化硅氧烷化合物的设计及合成方法,并通过研究可拉伸聚硅氧烷弹性体、硅氧烷化合物涂层、新型驱油用硅氧功能材料等多种方式以增进硅氧烷化合物在生物医学、航空航天、功能材料及三次采油方面的应用进展。     

晶体学在有机光电材料研究中的应用

基于有机分子的光、电、磁功能材料因其巨大的产业化前景,一直受到科技界的高度关注,已成为21世纪重要研究方向之一,也取得了一系列重大进展。有机光电功能材料的研究涉及物理、化学、材料、信息、电子乃至生物学等,是名副其实的交叉学科。在有机光电材料的研究中,一个重要的目标就是搞清材料结构与性能之间的关系,利用一系列先进的方法和工具,对材料的各级结构信息进行表征,找到材料特定性能与结构之间的内在联系,为材料的设计与改性提供必要的依据。晶体学作为一个已经诞生100年的方法,在这一进程中起到了关键的作用。从对材料作用机理的认识,到结构与功能关系的探讨,以及功能材料的定向设计,都需要晶体学知识和方法。结合作者课题组近年来在有机光电材料研究过程中一些比较有代表性的研究工作,阐述晶体学在探明材料的光电性能与结构之间的关系,特别是研究一些材料特殊性质的起源中不可或缺的作用。     

Organogels and Low Molecular Mass Organic Gelators

Nanostructured materials have many forms. One that has been somewhat neglected until recently is organogels, especially those whose three‐dimensional structural networks are based on the self‐assembly of low molecular‐mass organic gelators (LMOGs). These thermoreversible materials consist of a small amount of LMOG and an organic liquid. Because of the wide diversity of structures of molecules known to function as LMOGs and the dearth of direct information concerning how they pack in their gel assemblies, it has been difficult to decipher the salient features that constitute an LMOG. The stepwise simplification of LMOG structures and the development of methods to determine their packing in organogels at the micrometer‐to‐angstrom distance regimes are discussed for the simplest known LMOGs to the more complex, such as CEP (see Figure), which is known to form molecular wires when gelling chloroform. Work from the laboratory of the authors is emphasized. In addition, an overview of current and potential applications for these materials is presented.     

Modeling organic surfaces with self-assembled monolayers

The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long-chain thiols, HS(CH₂)_nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. More, complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecular scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest.     

Pressure effects on the structural and electronic properties of ABX4 scintillating crystals

Studies at high pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as, metals, semiconductors, superconductors, or minerals. In particular, the phase behaviour of ABX₄ scintillating materials is a challenging problem with many implications for other fields including technological applications and Earth and planetary sciences. A great progress has been done in the last years in the study of the pressure-effects on the structural and electronic properties of these compounds. In particular, the high-pressure structural sequence followed by these compounds seems now to be better understood thanks to recent experimental and theoretical studies. Here, we will review studies on the phase behaviour of different ABX₄ scintillating materials. In particular, we will focus on discussing the results obtained by different groups for the scheelite-structured orthotungstates, which have been extensively studied up to 50 GPa. We will also describe different experimental techniques for obtaining reliable data at simultaneously high pressure and high temperature. Drawbacks and advantages of the different techniques are discussed along with recent developments involving synchrotron X-ray diffraction, Raman scattering, and ab initio calculations. Differences and similarities of the phase behaviour of these materials will be discussed, on the light of Fukunaga and Yamaoka’s and Bastide’s diagrams, aiming to improve the actual understanding of their high-pressure behaviour. Possible technological and geophysical implications of the reviewed results will be also commented.     

磁性金属有机复合材料的合成与应用

金属有机框架材料(MOFs)是一种将金属离子中心与有机配体通过配位键结合起来的一类具有网格结构的材料。由于金属离子以及有机配体的多样性,MOFs的结构也具有多样性。磁性金属有机复合材料是一种新型的复合材料,既结合了MOFs的网状结构及结构多变性的优点,又结合了磁性材料易于分离且可重复利用的特性,使得这种材料在药物载体、多相催化、选择吸附等多种方面都有着较为广泛的应用。以经典的几类MOFs为分类依据,研究了它们与磁性材料结合形成新型复合材料的方法,同时概括了这些新型复合材料在不同领域的应用,最后提出了该材料目前所存在的问题,并对今后的研究方向进行了展望。     

Quantitative analysis of SiC polytype distributions by the Rietveld method

The quantitative determination of SiC polytype distributions is very difficult using traditional X-ray powder diffraction quantitative analysis methods, because the diffraction patterns of the various polytypes partially superimpose. The whole pattern fitting technique encapsulated in the Rietveld method is introduced to solve this problem. The detection limits for each polytype can be estimated based on their standard deviations. The results of both synthesized and experimental diffraction data show that the Rietveld method can give precise and accurate percentage compositions of the four most common SiC polytypes. This approach provides a practical tool to relate the preparation conditions to performance properties of SiC-based materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Perspective on Structural Evolution and Relations with Thermophysical Properties of Metallic Liquids

The relationship between the structural evolution and properties of metallic liquids is a long‐standing hot issue in condensed‐matter physics and materials science. Here, recent progress is reviewed in several fundamental aspects of metallic liquids, including the methods to study their atomic structures, liquid–liquid transition, physical properties, fragility, and their correlations with local structures, together with potential applications of liquid metals at room temperature. Involved with more experimentally and theoretically advanced techniques, these studies provide more in‐depth understanding of the structure–property relationship of metallic liquids and promote the design of new metallic materials with superior properties.     

Microstructural changes taking place during dynamic compaction of aluminium powders

The dynamic compaction of powders is characterized by a complex sequence of pressure loading and shear deformation, adiabatic temperature rise and structural annealing. The relative effects of these factors have been examined by studying the hardness and microstructure of compacted aluminium samples. Particle interiors are considerably shock-hardened, and then partially softened by dislocation recovery. The recovered structures appear to be relatively stable against further softening and recrystallization. The melted zones which occur as pore-filling between powder particles takes place are extremely hard and thermally stable. These zones possess a fine-grained structure after rapid resolidification, contain an oxide dispersion from the oxide films on the original powder surfaces, and are hardened by microvoids from the shrinkage occurring during solidification. The final material structures and properties are therefore variable throughout a given compacted samples, and depend sensitively on many aspects of the material, the powder and the consolidation conditions.     

Structural and optical properties of both pure poly(3-octylthiophene) (P3OT) and P3OT/fullerene films

We have investigated the structural and optical properties of P3OT and P3OT/fullerene thin films in view of their application as active layer in plastic solar cells. Films of these materials were prepared by spin coating from toluene solutions onto silicon substrates. Their optical properties were studied by spectroscopic ellipsometry, which provides the anisotropic dielectric function of the films. Moreover, structural properties were studied using X-ray diffraction. A close correlation between the results obtained by both methods could be found. Especially, the strong optical anisotropy of the films can be explained in terms of a preferable orientation of the polymer chains parallel to the substrate. The effect of the optical anisotropy on the performance of optoelectronic devices is discussed.     

Digital Light-Processing (DLP) 3D Printing of Magnesium Phosphate Minerals and Cements

Digital light processing (DLP) enables the fabrication of complex 3D structures based on a photopolymerizable resin usually containing a photo initiator and an UV or photo absorber. The resin and thus the final properties of the printed structures can be adjusted by adding fillers like bioceramic powders relevant for bone-regeneration applications. Herein, a water-based and biocompatible poly(ethylene glycol diacrylate) (PEGDA) resin containing the photo initiator lithium-phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) enables the production of 3D structures via DLP. The addition of calcium magnesium phosphate cement (CMPC) powder, acting as photo absorber, leads to higher accuracy of the final structures. After curing the printed construct in a diammonium–hydrogen phosphate (DAHP) bath for hardening, the resulting mechanical properties can be adjusted without post-process sintering. Solid loading of up to 40 wt% CMPC powder is possible, and the resins are investigated regarding their rheological behavior and printability. The resulting constructs are analyzed in respect to their surface morphology using scanning electron microscope (SEM), porosity, phase composition using X-ray diffraction (XRD) methods, as well as mechanical properties influenced by the hardening process using DAHP for different durations.     

One-pot polymorphism of nonlinear optical materials. First example of organic polytypes

Two nonlinear optical materials 1,1-dicyano-4-(4-dimethylaminophenyl)-1,3-butadiene (I) and 3-methoxydicyanovinylbenzene (II) have been investigated with regard to crystal growth, polymorphism, structure characterization and physical properties. It was found that both compounds form one-pot (concomitant) polymorphs, that might also be described as organic polytypes. Both polymorphs of compounds I and II form identical molecular layers with molecules located in the layer plane. In the two phases of compound I the layers superposition and their sequence differs significantly. This circumstance most probably determines the different color of the two crystal modifications. In the two crystal phases of compound II molecular layers and their superposition types are almost identical. The only difference between the phases is the sequence of the layer superposition. Single crystal and powder X-ray diffraction techniques, powder test for second harmonic generation, UV spectroscopy, and computational methods were used for characterization of these compounds.     

Image Formation by Magneto-optic Effects

The observation of magnetic domain structures by means of the Faraday and Kerr effect is described in terms of geometrical and physical optics; experimental techniques and theoretical explanations depend on the magnetic and optical properties of the investigated materials. Two methods are presented considering magnetization objects by means of Kirchhoff's diffraction theory. Starting from the calculation of intensities and polarization states of the diffraction by a magnetization grating, experiments are discussed concerning Fraunhofer diffraction and dark-field observation, artefacts in the image of domains, multiple diffraction by thick specimens, object functions available by magnetic structures and the applications for the investigation of magnetization reversal processes and data processing.