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Many crystalline solids cannot be prepared in the form of single crystals of sufficient size and/or quality for investigation using single‐crystal X‐ray diffraction techniques, and the opportunity to carry out structure determination using powder diffraction data is therefore essential to understand the structural properties of such materials. Although the refinement stage of the structure determination process can be carried out fairly routinely from powder diffraction data using the Rietveld profile refinement technique, solving crystal structures directly from powder data is associated with several intrinsic difficulties. Nevertheless, substantial progress has been made in recent years in the scope and potential of techniques in this field. This article aims to highlight the types of structural problems for which structure determination may now be tackled directly from powder diffraction data, and contemporary applications across several chemical disciplines are presented. A brief survey of the underlying methodologies is given, with some emphasis on recently developed techniques for carrying out the structure‐solution stage of the structure‐determination process.

A wide range of important crystalline solids cannot be prepared in the form of single crystals of suitable size and quality for structural characterization by conventional single-crystal X-ray diffraction methods. The development of techniques for crystal structure determination from powder diffraction data is clearly important for allowing the structural characterization of such materials. Although the structure refinement stage of the structure determination process can now be carried out fairly routinely using the Rietveld profile refinement technique, structure solution directly from powder diffraction data is associated with several intrinsic difficulties. The article surveys the field of crystal structure determination from powder diffraction data. Particular emphasis is given to the challenging structure solution stage of the structure determination process, with illustrative case studies highlighting the features of each of the main methods that are currently used for structure solution from powder diffraction data. The current scope and future potential of powder diffraction as an approach for crystal structure determination are discussed, and contemporary applications of this approach across several disciplines within materials chemistry are reviewed.

Fluorescein has been widely used in biomolecular sciences, ophthalmology, and other scientific disciplines, since its first preparation more than a century ago."] There are various solid forms of fluorescein that exhibit a wide selection of properties, and differ substantially in color. Progress in understanding the properties of fluorescein in particular, and crystalline solids in general, depends on knowledge of their crystal structures; however, to our knowledge, the different forms of solid fluorescein can only be obtained as microcrystalline powders, thus prohibiting crystal structure determination by conventional single-crystal diffraction methods. IR spectroscopyfz1 and solid-state 3C NMR spectroscopyr3] indicate that the different forms of fluorescein can be grouped into three categories (Scheme 1): 1) red fluorescein with quinoid structure I, 2) yellow fluorescein with OH I 11 Scheme 1. HO Illzwitterionic structure 11, and 3) colorless fluorescein with lactoid structure 111. We focus here on crystal structure determination of the red form of fluorescein from powder diffraction data. It has been reported previouslyrz*41 that samples of red fluorescein prepared by different methods give different powder diffraction patterns, and there has been much speculation[31 regarding the arrangement of fluorescein molecules in these different materials.As illustrated by fluorescein, many important crystalline materials cannot be prepared as single crystals of sufficient size and quality for single-crystal X-ray diffraction studies, and for these materials it is essential that structural information can be established from powder diffraction data.''] However, there are several intrinsic difficulties associated with solving crystal structures from powder diffraction data, originating primarily from extensive peak overlap in the powder diffractogram. This peak overlap limits the reliability of intensity information that can be extracted directly from the powder diffractogram, and, as the traditional approaches (e.g. for structure solution require accurate intensity data of this type, the success of these approaches can be severely restricted. The solution of the crystal structure for "equal-atom'' organic molecular solids (that is those containing only C, H, N, or 0) from X-ray powder diffraction data presents the greatest challenges, as there is substantially less data of significant intensity at high diffraction angles and, for these materials, a substantial fraction (at least 50%) of the atoms in the structure must generally be located correctly in the structure solution stage in order for subsequent structure refinement to be successful. As a consequence of these challenges, the structures of only a few previously-unknown "equal-atom'' compounds have been determined from powder diffractionIn all but one@' of these cases, prior knowledge of the molecular geometry was exploited by use of a rigid molecular fragment in the structure solution calculation. Clearly the extension to nonrigid molecular fragments is essential...

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Calcium phosphate cements (CPCs) are usually modified by organic/inorganic additives to improve their mechanical performance and to adjust their rheological and setting properties to clinical requirements. In this work we used several nontoxic and biocompatible R-hydroxylated organic acids (glycolic, lactic, malic, tartaric, and citric acids) and their calcium and sodium salts for the modification of CPC. The unmodified cement used in this study consisted of an equimolar powder mixture of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) mixed with water at a powder mass/liquid volume ratio of 3.3 g/mL. It had a compressive strength of 38 MPa and an initial setting time of 8 min. The free acids as cement liquids had mainly a detrimental effect on the strength of the cement and led to a decreased setting time around 2-4 min, while the calcium salts did not significantly alter the cement properties. However, the sodium salts of the oligocarboxylic acids (malic, tartaric, and citric acids) resulted in a liquefying effect combined with a strong reinforcement of the mechanical strength, such that compressive strength increased to 78-99 MPa. The liquefying effect and prolonged setting time of these compounds was thought to derive from a strong increase in the surface charge of both reactants and the reaction product hydroxyapatite as determined by ζ potential, which increased from about -15 and -18 mV in pure water for TTCP and DCPA, respectively, to values around -40 to -50 mV. In contrast, the calcium salts did not alter ζ potentials due to the formation of neutral and stable complexes in aqueous solution.

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A simultaneous experimental and computational search for polymorphs of chlorothalonil (2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile) has been conducted, leading to the first characterization of forms 2 and 3. The crystal structure prediction study, using a specifically developed anisotropic atom-atom potential for chlorothalonil, gave as the global minimum in the lattice energy a structure that was readily refined against powder diffraction data to the known form 1 (P2(1)/a). The structure of form 2 was solved and refined from powder diffraction data, giving a disordered structure in the Rm (166) space group (Z = 3). It could also be refined against a P1 ordered model, starting from a low-energy hypothetical sheet structure found in the computational search. This shows that the disorder could be associated with the stacking of ordered sheets. The disordered structure for form 2 was later confirmed by single-crystal X-ray diffraction. The structure of form 3, determined from single-crystal diffraction, contains three independent molecules in the asymmetric unit in P2(1) (4) (Z = 6). Powder diffraction showed that this single-herringbone structure was similar to two low-energy structures found in the search. Further analysis confirmed that form 3 has a similar lattice energy and contains elements from both these predicted structures, which can be considered as good approximations to the form 3 structure.