The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.

Crystalline Polymorphism of Organic Compounds

Bone is a hierarchically structured composite material which, in addition to its obvious biological value, has been well studied by the materials engineering community because of its unique structure and mechanical properties. This article will review the existing bone literature, with emphasis on the prevailing theories regarding bone formation and structure, which lay the groundwork for proposing a new model to explain how intrafibrillar mineralization of collagen can be achieved during bone formation. Intrafibrillar refers to the fact that growth of the mineral phase is somehow directed by the collagen matrix, which leads to a nanostructured architecture consisting of uniaxially oriented nanocrystals of hydroxyapatite embedded within and roughly [0 0 1] aligned parallel to the long collagen fibril axes. Secondary (osteonal) bone, the focus of this review, is a laminated organic–inorganic composite composed primarily of collagen, hydroxyapatite, and water; but minor constituents, such as non-collagenous proteins (NCPs), are also present and are thought to play an important role in bone formation. To date, there has been no clear understanding of the role of these NCPs, although it has been generally assumed that the NCPs regulate solution crystal growth via some type of ‘epitaxial’ relationship between specific crystallographic faces and specific protein conformers. Indeed, ‘epitaxial’ relationships have been calculated; but in practice, it has not been demonstrated that intrafibrillar mineralization can be accomplished via this route. Because of the difficulty in examining biomineralization processes in vivo , the authors of this article have turned to using in vitro model systems to investigate the possible physicochemical mechanisms that may be involved in biomineralization. In the case of bone biomineral, we have now been able to duplicate the most fundamental level of bone structure, the interpenetrating nanostructured architecture, using relatively simple anionic polypeptides that mimic the polyanionic character of the NCPs. We propose that the charged polymer acts as a process-directing agent, by which the conventional solution crystallization is converted into a precursor process. This polymer-induced liquid-precursor (PILP) process generates an amorphous liquid-phase mineral precursor to hydroxyapatite which facilitates intrafibrillar mineralization of type-I collagen because the fluidic character of the amorphous precursor phase enables it to be drawn into the nanoscopic gaps and grooves of collagen fibrils by capillary action. The precursor then solidifies and crystallizes upon loss of hydration waters into the more thermodynamically stable phase, leaving the collagen fibrils embedded with nanoscopic hydroxyapatite (HA) crystals. Electron diffraction patterns of the highly mineralized collagen fibrils are nearly identical to those of natural bone, indicating that the HA crystallites are preferentially aligned with [0 0 1] orientation along the collagen fibril axes. In addition, studies of etched samples of natural bone and our mineralized collagen suggest that the long accepted “deck of cards” model of bone's nanostructured architecture is not entirely accurate. Most importantly, this in vitro model demonstrates that a highly specific, epitaxial-type interaction with NCPs is not needed to stimulate crystal nucleation and regulate crystal orientation, as has long been assumed. Instead, we propose that collagen is the primary template for crystal organization, but with the important caveat that this templating occurs only for crystals formed from an infiltrated amorphous precursor. These results suggest that the 25-year-old debate regarding bone formation via an amorphous precursor phase needs to be revisited. From a biomedical perspective, in addition to providing possible insight into the role of NCPs in bone formation, this in vitro system may pave the way toward the ultimate goal of fabricating a synthetic bone substitute that not only has a composition similar to bone, but has comparable mechanical properties and bioresorptive potential as natural bone. From a materials chemistry perspective, the non-specificity of the PILP process and capillary infiltration mechanism suggests that non-biological materials could also be fabricated into nanostructured composites using this “biomimetic” strategy.

Bone structure and formation: A new perspective

The efficiency of the successive dichotomy method for powder diffraction pattern indexing [Louer & Louer (1972). J. Appl. Cryst. 5, 271–275] has been proved over more than 30 years of usage. Features implemented in the new version of the computer program DICVOL04 include (i) a tolerance to the presence of impurity (or inaccurately measured) diffraction lines, (ii) a refinement of the `zero-point' position, (iii) a reviewing of all input lines from the solution found from, generally, the first 20 lines, (iv) a cell analysis, based on the concept of the reduced cell, to identify equivalent monoclinic and triclinic solutions, and (v) an optional analysis of input powder data to detect the presence of a significant `zero-point' offset. New search strategies have also been introduced, e.g. each crystal system is scanned separately, within the input volume limits, to limit the risk of missing a solution characterized by a metric lattice singularity. The default values in the input file have been extended to 25 A for the linear parameters and 2500 A3 for the cell volume. The search is carried out exhaustively within the input parameter limits and the absolute error on peak position measurements. Many tests with data from the literature and from powder data of pharmaceutical materials, collected with the capillary technique and laboratory monochromatic X-rays, have been performed with a high success rate, covering all crystal symmetries from cubic to triclinic. Some examples reported as `difficult' cases are also discussed. Additionally, a few recommendations for the correct practice of powder pattern indexing are reported.

Powder pattern indexing with the dichotomy method

A detailed analysis of the microstructural properties of nanocrystalline zinc oxide powders produced by thermal decomposition of four different precursors (hydroxide nitrate, oxalate, hydroxide car

X-ray diffraction study of the early stages of the growth of nanoscale zinc oxide crystallites obtained from thermal decomposition of four precursors. General concepts on precursor-dependent microstructural properties

A novel approach to the determination of crystallite size and lattice strain by means of Total Pattern Analysis is described. Parameters to define the position, magnitude, breadth and shape of individual peaks are obtained by an adaptation of the pattern fitting program of Sonneveld and Visser (J. Appl. Cryst. 8, 1–7, 1975). A rapid assessment of the nature of the specimen broadening is given by a Williamson-Hall Plot. This leads to a more detailed study of line breadths by, for example, Voigt analysis applied to several orders of reflections or to single lines. Preliminary results are given for the application of this procedure to ‘size only’ and ‘size-strain’ samples of ZnO.

Applications of Total Pattern Fitting to a Study of Crystallite Size and Strain in Zinc Oxide Powder

Two new lanthanum sulfates have been synthesised under hydrothermal conditions in the presence of piperazine or ethylenediamine. The compounds have been structurally characterised from single-crystal X-ray diffraction. La2(H2O)2(C4H12N2)2(SO4)4, 1, and La2(H2O)2(C2H10N2)3(SO4)6·4H2O, 2, crystallise with monoclinic symmetry, space group P21/c. Compound 1 exhibits a three-dimensional anionic open-framework constituted by lanthanum atoms and sulfate groups. The charge compensation is ensured by the presence of piperazinium cations occluded in the cavities of the structure. Compound 2 has a layered structure, built from inorganic anionic sheets of lanthanum sulfates between which are located the ethylenediammonium cations and water molecules. The thermal behaviour of the two compounds has been studied by temperature-dependent powder X-ray diffraction and thermal analyses. Intermediates have been characterised during the dehydration stages. The decomposition temperature of the diamines is also discussed with regard to their hydrogen bonding to the framework. The products obtained at the final stages of the decomposition are La2(SO4)3 and La2O2SO4.

Two new diamine templated lanthanum sulfates, La2(H2O)2(C4H12N2)(SO4)4 and La2(H2O)2(C2H10N2)3(SO4)6·4H2O, with 3D and 2D crystal structures

The chemistry of thorium phosphate reported in the literature has been found to be erroneous. It was reconsidered in terms of careful chemical preparations and specific analytical methods. Special attention has been paid to the atom ratio value referred to r = thorium/phosphorus, which was experimentally fixed in order to obtain the correct composition of the final compound. A new compound with the chemical formula Th4(PO4)4P2O7, derived from the crystal structure determination, has been obtained. The unit cell parameters were obtained from powder and single-crystal X-ray diffraction data. It is orthorhombic (space group Pcam, Z = 2) with the cell dimensions a = 12.8646(9) A, b = 10.4374(8) A, c = 7.0676(5) A, and V = 949.00(9) A3. The atomic positions were derived from Patterson and Fourier methods and the structure was refined to an R value of 0.039. The structure consists of layers parallel to (010) containing both PO4 and P2O7 groups. These layers alternate with planes of Th atoms. The coordination sp...

Th4(PO4)4P2O7, a New Thorium Phosphate: Synthesis, Characterization, and Structure Determination

The microstructure of ZnO powder, obtained from thermal decomposition of the oxalate and studied previously by electron microscopy and adsorption calorimetry, was investigated by means of X-ray powder diffraction pattern decomposition. A Williamson–Hall plot revealed that some lines were broadened solely due to the effects of crystallite size, whereas other breadths included a contribution due to stacking faults. Spherical and cylindrical models are used to describe the form of the crystallites and procedures are presented for separating 'size' effects from `mistake' broadening. This leads to estimates of the mean dimensions of the crystallites and the stacking-fault probability. The analysis demonstrates that, with good-quality data for a large number of reflections, a considerable amount of detailed information can be obtained about microstructure. On the other hand, it reveals some of the limitations of current procedures for modelling diffraction line profiles.

Daniel Louër

Papers

X-ray diffraction study of the early stages of the growth of nanoscale zinc oxide crystallites obtained from thermal decomposition of four precursors. General concepts on precursor-dependent microstructural properties

Applications of Total Pattern Fitting to a Study of Crystallite Size and Strain in Zinc Oxide Powder

Two new diamine templated lanthanum sulfates, La2(H2O)2(C4H12N2)(SO4)4 and La2(H2O)2(C2H10N2)3(SO4)6·4H2O, with 3D and 2D crystal structures

Th4(PO4)4P2O7, a New Thorium Phosphate: Synthesis, Characterization, and Structure Determination

The use of pattern decomposition to study the combined X-ray diffraction effects of crystallite size and stacking faults in ex-oxalate zinc oxide