Dichroism in helicoidal crystals

Accounting for the interactions of light with heterogeneous, anisotropic, absorbing, optically active media is part of the characterization of complex, transparent materials. Stained biological structures in thin tissue sections share many of these features, but systematic optical analyses beyond the employ of the simple petrographic microscopes have not be established. Here, this accounting is made for polycrystalline, spherulitic bundles of twisted d-mannitol lamellae grown from melts containing light-absorbing molecules. It has long been known that a significant percentage of molecular crystals readily grow as helicoidal ribbons with mesoscale pitches, but a general appreciation of the commonality of these non-classical crystal forms has been lost. Helicoidal crystal twisting was typically assayed by analyzing refractivity modulation in the petrographic microscope. However, by growing twisted crystals from melts in the presence of dissolved, light-absorbing molecules, crystal twisting can be assayed by analyzing the dichroism, both linear and circular. The term "helicoidal dichroism" is used here to describe the optical consequences of anisotropic absorbers precessing around radii of twisted crystalline fibrils or lamellae. d-Mannitol twists in two polymorphic forms, α and δ. The two polymorphs, when grown from supercooled melts in the presence of a variety of histochemical stains and textile dyes, are strongly dichroic in linearly polarized white light. The bis-azo dye Chicago sky blue is modeled because it is most absorbing when parallel and perpendicular to the radial axes in the respective spherulitic polymorphs. Optical properties were measured using Mueller matrix imaging polarimetry and simulated by taking into account the microstructure of the lamellae. The optical analysis of the dyed, patterned polycrystals clarifies aspects of the mesostructure that can be difficult to extract from bundles of tightly packed fibrils

Resorcinol crystallization from the melt: a new ambient phase and new “riddles”

Structures of the alpha and beta phases of resorcinol, a major commodity chemical in the pharmaceutical, agrichemical, and polymer industries, were the first polymorphic pair of molecular crystals solved by X-ray analysis. It was recently stated that "no additional phases can be found under atmospheric conditions" (Druzbicki, K. et al. J. Phys. Chem. B 2015, 119, 1681.). Herein, is described the growth and structure of a new ambient pressure phase, epsilon, through a combination of optical and X-ray crystallography evaluated by computational crystal structure prediction algorithms. alpha-Resorcinol has long been a model for mechanistic crystal growth studies from solution and the vapor because prisms extended along the polar axis grow much faster in one direction than in the opposite direction. Research has focused on identifying the absolute sense of the fast direction – the so-called ‘resorcinol riddle’ – with the aim of identifying how solvent controls crystal growth. Here, the growth velocity dissymmetry in the melt is analyzed for the ? phase. The epsilon phase only grows from the melt, concomitant with the beta phase, as polycrystalline, radially growing spherulites. If the radii are polar, the sense of the polar axis is an essential feature of the form. Here, this determination is made for spherulites of beta resorcinol (epsilon, point symmetry 222, does not have a polar axis) with additives that stereoselectively modify growth velocities. Both beta and epsilon have the additional feature that individual radial lamellae may adopt helicoidal morphologies. We correlate the appearance of twisting in beta and epsilon with the symmetry of twist-inducing additives

Optically Anomalous Crystals

Optical anomalies in crystals are puzzles that collectively constituted the greatest unsolved problems in crystallography in the 19th Century. The most common anomaly is a discrepancy between a crystal’s symmetry as determined by its shape or by X-ray analysis, and that determined by monitoring the polarization state of traversing light. These discrepancies were perceived as a great impediment to the development of the sciences of crystals on the basis of Curie’s Symmetry Principle, the grand organizing idea in the physical sciences to emerge in the latter half of the 19th Century. Optically Anomalous Crystals begins with an historical introduction covering the contributions of Brewster, Biot, Mallard, Brauns, Tamman, and many other distinguished crystallographers. From this follows a tutorial in crystal optics. Further chapters discuss the two main mechanisms of optical dissymmetry: 1. the piezo-optic effect, and 2. the kinetic ordering of atoms. The text then tackles complex, inhomogeneous crystals, and the complex optical properties resulting from the superposition of anomalies having various etiologies. The book treats the literature comprehensively, but uses illustrations from the authors’ laboratories as the subjects of detailed analyses. This is an invaluable text for crystallographers, mineralogists, and petrologists interested in the growth of minerals and synthetic crystals, and their optical properties. It is also ideally suited to students of optical mineralogy, professional scientists and engineers as well as historians of science

Punin Ripening and the Classification of Solution-Mediated Recrystallization Mechanisms

Ripening (also called recrystallization) is a process that occurs commonly in nature and industry that shifts the size distribution of an ensemble of crystals toward a smaller number of larger crystals. Ostwald ripening is by far the best known recrystallization mechanism and sometimes is mistakenly considered as the only mechanism for shifting the crystal size distribution. Ostwald ripening accounts for recrystallization under thermodynamic control and is driven only by the well-known size dependence of solubility. There are, however, other recrystallization mechanisms that can be observed on laboratory timescales for crystals of any size under certain conditions. Internal stress dispersion is a thermodynamic ripening mechanism that depends not on surface energies but rather on crystal defects. In addition, there are two other mechanisms that are kinetic in nature. The most efficient is driven by the size dependence of growth and dissolution rates at low supersaturation. Finally, a mechanism proposed by Punin is driven by the difference between growth and dissolution rates due to crystal defects. All the four mechanisms can be at work simultaneously. The efficiency of ripening can be enhanced by temperature oscillations, but only the thermodynamic mechanisms can work at constant temperature. In this paper, we discuss the fundamentals of these four ripening mechanisms and revisit in detail Punin's mechanism because it is the least well articulated in the literature.This research was supported by the ACS Petroleum Research Fund (award #61270-ND10). This work was supported partially by the MRSEC Program of the National Science Foundation under award number DMR-1420073. J.M.G.R. acknowledges a grant of the program Salvador de Madariaga (Ministry of Economy and Competitiveness of Spain) and the Molecular Design Institute of NYU for hospitality

Nanoscale observations of the epitaxial growth of hashemite on barite (001)

The heteroepitaxial growth of hashemite BaCrO4 on barite BaSO4(001) from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows via two-dimensional nucleation easily forming a complete epitaxial layer,which is likely to have a low level of intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower rates,and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer,dendrite-like shape of nuclei and steps,preferable formation of nuclei at the step edges). As a result,the initially flat surface becomes rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous solutions.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Mechanics of twisted hippuric acid crystals untwisting as they grow

Spontaneous twisting of single crystals is a common growth induced deformation. But as twisted crystals thicken they can untwist, restoring a straight form. The mechanics of this process was studied for vapor grown needle-like crystals of N-benzoylglycine (hippuric acid) and N-(2-thienylcarbonyl)glycine, and analyzed by phenomenological models. The elastic stress at the crystal tip undergoes plastic relaxation leading to the twisting deformations. As the crystal grows and thickens it partially untwists showing linear increases of the twist period with crystal thickness. Such behavior was simulated with a model that assumes the constant density of defects in successive growth layers. However, transmission electron microscopy does not reveal any dislocations or other extended defects typically associated with plastic deformation. Published data on other materials show the linear dependencies of pitch on thickness suggesting comparable untwisting mechanisms for different materials.publishe

Crystallography of Contemporary Contact Insecticides

The active forms of contact insecticides used for combatting mosquito-borne infectious diseases are typically crystalline solids. Numerous molecular crystals are polymorphic, crystallizing in several solid forms characterized by different physicochemical properties, including bioavailability. Our laboratory recently found that the activity of crystalline contact insecticides is inversely dependent on the thermodynamic stability of their polymorphs, suggesting that efficacy can be enhanced by the manipulation of the solid-state structure. This paper argues that crystallography should be central to the development of contact insecticides, particularly because their efficacy continues to be compromised by insecticide resistance, especially among Anopheles mosquito populations that spread malaria. Although insecticidal compounds with new modes of action have been introduced to overcome resistance, new insecticides are expensive to develop and implement. The repurposing of existing chemical agents in metastable, more active crystalline forms provides an inexpensive and efficient method for ‘evergreening’ compounds whose risks are already well-established. We report herein seven new single-crystal structures of insecticides used for controlling infectious disease vectors. The structures reported herein include pyrethroid insecticides recommended by the WHO for indoor residual spraying (IRS)-bifenthrin, β-cyfluthrin, etofenprox, α-cypermethrin, and λ-cyhalothrin as well as the neonicotinoid insecticide thiacloprid