Table of contents

A summary is given of theories founded on concepts such as roughening transitions, kinetic roughening, and connected nets which in principle allow the prediction of the conditions under which a crystal face (hkl) will grow as a flat face, by a layer mechanism, or as a rough face. Starting with Jackson's theory based on the alpha concept recent developments are treated.

The underlying crystal-growth theory and structural molecular chemistry important in the understanding of the methodology behind the theoretical prediction of crystal morphology is presented together with its application to the molecular solid benzophenone. Benzophenone crystallizes in the orthorhombic space group P2₁2₁2₁ with a well defined morphology dominated by large (110) faces with smaller (021), (011), (101), (111), (002) and (020) faces, and forms in a habit elongated along the c crystallographic axis. A comparison of this observed morphology with that predicted from lattice geometry, PBC analysis, attachment energy and Ising models reveals a much more squat habit, with only the (110), (011) and (101) forms predicted. Calculations of the Ising temperatures reveal that only the (110) crystal form should grow below the roughening transition, in direct contradiction to the experimental data. These discrepancies are rationalized through a consideration of the change in the molecule conformation experienced by the benzophenone molecule during the growth process, as revealed from a comparison between the crystallographic structure and that calculated using semi-empirical molecular-orbital methods for the free molecule. Examination of the molecular packing in the solid state reveals that this conformational change is easier to accommodate in the (hk0) and (00l) faces where the surface binding sites are unconstrained and where the number of significant atom-atom interactions is small. This is in contrast to the pyramidic (hkl) faces where the conformational change appears to result in stronger surface adsorption which leads, in turn, to an underestimation of the surface attachment energy if the molecular arrangement in the solid-state structure is assumed. The implication of the additional conformation-change enthalpy involved in crystal growth at the crystal-solution interface for the proportionality, surface/bulk equivalence and equivalent wetting assumptions used in modelling theory are discussed.

Tailor-made molecular auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes which include morphological engineering, reduction of crystal symmetry, assignment of absolute structure of polar crystals, elucidation of the effect of solvent on crystal growth and crystallization of a desired polymorph. As for crystal growth promoters, Langmuir monolayers on water have been used to induce growth of 3D crystals at the monolayer-solution interface by means of structural match, molecular complementarity or an electrostatic interaction. The 2D crystalline structures of these monolayers have been studied by grazing-incidence X-ray diffraction (GIXD). It has become possible to monitor, by GIXID, the growth and dissolution of self-aggregated crystalline monolayers affected by the interaction of solvent molecules in the aqueous subphase with the monolayer headgroups.

The pseudosymmetric chiral sulphoxide 1 was designed with two segments of nearly identical shape but with significantly different electron-donor/acceptor properties. Based on the known high statistical preference for organics to pack in one of the centrosymmetric space groups, formation of molecular crystals of enantiomerically pure 1 was predicted to occur with pseudocentrosymmetry with 1 also playing the role of its enantiomer and packing as would the racemate. Such a packing motif would lack true centrosymmetry, resulting in a polar axis for the crystal and net additivity of the vectors from nitrogen to sulphur (the direction of polarizability for the molecules). Enantiomeric sulphoxide 1 does from molecular crystals with false centrosymmetry, mimicking P2₁/c, and with a substantial net directionality of polarizability vectors. In contrast, enantiomeric sulphoxides 3 and 4 form molecular crystals where the vectors from nitrogen to sulphur in neighbouring molecules are essentially opposed.

Crystal morphology is a major consideration in the design and optimization of industrial crystallization processes. Control of crystal morphology allows the optimization of particle properties such as filterability, flowability, and attrition resistance. Advances in morphology prediction are making it a useful tool, reducing the number of labour intensive experiments necessary to develop an optimum morphology. To be useful, however, experimentally verified morphology predictions must be made available quickly and efficiently. The authors report a unified approach to crystal habit prediction and verification. Up to now, crystal morphology prediction has been demonstrated only for idealized laboratory systems. They have applied this powerful technique to a model system, glutaric acid (CH₂)₃(COOH)₂. Morphologies are predicted based on lattice geometry and lattice energy calculations and compared with actual morphologies of crystals grown batchwise in a 'microcrystallization' cell. Advanced digital image analysis is used to analyse video images from in situ optical microscopy. The approach is ideal for industrial application because modelling minimizes the experiments required. The experiments required for model verification are rapid, simple, and no complicated powder sampling is required.

During the growth of optically active crystals (host), trace quantities of the opposite enantiomer (guest) present in the solution may be taken up by the host. However, the enantiomeric guest, due to its opposite chirality, may distort the crystal lattice of the host. Thus, the incorporation of the guest may lead to changes in the pharmaceutically important properties of the host, such as dissolution rate and thermodynamic properties, perhaps through lattice disruption. This hypothesis was tested by growing crystals of a model compound, (-)-ephedrinium 2-naphthalenesulphonate, from aqueous crystallization media containing various trace quantities of its opposite enantiomer. A stereoselective HPLC method was developed to determine the trace amounts of the opposite enantiomer within and on the surface of the homochiral host crystals. Increasing concentrations of the guest in solution led to its increasing incorporation into the host crystals and to increasing intrinsic dissolution rate of the host. The enthalpy and entropy of fusion of the host decreased with increasing incorporation of the guest, suggesting the disruption of the crystal lattice of the host and an increase in its lattice strain. The water content and the melting point were not significantly affected. At higher levels of the incorporated guest, the enthalpy and entropy of fusion increased, suggesting a relaxation of the lattice strain, and subsequently reached a plateau value. A value of 20.6 was obtained for the disruption index, which is defined as the rate of change of the difference between the entropy of the solid and that of the liquid, with respect to the ideal entropy of mixing of the host with the guest. This relatively high disruption index indicates significant disruption of the crystal lattice of the host by the incorporated guest, suggesting changes in the nature and concentration of the crystal defects.

Phenytoin crystal growth kinetics have been measured in phosphate buffer as a function of pH, supersaturation, and temperature. Incorporation of growth units into the crystal lattice is controlled by surface integration and follows a screw-dislocation mechanism. This mechanism explains the observed dependence of growth rate on supersaturation and pH. A prediction of phenytoin crystal morphology was made by two methods: the Bravais-Friedel-Donnay-Harker (BFDH) model and attachment energy calculations. The results from both methods compare well with observed phenytoin morphology resulting from growth in certain supersaturation ranges. Observed phenytoin crystal morphology is dominated by the (020) and (011) faces. Both faces were predicted to be morphologically important with the BFDH and attachment energy models. The (002) and (200) faces were also predicted to be morphologically important. However, the (002) was not observed and the (200) was only observed on crystals grown at low supersaturation ( sigma <or=0.4). Solvent-solvent interaction at the growing crystal faces and the types of intermolecular bond formed are proposed as the source of deviation between predicted and observed morphology.

A statistical analysis of molecular dipole moments within three space groups (P1, P1, and P2₁) was carried out. The magnitude of the molecular dipole moment does not vary significantly between centrosymmetric (P1) and noncentrosymmetric (P1 and P2₁) space groups and does not correlate with relative molecular orientations within the P2₁ space group. Thus, the high preference for organic molecules to crystallize in one of the centrosymmetric arrangements cannot be attributed to dipole-dipole interactions.

The crystallinity of samples of beta -succinic acid crystallized from aqueous solution has been observed to be influenced by the initial supersaturation and agitation intensity. Samples were prepared under controlled conditions from three initial supersaturations and at four stirrer speeds. For any initial supersaturation level the crystallization onset temperature decreased with increasing stirrer speed. This effect was attributed to a decrease in the width of the metastable zone. The increase in sample crystallinity as indicated by X-ray powder diffractometry was considered to result from differences in the predominating growth mechanism caused by variations in the supersaturation level for crystal growth. Scanning electron microscopy indicated a change in surface rugosity and a habit change for crystals prepared under one set of extreme conditions.

Understanding the role of structure in determining the properties of materials is a crucial aspect of the design of new materials. The existence of polymorphic crystal structures provides a unique opportunity to study structure-property relationships, since the only variable among polymorphic forms is that of structure, and variation in properties must be due to differences in structure. Systematic characterization of the polymorphic forms and acquisition of the ability to grow crystals of a desired form are additional elements in the design strategy of new materials. The conditions and techniques required to obtain a particular polymorph, combined with knowledge of the crystal structures, can also provide information on the relative stability of the different structures. Studies of representative systems which illustrate the intimate connections between polymorphism, structure-property relations and crystal growth are presented.

Polymorphism has critical implications in material design of organic crystals for elucidating specific functions in optics, pharmaceuticals, foods, biology etc. This is because diversified properties are revealed in polymorphic modifications. Depending on the selection of the specific polymorphs, one has to deal with two tasks: crystallization of the specific polymorph in an exclusive manner, and preservation of that form after or even during the crystallization in cases where polymorphic transformation easily occurs. The polymorphic crystallization is primarily determined by the nucleation process, in which chemical potential difference and interfacial energy are predominant factors. Both factors differ between polymorphs, giving rise to complexity in polymorph-dependent nucleation. The polymorphic transformations occur through four schemes: solid state, solution mediation, melt mediation and interface mediation. The last three processes can occur more rapidly than the solid-state transformation, particularly in organic crystals, since steric hindrance for the solid-state transformation is critically serious. Recent studies on polymorphic transformations in organic crystals mediated by melt, solution and interface are reviewed.

L-706,000, a class III antiarrhythmic compound, has been found to exist in several different crystalline structures including two anhydrous polymorphs, two dihydrated enantiotropic polymorphs, a monohydrate, and several organic solvent solvates. The isolation of the desired crystal modification, dihydrate type A, can be accomplished under thermodynamic or kinetic control depending on the conditions. Under kinetic control, the isolation depends on a suspended transformation of a metastable state, L-700,462, a thrombotic agent, has been found to exist in three crystalline structures: a monohydrate and two anhydrous monotropic polymorphs. Both anhydrous polymorphs, when hydrated, yielded the single monohydrate. Drying of the monohydrate, depending on the conditions and sample, will give either anhydrous form. The varying results obtained upon drying are, once again, indicative of the presence of metastable states and suspended transformations in connection with the solid state of L-700,462.

The effect of solvent in crystallization of polymorphs has been studied using the drug sulphathiazole as a model compound. The solubilities of the four polymorphic forms of sulphathiazole were determined as a function of temperature in various solvents. Within the temperature ranges studied, the rank order of solubility of the polymorphs was the same in all solvent systems. On the basis of this knowledge of the temperature dependence of the solubilities, recrystallization experiments, in which the supersaturation was systematically varied, were carried out in an endeavour to isolate each of the polymorphic forms from each solvent system. These recrystallization experiments reveal that not all of the know polymorphic forms can be crystallized from any given solvent by varying the supersaturation. Indeed some solvents selectively favour the crystallization of a particular form of forms. The authors conclude that thermodynamic effects are not responsible for the selective behaviour of a solvent. A kinetic mechanism is proposed. It is considered that the solvent acts by selective adsorption to certain faces of some of the polymorphs, and thereby either inhibits their nucleation or retards their growth to the advantage of others.

Starburst molecules based on pi -electron systems for making amorphous molecular materials, 1,3,5-tris(2-methylphenylphenylamino)benzene and 1,3,5-tris(4-methylphenylphenylamino)benzene, have been found to show polymorphism depending upon the history of heat treatment, which involves crystallization via amorphous glasses, as characterized by differential scanning calorimetry, X-ray diffraction, and polarizing microscopy.

Protein crystal growth experiments have been performed on fourteen space shuttle missions between April 1985 and June 1992. These space shuttle missions have been used to grow crystals of a variety of proteins using vapour diffusion, liquid diffusion, and temperature induced crystallization techniques. The United States Microgravity Laboratory-1 mission (june 25-July 9, 1992) was a space lab mission dedicated to experiments involved in materials processing. New protein crystal growth hardware was developed to allow in-orbit examination of initial crystal growth results, the knowledge from which was used on subsequent days to prepare new crystal growth experiments. The hardware developed specifically for the USML-1 mission is discussed along with preliminary experimental results.

Protein and virus crystals commonly grown in the laboratory on earth have also been grown in a microgravity environment on the US Space Shuttle. Two techniques for growth of the crystals were investigated: vapour diffusion and direct liquid-liquid diffusion. Crystals produced in microgravity, although generally isomorphous with earth-grown crystals, frequently show positive changes in habit, size and degree of perfection. X-ray diffusion analyses of crystals grown free from gravity usually show an improvement in the intensity-to-background ratio for all resolution ranges, and in some cases an extension of the resolution of the diffraction patterns to a higher limit. Both findings suggest an overall improvement in the quality of macromolecular crystals grown in space. A possible source of this improvement is the formation of depletion zones around crystals growing in microgravity that remain stable in the absence of convection and thereby allow more self-regulated growth. This in turn may be manifested as a more uniform and general ordering of the molecules in the crystal lattice and/or a reduction in gross crystal imperfections. Some illustrative results obtained in two kinds of macromolecular crystal growth experiments on International Microgravity Laboratory-1 are described.

The authors have adapted a dialysis technique which provides aggregate concentrations of protein molecules in solutions which lead to crystal growth. In dialysis, the flux across a semipermeable membrane is directly proportional to the concentration of the diffusible solute inside the bag provided that the solute concentration in the bulk solution is infinitely dilute. Using membranes of varying porosity, the concentrations of different size solutes can be measured by measuring the flux rate across the membrane. They have used this technique to independently measure the concentrations of monomers, dimers, trimers and higher aggregates of lysozyme in both 1% NaCl and 3% NaCl (0.1 M NaAc, pH4) using 25 K and 50 K molecular weight cut-off membranes. They compare these concentration profiles with (110) face growth rate data under the same conditions.

Hen egg-white lysozyme has been studied as a model system to investigate the problems of protein crystallization. An analysis is made of the induction time in new phase formation. This is the time that elapses between the achievement of supersaturation or undercooling and the appearance of a solid phase. The laser diffraction technique provides a means of detection of the appearance of nuclei by following the evolution of obscuration. The results show that the induction period increases exponentially with decreasing supersaturation of the solution. A general expression for the induction time, which is valid for any number of nuclei appearing and growing in the parent phase, is applied.

Metastable state relaxation in a gravitational field is investigated in the case of noncritical binary solutions. A relaxation description is presented in terms of the time-dependent Ginzburg-Landau formalism for a nonconserved order parameter. A new ansatz for solution of the corresponding partial nonlinear stochastic differential equation is discussed. It si proved that, for the supersaturated solution under consideration, the metastable state relaxation in a gravitational field leads to formation of solute concentration gradients due to the sedimentation of subcritical solute clusters. The pure discussion of the possible methods to compare theoretical results and experimental data related to solute sedimentation in a gravitational field is presented. It is shown that in order to describe these experiments it is necessary to deal both with the value of the solute concentration gradient and with its formation rate. The stochastic nature of the sedimentation process is shown.

Atomic force microscopy was used to obtain images of lysozyme crystals growing in their native solution environment. The authors growth by both two-dimensional nucleation and screw dislocation mechanisms. Nucleation centres appeared to occur randomly over the surface, and were uncorrelated from layer to layer. Surface defects on the (110) face were found in the form of double-arm spirals, opposing pairs of such spirals, or more complex structures. The formation of defects as layers grew over foreign particles, and occasional later healing of the defects, were followed. At sufficiently low supersaturation, defect-mediated growth dominated over growth by two-dimensional nucleation.

The authors present results of an X-ray scattering study on the rotator phases of normal alkanes: CH₃-(CH₂)_n-2-CH₃ (20<or=n<or=33). These are layered plastic-crystal phases in which the long axes of the molecules are all parallel and can be either parallel to or tilted from the layer normal. They have characterized a new tilted rotator phase and measured the distortion, area per molecule and tilt parameters in all five rotator phases. There is no strong even-odd chain effect within the rotator phases, but there is a significant change in phase sequence in the C₂₆-C₂₇ carbon number region. A typical phase sequence for the shorter chains is L-R_II-R_I-R_V-crystal. For longer chains the sequence is L-R_IV-R_III-crystal. In the R_V, R_IV and R_III phases the molecules are tilted. There is a general trend towards decreasing distortion and increasing magnitude of tilt with increasing carbon number. An unusual decrease in area per molecule with increasing temperature for the high-carbon-number phases suggests that defects may play a role in the R_IV phases.

The value of electron microscopy in the study of organic crystals is reviewed, together with its limitations in terms of radiation damage. The mechanism of formation and growth of crystals on a substrate is discussed, on the basis of electron microscopic observations, of both small crystals and the molecular structure of growth boundaries. Several examples of polycyclic hydrocarbons and phthalocyanines are described. A distinction is made between the information obtainable about disorder in a crystal, and precise measurements made by electron crystallography involving structural averaging.

Urea single crystals were grown by solution growth and melt growth by the Czochralski method. The perfection of the grown crystals was evaluated by X-ray topography. The crystal defects, especially dislocations, in the crystals were characterized by analysis of their images. As a result, the Burgers vectors of the dislocations in urea crystals were determined to be (001). Almost every long straight dislocation in the crystals was a type of screw dislocation.

The importance of crystal morphology in melt crystallization and its control are described. The morphologies of m-chloronitrobenzene (mCNB) crystals grown from pure, binary, ternary and quaternary melts are reviewed and systematically summarized. The effects of components other than mCNB and of their concentration on the mCNB morphology are discussed. An attempt is also made to correlate crystal morphologies with the physical properties of the system. Finally, from the relative growth rates of the major faces, the morphology changes in melt crystallization operations are illustrated.

Melt crystallization processes by solid layer techniques are predominantly used to purify organic materials. On top of the separation by the actual crystallization process a further purification can be achieved by additional sweating and/or washing operations. However, both operations depend strongly on the previous crystallization process. To understand the two mentioned operations it is necessary to know the impurity distribution within a crystalline layer prior to their application. Another question is what happens within the layer when the operations are executed. The authors present experimental results for the impurity distribution within crystal layers in laboratory equipment (tube crystallizer of 1.5 m length, with the crystalline layers on the inside or the outside). Different growth rates and fluid dynamic conditions as well as two distinct systems (materials) are other varied parameters. First explanations for the experimental data are also given. These explanations can help both the understanding of the phenomena and the modelling of solid layer processes.

The growth kinetics of m-chloronitrobenzene crystals from acetone solutions were measured and analysed to see the effects of the presence of microcrystals in the solution. The microcrystals generated by the added fine crystals were found to cause significant enhancement of the growth rate by a factor of 2-5. Possible mechanisms of the enhancement are discussed and incorporation of the microcrystals as macrogrowth units is suggested to be a likely one, which is identical to the case of NaCl crystals growing from aqueous solutions.

The measurement of ultra-micro-hardness is an attempt to find a physical property which can be used for the description of the so-called abrasion resistance of crystals produced in chemical engineering equipment. Such approaches are necessary in order to compare calculations of the secondary nucleation rates for different substances. Abrasion is the main source of the existing secondary nucleation. The main purpose of the paper is to show that it is possible to obtain ultra-micro-hardness data for organic and inorganic substances, and such data are presented. All of these substances are commonly processed in industrial crystallization processes. The results for the Vickers-hardness measurements of the substances are presented as hardness-indentation force-dependency diagrams. The pictures of indentations at different indentation forces show that measurements of the Vickers hardnesses with indentation forces higher than 0.04-0.1 N (with respect to the substance) lead to cracks at the Vickers pyramid of the indentation and therefore to irreproducible results for the Vickers-hardness measurements.

Various solution growth techniques were tested in an attempt to grow single crystals of four new partial esters of clodronic acid for X-ray structural studies. Gel (tetramethoxysilane) and vapour diffusion techniques proved to be successful. The starting materials and crystals were examined by infrared spectroscopy and thermogravimetric analysis and the structures of the starting materials were confirmed by ³¹P NMR and elemental analysis. Preliminary morphological studies were carried out by optical microscopy. These new bisphosphonate derivatives are of pharmaceutical interest as inhibitors of osteoclastic bone resorption.

The production of uniform fine crystalline particles is highly desirable for pharmaceutical products in order to enhance stability and dissolution rates and, thereby, to favour high bioavailability. Precipitation under high-supersaturation conditions favours nucleation over growth processes and will result in smaller particles. Little fundamental information is available on the kinetics of nucleation and growth of organic compounds when precipitated under high-supersaturation conditions by the addition of a non-solvent. In order to accurately measure such rapid kinetics a technique for rapid mixing prior to any nucleation events is required. The authors report the use of a grid mixing device with a mixing time of less than 3 ms to measure the nucleation and growth rates of L-asparagine and lovastatin when precipitated by non-solvents at high supersaturation ratios. Nucleation induction times as small as 60 ms are observed, and the growth of asparagine at high supersaturation is found to be described by the same rate law as previously determined at low supersaturations.

It is frequently necessary to ensure that pharmaceutically active materials of low aqueous solubility meet minimum requirements for specific surface in order to achieve their desired therapeutic activity. The paper describes work on a compound with a very slight aqueous solubility where the need for control over specific surface is indicated. It is customary and often convenient to establish the relationship between surface area and particle size, and to determine the latter, as modern instrumentation allows the rapid measurement of size distribution. For this compound this approach was not feasible as particle size determination proved difficult. The determinations were therefore made directly as surface area measurements using the dynamic flow technique based on the BET nitrogen adsorption technique. Laboratory scale preparations of the compound achieved a sufficiently high specific surface consistent with small particle size, and crystal shape. As these materials were used in early bioavailability studies, it was necessary to achieve a comparable specific surface on scale-up. This initially proved difficult to achieve. A programme of work has been conducted to establish the causes of the lower specific surface observed in these preparations, and to examine the conditions of the precipitation isolation process. The conclusions from the study allowed successful further scale-up of the precipitation to pilot plant scale, achieving acceptable specific surface values.

A high-purity dioctadecyl adipate (OAO) single crystal, of dimensions 30 * 20 * 0.1 mm³, can be grown over a period of fourteen days using the appropriate method. Diffraction studies on the crystal have indicated the suitability of this system in the dispersing of ultrasoft X-rays in X-ray spectroscopy; large 2d spacing (94.85 AA), high resolution and reflection properties. The OAO crystal was employed in an X-ray spectrometer to obtain the spectra of pure boron at the B K-edge and diamond at the C K-edge; the results from the experiments clarify the suitability of an OAO single crystal as an ultrasoft X-ray dispersant.

Single crystals of octadecyl hydrogen succinate (OHS, 2d = 90.94 AA) up to 70 * 30 * 1.0 mm³ in size have been grown by the solution cooling method. The crystal data and X-ray-diffraction characteristic parameters of OHS were determined. The results of measuring normal samples of carbon and boron using OHS in an electron probe spectrometer are reported and compared with those obtained using the soap film 'pseudo-crystal' of lead stearate (STE).

Amorphous molecular films of three types of organic compounds were prepared by vacuum evaporation onto cold metal substrates. Structural relaxations in the films were studied by Raman spectroscopy during the annealing process. A general view on this study is presented along with some experimental results which illustrate the scope of the study. Relaxation and the potential energy surface of amorphous molecular systems are briefly discussed.

The quest for more efficient nonlinear optical materials of increased optical quality is being spurred by the development of optical communication systems that require ultrafast broad band optical signal processing functions. Conversely, nonlinear optical phenomena enlarge traditional spectroscopic approaches to encompass more complex and informative multiphotonic pathways. In this stimulating context, organic nonlinear materials have been recognized as forefront candidates for fundamental and applied investigations involving, in a joint effort, chemists, material scientists and optical physicists. After reviewing the molecular engineering foundations of the domain, specifically with respect to an efficiency-transparency trade-off, the author concentrates on a more specific case that embodies, from the definition of the molecule all the way down to applied physics endgoals, the key concepts and methodological tools in the field. A crucial part of such study is the definition and implementation of crystal growth methods leading to the high optical quality demanded by parametric phenomena. This approach is best exemplified in the paradigmatic case of N-4-nitrophenyl-(L)-prolinol (NPP) which, after almost a decade of effort involving crystal growth and ultrafast time resolved spectroscopy, has recently reached sufficient quality to lead to optical parametric oscillation. Stable coherent tunable oscillation in the near IR from 0.9 to 1.7 mu m has thus been demonstrated for the first time in an organic molecular crystal, with specific advantages over mineral candidates, and room for further performance improvement. The author recalls in conclusion that new molecular and crystalline pathways are still opening up, such as those based on molecular nonlinearities of octupolar origin or organomineral crystals.

The information required from optical assessment of nonlinear organic materials differs widely depending on the application envisaged for the material in question. Accordingly, a suitable assessment scheme must be devised which will give a reliable indication of the promise of these materials in a wide variety of applications. Such a scheme is described, beginning with the assessment of microcrystalline powders and ending with predictions of wavelength and angle criticality of phase matching in single crystals. Electro-optical assessment at low frequencies is also described. It is shown that the performance of a material cannot be reliably expressed by any single parameter, but that it must be examined afresh for each application considered. Although several examples are discussed, the literature contains very few comprehensive nonlinear optical assessments. Based on the available data, some tentative structure-property relationships are proposed.

A novel organic crystal, (-)-1-(4-dimethylaminophenyl)-2-(2-hydroxypropylamino)cyclobutene-3,4-dione (DAD), has been grown and its linear and nonlinear optical properties have been characterized. The DAD crystal belongs to a space group P1 with a large second-order nonlinear optical coefficient d₁₁, which is measured by the Maker-fringe method to be 200 +or- 40 pm V^-1 at a fundamental wavelength of 1.064 mu m.

Acceptor-substituted carbazole derivatives were newly designed as novel nonlinear optical materials with photoconductive properties. Various 3-, 6- and 9-substituted carbazole derivatives were synthesized systematically and their optical second-harmonic generation (SHG) activity and crystallographic structures were examined. Enhancement of molecular hyperpolarizability ( beta ) was achieved and an intracharge-transfer band was observed by introducing electron-withdrawing groups in 3 and/or 6 positions. The hydrogen bonding in the 9-hydroxyethyl substituent plays an important role in generating non-centrosymmetric packing of carbazole molecules. Single crystals of 9-hydroxyethylcarbazole (HEK), 3-nitro-9-hydroxyethylcarbazole (NHEK) and 3,6-dinitro-9-hydroxyethylcarbazole (DNHEX) can be obtained by the solution growth technique. The HEK and NHEK crystals were determined to have non-centrosymmetric crystallographic structures with tetragonal and monoclinic unit cells. These crystals have unique space groups I4₁ and P2₁, and hydrogen bonding was formed in adjacent hydroxy groups. In these crystals each carbazole ring is aligned along the same direction. On the other hand, DNHEK has a centrosymmetric unit cell with space group P1. In this case the hydrogen bond involves a hydrogen on a hydroxy oxygen interacting with the nitro oxygen in an adjacent molecule. The highly aligned carbazole system is attractive in terms of structure related properties such as the carrier transport process and second-order nonlinear optical responses.

The nonlinear optical behaviour of the organic crystal MBA-NP has been investigated at 1064 and 532 nm, with both nanosecond and picosecond pulses. The second-order effective susceptibility for second harmonic generation, and the damage thresholds compare favourably with those of most inorganic crystals. Two-photon absorption is rather small, while stimulated Raman scattering, which can be excited only with the 532 nm radiation, is strongly dependent on the orientation of the sample.

Certain organic crystals have attracted much attention owing to their nonlinear optical coefficients, at least one to two orders of magnitude above those of inorganic nonlinear optical materials. However, none of them has so far actually been applied in frequency conversion devices used with high-, medium- or low-power lasers. In this article the authors discuss the major factors affecting frequency conversion efficiency and examine characteristics of some frequently mentioned organic crystals. The properties and structure characterization of a new type of nonlinear optical crystal are also reported and discussed.

The authors designed and synthesized five di-substituted benzophenone derivatives: 4,4'-diaminobenzophenone (DABP), 4-amino-4'-fluorobenzophenone (AFBP), 4-amino-4'-chlorobenzophenone (ACBP), 4-amino-4'-bromobenzophenone (ABBP) and 4-aminomethylbenzophenone (AMBP). Their crystal structures were determined, and the relationship between their crystal structures and nonlinear optical properties are discussed.

The influence of solvents (acetone-water mixtures) on the crystal growth kinetics of 3-methyl-4-nitropyridine-1-oxide (POM) has been investigated. Variation of the ratio of solvent components changes the solubility, with its maximum being at a water mole fraction of about 0.5, and the growth rates of individual crystal faces, and also affects the processes of defect formation. It has been shown by IR and NMR spectroscopy that POM molecules in solution interact with those of the solvent. The surface entropy factor ( alpha ) for the faces of POM crystal for three different solvents has been estimated. The principal kinds of defects and their distribution in the grown crystals have been studied relative to solution compositions.

The authors have obtained single-crystal flakes with dimensions approximately 5 mm * 1.0 mm. The major impurity in commercial coronene has been removed by recrystallization from benzene solution. Other unknown impurities were removed by zone refining combined with a sublimation technique. The molecules in the coronene crystal are stacked closely and parallel to each other. A feature that one expects from the stacked molecular arrangement is a strong exciton-phonon interaction. Luminescence spectra were consistent with this suggestion, indicating that free excitons relax quickly to a self-trapped state which is shallow.

Growth of thin single crystals from the melt of a diacetylene monomer, 2,4-hexadiynylene di-p-toluenesulphonate (PTS), between a substrate and a glass plate, has been investigated. Among five kinds of substrate examined, pseudo-homoepitaxy on poly-PTS substrate attained the best performance of growth control to give thin single crystals of PTS with a developed surface of over 1 mm² as well as uniaxial orientation, whereas the other four substrates, e.g. KC single crystals and rubbed polyimide films, resulted in the formation of microcrystallites without any regular orientation under the same crystal growth conditions. The PTS crystals were successfully converted into the single-crystalline polymer, with a quality suitable for nonlinear optical device fabrication.

The authors have succeeded in controlling organic-crystal orientation in glass capillaries using the modified Bridgman-Stockbarger method, in which the pulling direction of the capillary is at an angle theta to the temperature gradient. The controllability of the crystal orientation in the organic-crystal-cored fibre (OCCF) of 2-methyl-4-nitroaniline (MNA) was examined by the measurement of the second-harmonic generation power. It was confirmed that the second-harmonic power of the controlled OCCF is three times higher than that of the uncontrolled OCCF, since the axis with the largest second-order nonlinear optical susceptibility coefficient is tilted from the fibre axis in the controlled OCCF. The control mechanism for the crystal orientation is also discussed.