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The effective quadratic coefficients 2-amino-5-nitropyridinium chloride (2A5NPCl) are measured by phase matched second harmonic generation. 2A5NPCl is an interesting material for quadratic nonlinear optics due to the quasi-optimized arrangement of 2A5NP units in herringbone motifs and the high density of nonlinear 2-amino-5-nitropyridinium chromophore per unit volume. The maximum effective coefficient is obtained as 9.7 pm/V at 1320 nm. Since 2A5NPCl crystal is transparent from 410 to 1650 nm, sum frequency generation is also carried out with incident beams of 1320 and 660 nm. In this study, 11% conversion efficiency from 660 to 440 nm is obtained for 1.6 mm interaction length.

The evolution of scientific requirements to improve knowledge and understanding of sophisticated phenomena occurring during microgravity experiences leads to implement a new generation of hardware with more than classical instrumentation such as temperature or pressure measurement; optical diagnosis for in situ characterization is the unavoidable answer to this problem. However, there are many ways to “observe” one experiment, and optical specifications are so different among the great panel of microgravity experiments. This paper deals with a survey, performed by CNES, for identifying each unique optical system with respect to each optical diagnosis selected to satisfy experimental scenarios such as crystal growth, directional solidification, fluid physics and physical chemistry. The survey pointed out the needs of high resolution video data links and “real time holography”. A new generation of advanced technology was raised from this study, especially the use of photorefractive crystals and polymer resins for hologram recordings. Such optical diagnosis will be implemented in a new multi-user facility (CHEOPS) developed by CNES for the next Columbus Orbital Facility.

Measurements of refractive index as a function of time have been done in supersaturated lysozyme solutions. They reveal that the refractive index decreases in free solutions and increases in gelled solutions during prenucleation. These results are interpreted assuming molecular associations that sediment in free solution and levitate in gel medium.

Gel growth of 2-amino-5-nitropyridinium dihydrogen phosphate (2A5NPDP) has been followed first by optical micrography, which allows to determine growth rates in given directions ([001] and [100]), and second by holographic interferometry, which allows to characterize the evolution of the growth medium through the calculations, at every time, of the supersaturations at the crystal-solution interface. Two growth conditions obtained by varying the solvent concentration have been tested. The growth mechanism appears to be the same governed by a slow surface integration and a slow diffusion of matter.

2-Amino-5-nitropyridinium dihydrogen phosphate (2A5NPDP) is an organomineral material presenting interesting quadratic optical nonlinear properties, in particular towards second harmonic generation. Single crystals have been grown by spontaneous nucleation in tetramethoxysilane gel by slow cooling. X-ray characterization provides evidence of their good crystalline quality. Second harmonic generation experiments at 1.34 μm have confirmed the efficiency of these crystals and permitted inferring the d15 and d24 coefficients from the susceptibility tensor.

Twin structure of triclinic sodium oxalate monohydrate was determined by means of X-ray techniques. The twin is formed by reticular pseudomerohedry, with [101] as twin axis and (101) as contact plane. On a determine aux rayons × la structure de mǎcle de cristaux tricliniques d'oxalate hydrate de sodium, pousses en gel. Il S'agit d'une mǎcle par pseudomeriedrie avec [101] comme axe de mǎcle et (101) comme plan de contact.

Two populations of aggregates are generally indentified in supersaturated solutions of biological macromolecules: small aggregates of a size which is less than 5 nm and large aggregates, the largest of which are at least one order of magnitude bigger. In order to understand the role played by the microporous network of a gel in the growth and behaviour of these different species in the prenucleation period, an in situ observation of nucleation has been carried out using either free solutions or solutions trapped in agarose gels. In a previous study, free solutions were investigated by small-angle neutron scattering (SANS) to identify the small aggregates. Optical observations, made under the same conditions, revealed the formation of an amorphous precipitate which disappeared at the end of the experiment. The sedimentation of this phase, which occurs in free solution but never occurs in gelled solution, depletes the solution bulk and this could explain why the nucleation density is higher in agarose gel than in free solution. The case of silica gel, the behaviour of which is completely different with respect to nucleation, will be discussed.

A procedure which allows an investigator to supply a crystal with fresh mother material without inducing significant growth defects is described. This technique requires that the crystal is grown in a gelled hanging or sitting drop. An example concerning a model macromolecule, hen egg-white lysozyme, is given. Extension of this procedure to other macromolecules is discussed.

In order to investigate how macromolecular aggregation proceeds for obtaining nucleation and crystal growth, a series of hen egg white (HEW) lysozyme solutions representative of a large supersaturation range have been investigated by small angle neutron scattering (SANS). In all these solutions, the signal well corresponds to species of radii of gyration Rg below 50 A. Moreover, the effective values Rg are compatible with a rather monodisperse distribution of species the size of which increases when the supersaturation increases; for example, saturated solutions correspond to dimer populations. The largest size which has been identified corresponds to octomers which seems a limit beyond which nucleation and growth occur. The growth units are larger than dimers and probably correspond to tetramers or octomers. SANS allows one to study kinetic aspects. We observe that when a given supersaturation is quickly established, the radius of gyration increases with time. In the light of these results, it appears that SANS affords a powerful tool to study aggregation phenomena occurring in the metastable zone.

Silica gel provides a useful medium for crystal growth; solution growth is confined to pores left free by the polymer during its development. All growth steps depend on the gel structure, which is not completely known for crystal growth conditions. Therefore, a three-dimensional (3-D) visualization has been performed for two TMOS aqueous gels, which are rather fragile: the quick-freeze, deep-etching, rotary-replication method has been applied for sample preparation. An original surface labeling technique has been used for surface recognition. The results concern the distribution of macropores that are responsible for crystal nucleation; micropores whose total volume is larger have not been visualized due to the limits of the method. These results are discussed in comparison with previous data performed by light scattering.

Characterization of growth defects by X-ray topography of Guanidine aluminium sulfate hexahydrate crystals allows to set a close correlation between ferroelectric domain and growth sector patterns. The crystalline quality of prismatic sectors is much better than that of basal sectors. La caracterisation des defauts de croissance par topographie aux rayons × des cristaux de sulfate d'aluminium et de guanidine permet d'etablir une correlation etroite entre la repartition des domaines ferroelectriques et celle des secteurs de croissance. La qualite cristalline des secteurs prismatiques est bien meilleure que celle des secteurs basals.

3‐methyl‐4‐nitropyridine‐1‐oxide is an organic material presenting high optical nonlinear behavior especially for second‐harmonic generation. Single crystals are obtained by spontaneous nucleation and grown by slow cooling in tetramethoxysilane gel. Coupled phase‐matched second‐harmonic generation and x‐ray topography experiments allow one to relate nonlinear optical performance to crystalline quality.

Silicon tetramethoxide monomers are dissolved in water, hydrolyzed, and condensed to form a mesh of siloxane polymers. The growth and aging of this gel are examined through light and X-ray scattering. From interferences at short distances, the fractal dimension, D, of the growing polymers is measured. From interferences at large distances, the mesh size, ξ, of the resulting network is measured, which scales with concentration: ξ is 400 A when the volume fraction of polymer is φ = 0.04, and 10 000 A when φ is 0.004. The scaling laws which are determined from the final state of the gel differ from those which describe its growth patterns.

The crystallline quality of 2-amino-5-nitropyridinium chloride gorwn in solution was examined by X-ray topography. Strong white beam coming from synchrotron was irradiated to an as-grown crystal to avoid an absorption of X-ray. Topographs were taken in transmission geometry (Laue setup). The image obtained by the reflections 3 0 3 , 601, 6 1 2 and 1 7 0 planes showed fine grey contrast in nearly the whole section. Inclusions and strained growth sector boundaries were clearly observed in these images. These defects disappeared far from the seed in the direction of c -axis. The whole section of the reflection 1 7 0 was full of fine lines along c -axis with white and black contrast. It was found that a dendritic growth leaded to the formation of whiskers slightly misoriented along c -axis.

Crystals grown in convective media often show defects related to solution flux. Traces of polyethylene oxide (PEO) added to potash alum crystal growth solution slightly reduce the growth rates but significantly improve the quality of the crystal. In order to interpret this action, an electrochemical simulation of mass transfer during growth was attempted. It has been shown that the PEO additive does not act as drag reducer but is adsorbed at the interface, forming a porous filter through which chemical species diffuse.

A gel-growth technique is used to grow highly nonlinear N-(4-nitrophenyl)-(l)-prolinol (NPP) single crystals of larger size with improved optical quality over those produced by other methods, such as solution or melt growth. High-quality samples are obtained by lowering the temperature of a NPP saturated water-acetonitrile mixture in a tetramethoxysilane gel. X-ray characterization gives evidence of the occurrence of twins, in agreement with the results of nonlinear optical experiments under phase-matched conditions. A close agreement between the effective interaction length with the actual physical thickness of the samples attests to their optical quality. The high quadratic nonlinear efficiency of NPP (25 times that of currently available 3-methyl-4-nitropyridine-1-oxide crystals) is confirmed. The low, monoclinic P${2}_{1}$ symmetry of NPP does not fully determine the orientation of the principal dielectric framework. A further inspection of the orientation within the crystal unit cell of the chiral prolinol groups, together with an oriented-gas description of molecular and crystalline polarizabilities, complements the partial crystallographic information, allowing us to account fully for the orientation of the ``symmetry''-free dielectric axes.

The quadratic nonlinear optical efficiency of the 2-amino-5-nitropyridinium dihydrogen chloride crystal is studied. The quadratic coefficients are determined by second-harmonic generation as d11=9 pm/V and d12=8 pm/V. The effective coefficients are obtained as deff=5.1 pm/V for type I and deff=9.7 pm/V for type II. Following the gas-oriented model, the dimensions of the hyperpolarizability tensor bIJK of the 2-amino-5-nitropyridinium are examined. Considering the arrangement of the organic and inorganic entities in the unit cell, the failure of the two-dimensional model can be due to the local electric field induced by the Cl- anions.

The HOLIDDO (holographic interferometry by double-direction observation) instrument is dedicated to the characterization of transparent media in a microgravity environment. Characterization is accomplished by holographic interferometry in two perpendicular directions in order to reconstruct the three-dimensional object. An Fe:LiNbO3 crystal is selected for storing holograms, which makes real-time interferometry possible. A depth of field of 30 mm allows us to study rather large objects with a resolution of better than 50 μm.

NH4H2PO4 (ADP) crystals have been grown in a tetramethoxysilane gel by temperature decrease. Nucleation and growth have been followed by interferometric holography which has allowed to determine the supersaturation so that the growth proceeds by a bidimensional mechanism without generation of dislocations.

A plane wave imaging technique has been applied to the characterization of growth defects in Ba(NO 3 ) 2 and Sr(NO 3 ) 2 crystals. Precise values of lattice parameter differences between cubic and octahedral sectors have been obtained in the case of Sr(NO 3 ) 2 . For Ba(NO 3 ) 2 , plane wave observation confirms the crystalline perfection revealed by Lang topography.

ADP crystals doped with chromium were obtained by temperature decrease of a saturated aqueous solution either stirred (solution-growth) or gelled (gel-growth). Characterization of the doping effect was made by atomic absorption, optical observations (tapering angle, colour) and X-ray Lang topography. The global incorporation of chromium is found to be higher in the gel-grown crystals; however, the lattice strains associated with the relative segregation of chromium in the prismatic and pyramidal sectors are reduced.

Sequences of I Incommensurate-II Commensurate-Ill Commensurate transitions in TMA2ZnCL4, were observed in real time by synchrotron X-ray topography. For I-II transition, the coexistence of commensurate and incommensurate phases causes, on a small temperature range near the lock-in transition, the formation of macroscopic domains bounded by defects which can be identified as deperiodisation lines. These lines have their emergence points on directions which can be indexed in the superspace.

Gypsum (CaSO4·2H2O2) crystals which are large and good enough to be characterized by X-ray topography have been grown from solution and from gel in order to study their twin growth.

An X-ray topographic study of potassium bichromate crystals shows that the crystalline quality is improved by increasing the growth temperature. Nearly perfect crystals can be obtained. In that case, it is observed that the presence of a few dislocations strongly influences the growth habit.

Single crystals of CaHPO 4 ·2 H 2 O and PbHPO 4 were grown in silica gel and tetramethoxysilane gels with different concentrations of the gel forming agent and different pH values. At a high growth rate of several mm per day the crystals exhibit a good quality. Low density gels yield at low pH values almost perfect crystals.

Différents aspects de la croissance des cristaux en solution sont successivement passés en revue pour montrer le rôle de l'environnement hydrodynamique.

X-ray topographs from hyfrothermally grown single crystals of calcite have given evidence for misorientations between growth sectors. Furthermore, strained areas revealed by cleavage show mismatching in a same growth sector between two parts, one grown from the center of the face, the other from the edges. According to the value of local supersaturation, one of the growth processes prevails over the other.

The characteristics of gel growth are compared to those of classical solution growth, showing the advantages and drawbacks of different growth media. The main techniques used to grow crystals in gels are illustrated in the case of hen egg white lysozyme crystals.

Facilities for topographic studies using Synchrotron Radiation Sources are reviewed and discussed as for growth defect characterization. Two types of studies are most used: kinetic studies performed by using white beam topography and lattice parameter mapping obtained by plane wave topography.

NH4H2PO4 crystals moderately doped with trivalent chromium (30–130 ppma in the mother solution) were obtained either in a stirred solution (solution growth) or in a tetramethoxysilane gel (gel growth). Characterization of the doping effect was made by means of traverse or section X-ray Lang topography, plane-wave synchrotron topography and secondary ion mass spectrometry in order to have direct correlation between impurity distribution and lattice distortion. It is shown that the preferential chromium segregation in the prismatic growth sectors is less pronounced for gel-grown than for solution-grown crystals and correlatively the lattice parameter variation Δcc is smaller. Strain distribution between prismatic and pyramidal growth sectors also differ: in gel-grown crystals, the lattice displacement gradient is localized at the growth sector boundary whereas, in solution-grown crystals, it is expanding through the bulk of the prismatic sectors.

Large single crystals of Na2CoGeO4 were grown under hydrothermal conditions. Two methods are used: crystallization on seed and spontaneous growth in temperature gradient conditions. The comparison of the defect distribution shows the best quality for crystals obtained with the most rapid growth rates.

Various experimental methods have been used in order to study the mass transport of NaCl in tetramethoxysilane (TMS) gels. The concentration of NaCl has been measured either through refractive index (“holographic interferometry”) or electrochemical potential (“extraction method” and “diffusion through a gel”). The values of the diffusion coefficient of NaCl in TMS gels have been deduced.

Holographic interferometry has been applied to the study of mass transfer induced by nucleation and growth in gel media. Real time obsrvations were made on growth of insoluble (CaHPO4, 2 H2O) and soluble (KDP) materials. L'holographie interferentielle a ete appliquee a l'etude du transfert de masse induit par la/nucleation et la croissance en gel. Des observations en temps reel ont ete faites lors de la croissance de materiaux insolubles (CaHPO4, 2 H2O) et solubles (KDP).

KDP crystals grown from boiling solutions present well developed prismatic sectors and show a good crystalline quality while being obtained at high growth rates.

Lead hydrogen phosphate (LHP) and lead nitrophosphate (LNP) crystals have been grown by counterdiffusion of lead nitrate and phosphoric acid solutions either in a gelled medium or in a free medium under the reduced gravity environment of Spacelab. Characterization of grown crystals allows to conclude that diffusive regimes are obtained in both xases. Dielectric measurements reveal significant differences which can be correlated with the crystalline quality.

Growth defects in single crystals of potash alum grown with various conditions have been examinated by X-ray topography. An asymmetrical distribution of defects can be related to the direction of the flux. From static solutions, large defect free areas can be obtained in sectors corresponding to slow faces.

A study of thermoluminescence and isothermal decay of calcite synthetic single crystals after electronic irradiation shows that a new, weak, low temperature emission precedes the “360 K” glow peak (related with manganese): it has the same emission spectrum as the glow peak, the intensity varies in inverse proportion to the time and does not depend on temperature in a wide range (LNT to 180 K). These two emissions are accounted for by a model with charge transfer taking place between traps and manganese centers through two competing tunneling processes. One starts from the fundamental level of the traps and is athermal. The other starting from a thermally excited level 0.62 eV higher, gives rises to the 360 K glow peak. Une etude de la thermoluminescence et du declin isotherme de monocristaux de calcite synthetique apres bombardement electronique montre qu'une nouvelle emission a basse temperature et de faible intensite precede le pic a 360 K attribue au manganese: elle a le měme spectre d'emission que ce pic, son intensite varie en raison inverse du temps et independamment de la temperature dans un grand domaine, de 77 a 180 K environ. Un modele rend compte de ces deux emissions par des transferts de charges s'effectuant entre les pieges et les centres manganese par deux effects tunnels competitifs: l'un, qui part du niveau fondamental du piege, est athermique, l'autre, qui part d'un niveau excite thermiquement a 0,62 eV au dessus, donne le pic de thermoluminescence a 360 K.

A real-time X-ray Synchroton study of (N(CH 3 ) 4 ) 2 ZnCl 4 singles crystals allows one to follow the evolution of a satellite reflection above the lock-in transition. Thermal cycles give evidence for relaxation effects associated with the defect distribution Une etude de monocristaux du compose du titre a permis de suivre, en temps reel, l'evolution d'une reflexion satellite au-dessus de la transition de blocage. Des effets de relaxation associes a la distribution des defauts ont ete mis en evidence lors de cycles thermiques

Water soluble crystals such as ADP and KDP have been grown in TMS gels by different techniques. A study of growth defects by X-ray topography shows that these gel grown crystals obtained by a very simple technique can be compared to crystals grown from solutions using elaborated systems.

In order to elucidate how defects appear during growth, known perturbations have been generated and the nature of the defects they induced was investigated by means of X-ray topograohy. They are essentially growth bands, inclusions and dislocations. The growth rates could be determined and it was shown that the presence of dislocations increased the growth rate inasmuch as they have a strong screw component. Furthermore, nearly dislocation free and perfect crystal has been observed on “S” type faces, according to Hartman's classification.

Hydrothermally grown single crystals of calcite have been examined. They are of two types: crystals grown from a rhomboedral seed and spontaneously nucleated plates. Lattice defects have been studied by X-ray Lang topography. The dislocation densities, their distribution and preferred orientations are described. An interpretation is proposed for the origin of the many dislocation bundles observed.

Artifical calcite single crystals are grown by the hydrothermal method. Their luminescence under cathode ray bombardment is studied. Cathodo-thermoluminescence gives results extending those known for natural calcite. The cathodoluminescence displays, besides the orange-red emission attributed to manganese, an emission around 373 nm. Near liquid helium temperature, the emission of manganese disappears. Des monocristaux de calcite synthetique, obtenus par voie hydrothermale, sont l'objet de mesures de luminescence sous bombardement electronique. En cathodo-thermolumines cence les resultats de la calcite naturelle sont retrouves et etendus. En cathodoluminescence on observe, en plus de l'emission rouge orange due au manganese, une emission autour de 373nm. A une temperature proche de l'helium liquide l'emission du manganese a disparu.

Potash alum crystals grown in solution with and without polyethylene oxide traces are compared: the crystalline quality is improved in the first case. An interpretation is attempted.

Nearly perfect brushite crystals can be grown quickly in gels as checked by X-ray topography.

Discrepancies in lattice parameter determinations of lead phosphate crystals are discussed in terms of crystal growth.