Synthesis of GaN Crystals by Nitrogen Pressure-Controlled Recrystallization Technique in Na Alloy Melt

GaN crystals are synthesized by recrystallization technique in Na-Li-Ca alloy melt under different N2 pressure. X-ray powder diffraction results confirm that the structure of crystals is GaN with wurtzite type and there still have raw powders remaining. The total mass of GaN decreases with the nitrogen pressure reduces. No GaN crystals are found in the solution under N2 pressure of 0.4 MPa. The morphologies of the crystal are mainly prism and pyramid. The size of the crystal increases when closer to the liquid surface. Raman spectra indicates that these crystals are stress-free and crystal grown at 3.6 MPa has high structural quality or low impurity concentrations. The results reveal that the solubility and supersaturation of the solution are controlled by N2 pressure. The principle of GaN crystal synthesis by recrystallization is discussed.

Ultraviolet Light Effects on Cobalt–Thiourea Complexes Crystallization

In this work, a cobalt–thiourea complexes crystal synthesis was carried out comparatively with and without ultraviolet light assistance (λ = 253 nm), and its effect was studied. Through the solvent evaporation technique, crystalline forms were obtained, which were analyzed and characterized by different techniques: Raman spectroscopy, X-ray diffraction (XRD), and digital optical microscopy. Crystal’s shape changes were observed when comparing those obtained from the solution with and without ultraviolet (UV) assistance. It was found that the UV light effect on the crystals causes a structural modification of the complex synthesized in the (022) (120) planes and without UV assistance in the (002), (111), (131¯), and (132¯) planes. It is also possible to observe an increase in intensity by Raman spectra identified as Co–S bonds (297 cm−1) for crystals synthesized with UV assistance.

The Impact of Halogen Substituents on the Synthesis and Structure of Co-Crystals of Pyridine Amides

Strategies for co-crystal synthesis tend to employ either hydrogen- or halogen-bonds between different molecules. However, when both interactions are present, the structural influence that they may exert on the resulting assembly is difficult to predict a priori. To shed some light on this supramolecular challenge, we attempted to co-crystallize ten aliphatic dicarboxylic acids (co-formers) with three groups of target molecules; N-(pyridin-2-yl)picolinamides (2Pyr-X), N-(pyridin-2-yl)nicotinamides (3Pyr-X), N-(pyridin-2-yl)isonicotinamides (4Pyr-X); X=Cl/ Br/ I. The structural outcomes were compared with co-crystals prepared from the non-halogenated targets. As expected, none of the reactions with 2Pyr-X produced co-crystals due to the presence of a very stable intramolecular N-H···N hydrogen bond. In the 3Pyr series, all six structures obtained showed the same synthons, –COOH···N(py) and –COOH···N(py)-NH, that were found in the non-halogenated parent 3Pyr and were additionally accompanied by structure directing X···O(OH) interactions (X=Br/I). The co-crystals of the unhalogenated parent 4Pyr co-crystals assembled via intermolecular –COOH···N(py) and –COOH···N(py)-NH synthons. Three of the analogues 4Pyr-X co-crystals displayed only COOH···N(py) and –COOH···N(py)-NH interactions. The three co-crystals of 4Pyr-X with fumaric acid (for which no analogues structures with 4Pyr are known) formed –COOH···N(py)-NH and –NH···O=C hydrogen bonds and showed no structure-directing halogen bonds. In three co-crystals of 4Pyr-I in which –COOH···N(py)-NH hydrogen bond was present, a halogen-bond based –I···N(py) synthon replaced the –COOH···N(py) motif observed in the parent structures. The structural influence of the halogen atoms increased in the order of Cl < Br < I, as the size of σ-holes increased. Finally, it is noteworthy that isostructurality among structures of the homomeric targets was not translated to structural similarities between their respective co-crystals.