Crystal Research and Technology

Existing and new results in nucleation in solutions are outlined from a unified point of view. The thermodynamics of the process is considered and expressions are given for the supersaturation, the nucleation work and the size of the nucleus in homogeneous or heterogeneous nucleation. It is shown how the nucleation theorem can be used for a model‐independent determination of the nucleus size from experimental data. The mechanism and kinetics of nucleation are also considered and formulae are presented for the supersaturation dependence of the monomer attachment frequency and the stationary rate of homogeneous or heterogeneous nucleation. General expressions for the induction time and the critical supersaturation ratio for crystallization are given. An approximate formula is derived for estimating the width of the metastable zone with the help of data for the solubility of the substance crystallized. Existing experimental data are used for verification of the validity of some of the presented theoretical dependences.

Single crystals LiGaX₂ (X = S, Se, Te) of optical quality were grown, with transparency ranges at 5 cm^‐1 absorption level of 0.32‐11.6 μm, 0.37‐13.2 μm and 0.54‐14.2 μm, respectively. The first two, LiGaS₂ and LiGaSe₂, have a wurtzite‐type structure whereas LiGaTe₂ is tetragonal (chalcopyrite lattice). The three refractive indices were measured in the whole transparency ranges of LiGaS₂ and LiGaSe₂ and n_a and n_c were found to be very close (quasi‐uniaxial optical anisotropy) with a crosspoint at 6.5 μm (LiGaS₂) and 8 μm (LiGaSe₂). Sellmeier equations were fitted and phase‐matching conditions for second harmonic generation (SHG) were calculated: the 1.467‐11.72 μm spectral range for the fundamental is covered by LiGaS₂ and LiGaSe₂.

Crystallization from solutions often starts in such a way that thermodynamically unstable phases appear first followed by recrystallization to thermodynamically stable phases. This empirical observation, called as Ostwald's rule of stages, is explamed here on the basis of structure changes of solutions. Experiments supporting this explanation have been carried out with aqueous solutions of citric acid, sodium hydrogen phosphate and ferrous sulphate, respectively.

Cobalt phthalocyanine nanowires with new crystal structure and broad optical absorption spectra were fabricated by using organic vapor phase deposition method. The morphology, crystal structure and optical properties of CoPc nanowires were characterized by SEM, X‐ray diffraction, Fourier transform infrared and UV‐visible spectroscopies. Analyses of X‐ray diffraction patterns and Fourier transform infrared spectra indicate that the crystal structure of CoPc nanowires represents a new polymorph, which is designated J‐CoPc. These J‐CoPc nanowires with high directionality (average diameter ∼50 nm) surprisingly possess much broader optical absorption spectra in the visible spectral region than those of the α‐ and β‐phase CoPc, enabling high potential for practical applications in novel molecular electronic/optical devices.

In this study the effects of citric acid concentration, used as organic emulsifier, on the perovskite phase formation of the nano strontium manganite or cobaltite samples were studied. Stoichiometric perovskites in the absence and presence of citric acid were prepared by drying a solution containing molar ratio of Sr(NO₃)₂/Mn(NO₃)₂·4H₂O and Sr(NO₃)₂/Co(NO₃)₂·6H₂O = 1 followed by calcination at 900 °C for 5 h. Citric acid concentration, selected to be 0.0, 0.3, 0.6, 1.0, 1.3, 2.5 and 5 times of the total number of moles of the nitrate ions. The results revealed that increase in the citric acid concentration, larger than number of moles of the nitrate ions equivalent, deteriorates the perovskite phase formation. Instead, a new phase of carbonates and metal oxides are appeared. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This work reports about the growth behavior and defect formation during the directional solidification of mono‐crystalline lab‐scale silicon ingots in which split seed crystals were used. Thereby the split seeds have certain crystallographic orientations which chosen to induce the growth of selected grain boundaries between the seeded regions. It will be shown that the grain boundary type which is formed inside the seed gap correlates quite well with the intentionally selected orientation relationship of the adjacent seeds. During further growth of the ingot the grain boundaries show a different behavior with respect to their structure and the formation of other crystal defects, e.g. dislocations. It was found that some of the grain boundaries continue straightly in growth direction without any change of structure or formation of other defects. In contrast, other grain boundaries split into several new grain boundaries with a simultaneous formation of dislocations around them. This behavior correlates very well with the energy content of the respective grain boundary, i.e. with its symmetry or coincidence lattice site parameter Σ. In consequence the use of special grain boundaries between the seeds can significantly reduce the defect generation above the seed joints during growth of quasi‐mono silicon ingots.

The use of variants to analyze fcc/bcc orientation relationships is demonstrated by EBSD data. Because of multiply occupied poles in the stereographic projections low indexed pole figures are not always suitable. This is mostly caused by the convolution of all scattered individual orientation data as the single poles cannot resolved in the pole figure. Pole figures of higher indexed lattice planes more reliably reflect the character of the orientation relationship since no overlapping of poles occurs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single crystal of 8‐hydroxyquinoline (8HQ) having chemical formula C₉H₇NO, an organic nonlinear optical (NLO) material has been successfully grown by slow evaporation solution growth technique at room temperature. The crystal system has been confirmed from the powder X‐ray diffraction (PXRD) analysis. The crystalline perfection was evaluated by high resolution X‐ray diffractometry (HRXRD). From this analysis we found that the quality of the crystal is quite good. However, a very low angle (tilt angle 14 arc sec) boundary was observed which might be due to entrapping of solvent molecules in the crystal during growth. Its optical behavior has been examined by UV‐Vis. analysis, which shows the absence of absorbance between the wavelengths ranging from 400 to 1200 nm. From the thermal analysis it was observed that the material exhibits single sharp weight loss starting at 113°C without any degradation. The laser damage threshold was measured at single shot mode and the SHG behavior has been tested using Nd:YAG laser as a source. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A new method has been developed for the synthesis of mixed‐valence ammonium vanadate crystals. Single crystals of (NH₄)₂V₃O₈ were synthesized on a large scale by hydrothermal reduction of NH₄VO₃ in ethanol‐H₂O solutions in the presence of triblock copolymer Pluronic P123. The crystals are shining thin plates with (001) cleavage planes. Calcination of the (NH₄)₂V₃O₈ crystals at 300°C or above resulted in pure phases of V₂O₅. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)