Crystal growth by sublimation

The FXe0S02F had sufficient kinetic stability for crystal growth by sublimation, but the Xe(0S02F)2 decomposed slowly and cleanly to provide the peroxide source of the SO3F radical, first made by F. B. Dudley and G. H. Cady. This route via Xe proved to be a safe and convenient laboratory synthesis for this powerful oxidizer. The perchlorates proved to be dangerously explosive ... 

Crystal growth by sublimation is extensively used for obtaining high purity crystals. The nucleation and growth of crystals from the vapor invariably occurs by grafting to some solid support, usually in practice some cold spot on the walls of a container. A likely hypothesis is then that the solid surface somehow acts as a nucleation catalyst. In principle, nothing forbids the simulation of the vapor-solid equilibrium along the same lines as for vapor-liquid equilibria. 

Powder Preparation. There are several routes to preparing SiC powders having variable purity levels, crystal stmcture, particle size, shape, and distribution. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. 

As a class, the azides present several problems when attempting to use the well-known techniques of crystal and thin-film growth. Their instability conflicts with the successful application of most thermal approaches, and their restricted solubilities limit the usefulness of growth from aqueous solutions. For instance, growth by sublimation techniques is not feasible because the azide ion decomposes at elevated temperatures. Melt growth is limited to the azides of K, Rb, Cs, and Tl, of which only KN3 is sufficiently stable at its melting point to permit routine success. Thus, while the azides must, in general, be crystallized from solutions, precautions are also necessary to avoid hydrolytic decomposition when water is used as the solvent. 

The base and impurity compositions of crystals are influenced by the crystal growth kinetics. Growth by sublimation is performed at rather low temperatures, below 0.7 T. In addition, the normal growth rates are considerably higher than those of self-diffusion of the components, i.e., of silicon and carbon, in the lattice. It follows that SiC crystal growth takes place in the... 

Tanaka et al. (13) have investigated AIN single crystal growth by the sublimation method devised by Slack and McNelly (12) in order to produce substrates for GaN-based diodes. They reported that the morphology of AIN crystals depended on the sublimation temperatures. 

In addition to sublimation or vaporization, we can also use chemical tremsport as a method of single crystal growth. For example, we could use either of the apparati of 6.12.1. or 6.12.4. to grow a crystal of ZnCl2 by the following reactions ... 

The growth of large crystals at the expense of small ones occurs, not only in solutions, but also under conditions which resemble even more closely the growth of large drops, i.e., by sublimation. The phenomenon has been observed in the case of sulphur and of sulphur trioxide in an evacuated space, and in the case of camphor crystals condensed from the vapour on a cold glass surface. 

The other method for nitride crystal growth is based on sublimation. Naoi et al [7] reported the RC for GaN crystals (of size up to 0.8 mm x 3 mm) grown by this technique. The measured FWHMs were... 

In some cases, new phases that may not be detectable by other methods may be detected optically (Chang et al. 1995). Solid state conversions and their monotropic (Burger et al. 1997) or enantiotropic nature (Henck et al. 2000), or the products of desolvations may be easily recognized (Schinzer et al. 1997). Intimate processes of polymorphic behaviour, such as nucleation, crystal growth, habit transformation, sublimation and properties of the melt (e.g. degradation) may be readily observed and video recorded (de Wet et al. 1998). 

Crystal growth from vapour, as in sublimation, makes the simplest approach to the subject, and the principles involved apply to the more complicated situation in solutions. Consider the flat 001 surface of a cubic crystal in contact with its vapour and partly covered by an incomplete layer (Fig. 114). 

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