Single crystals formation methods

The description of Pbl2(i) single-crystal formation in a gel suggests that any metal-anion combination is suitable. What limitations does this method have ... 

The formation of such materials may be monitored by several techniques. One of the most useful methods is and C-nmr spectroscopy where stable complexes in solution may give rise to characteristic shifts of signals relative to the uncomplexed species (43). Solution nmr spectroscopy has also been used to detect the presence of soHd inclusion compound (after dissolution) and to determine composition (host guest ratio) of the material. Infrared spectroscopy (126) and combustion analysis are further methods to study inclusion formation. For general screening purposes of soHd inclusion stmctures, the x-ray powder diffraction method is suitable (123). However, if detailed stmctures are requited, the single crystal x-ray diffraction method (127) has to be used. 

X-Ray Diffraction. Because of the rapid advancement of computer technology (qv), this technique has become almost routine and the stmctures of moderately complex molecules can be estabUshed sometimes in as Htde as 24 hours. An example illustrating the method is offered by Reference 24. The reaction of the acrylate (20) with phenyldiazo derivatives results in the formation of pyrazoline (21). The stereochemistry of the substituents and the conformation of the ring can only be estabUshed by single crystal x-ray diffraction. 

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

The crystal growth of metal borides by gas-phase methods permits preparation of products at moderate T (1000-1500°C). This is an important advantage since most borides melt at high T (ca. 3000°C), which makes their crystal growth from melts difficult. In addition, the gas-phase methods lead to the formation of single crystals and solid films of incongruently melting borides. 

Grain boundaries form junctions between grains within the particle, due to vacancy and line-defect formation. This situation arises because of the 2nd Law of Thermodjmamics (Entropy). Thus, if crystallites are formed by precipitation from solution, the product will be a powder consisting of many small particles. Their actual size will depend upon the methods used to form them. Note that each crystallite can be a single-crystal but, of necessity, will be limited in size. 

Obviously, the major difference in the single-crystal and polycrystalline (crystallite) state is a matter of size. For the single-crysted, the size is leu ge (> 10 cm), wherecis in the polycrystalline state, the size of the cr3rstals is small (10 (im = 0.001 cm.) The methods for obfriining one or the other differ considerably. They include formation from ... 

The simple drop casting crystallization method, discussed in Section 3.1, has also shown its validity in the determination of hole mobilities at least for the TTF-derivatives DTTTF (Mas-Torrent et al, 2004), EDT-TTF-(CONHMe)2 (Colin et al, 2004) and DBTTE (Mas-Torrent et al, 2005). In all cases, 1 cm s In this case the formation of high-quality organic/insulating interfaces is a matter of serendipity (see the discussion on the formation of nanovolcanoes at the surfaces of drop-cast single crystals in Section 3.1). 

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