Page 40 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

diffraction analysis, relatively few nuclei should be formed rather than multiple sites
of nucleation that will yield microcrystalline solids.
High quality crystals may only be obtained when the rate of deposition onto a
nucleation site is kept at a rate sufficiently low to allow oriented growth. A high
growth rate may lead to defects in the crystal, forming multi-branched or dendritic
crystallites through rapid growth in too many directions. As molecules in the gas-
phase or solvent interact with the surface of the growing crystal, they may or may
not be preferentially adsorbed. That is, a nucleation site that contains steps, ledges,
or surface depressions is able to provide more efficient crystal growth due to the
prolonged interaction of suspended molecules with a greater surface area.
Experimentally, the successful growth of single crystals on the order of
2
0.01–0.1 mm is not trivial, and has long been considered as a “black art”! Figure 2.6
illustrates common techniques that may be applied for crystal growth via sublima-
tion or from solution. Perhaps the most important starting point is with a solution
that is filtered to remove most suspended nuclei. For air-sensitive solutions, this
requires careful manipulation using filtering cannulas and Schlenk techniques. Most
of the solvent is then removed to create a nearly supersaturated solution, and then
left undisturbed. Another method that is used to grow single crystals from saturated
solutions consists of layering a “nonsolvent” onto the top of the saturated solution.
Since the compound of interest is not soluble in the layered nonsolvent, crystal
formation may begin at the interfacial region. If the nonsolvent is volatile, vapor
diffusion may provide another route for the growth of crystals.
Depending on the nature of the suspended molecules, crystal formation may begin
immediately, or may even take months to occur. Many organometallic chemists
have been surprised to find large crystals at the bottom of flasks placed in the back of
the freezer, after months of observation and concluding that no crystals would ever
be realized. Sometimes, fortuitous crystal growth may also be realized from unex-
pected sources. Crystals may be formed from the incorporation of impurities such as
dust or vacuum grease, or from surface scratches on the inside walls of the flask.
Surprisingly, NMR tubes are notorious for the formation of large crystals, discov-
ered only as the tubes are about to be cleaned! Quite often, chemists set these tubes
aside for weeks after the analysis, creating an undisturbed environment for crystal
growth. NMR tubes are long and narrow, suppressing convection currents, and
solvents very slowly evaporate through the low-permeable cap. Hence, the overall
take-home message for crystal growth is to exercise patience; in the process of
impatiently checking for crystal growth, additional nucleation sites are often intro-
duced, resulting in the formation of small crystals. Fortunately, many institutions
now possess CCD X-ray diffractometers that allow for enough data to be obtained
from even microcrystalline solids, in a fraction of the time required for older four-
circle instruments.
Although much crystallization from a solution is performed at low temperatures,
crystals may also be formed from molten solids. For example, the Czochralski (CZ)
method for purification of silicon uses a seed crystal on the surface of the melt
maintained slightly above its melting point. As the crystal is slowly pulled from the