Crystal Growth in Gels Steven L. Suib University of Connecticut, Storrs. CT 06268
The ability to prepare large single crystals of inorganic and organic compounds is important in several fields in science and in many industrial and technological products. With large single crystals a scientist is capable of accurately describing bond distances and coordination environments through the aid of X-ray diffraction and neutron diffraction methods. There are several uses of crystals with special physical properties. such as the oiezoelectric nature of quartz and the semicbnductiug properties of materials like silicon, germanium, and gallium arsenide. Several methods are used to grow large single crystals of inorganic and organic compounds. Growing crystals from aqueous solutions can he a hobby for children and is often one of the first experiences in science that young people encounter. An enormous numher of high school and undergraduate general chemistry experiments have been developed for the growth of large crystals and there are many hooks discussing this matter (1-3). Most of the simple procedures in crystal growth deal with aqueous solutions and crystallization that is either induced by cooling or by evaporation. Other solution mowth methods include flux erowth ( 4 ) . electrolvtic methods 75), hydrothermal synthesis c6) of m&rials likezeolites and eel " erowth (7). "~ More elal;oiate techniques such as the Czochralski method use nulline rods and melts for materials like calcium turnstate ( 8 ) . ' ~ h egridgeman-~tockhar~er furnace is a special apparatus for controlled melt erowth of cadmium fluoride crvstals (9). Finally, several methods for crystal growth include the use of the vapor phase such as vapor phase transport methods used in the production of metal dichalcogenide crystals (10) and sublimation techniaues. One ot'the simplrst, and yet relatively unknnwn, methods of crvstnl erowth in\.olves the EPI technique (7). Several experi&tli~involving crystal growth in gels are presented below. These areeasy to perform and require very littlr equipment. However, the romplrte understanding of the experiments is not straightforwnrd since it rrquires the knowledge of a numher of rather complex prope&es of gels, crystals, and the structures of these materials. Nevertheless, the simple experiments can he used to explain rather suhtle phenomena such as syneresis and nucleation (7). Once these are understood qualitatively, other phenomena surh Liesegang ring formation ( 1 1 ) and Ostwald ripening (12) can hestudied much more intelligently. The experiments are performed with an aqueous silicate gel, hut the phenomena observed are characteristic for many other types of gels such as agar or, for that matter, a common evervdav growth in gels attained . . substance. .iellv. . Cwstal . importanrr many years ago; for example, Pb12 crystals were erown in fruit iells a r k in the late 1800's (1.1).More recently ihe gel growth te&ique has heen used for the preparation of several semiconducting materials such as CsSn21S and RbSnBrs (14). These latter materials could he prepared by solution and melt growth techniques, hut the size of the crsstal?i was much sialler. For instance, CsSnllS crystals as large as 1 mm on an edgr ran he grown by the gel technique, hu
