A Flotation Method for Growing Large Single Crystals A popular technique for growing large single crystals is to tie a seed crystal to a string and suspend it in a. ssturated solution to effect unhindered growth in all directions.' With slow evaporation and rigid temperature control one can grow beautifully developed single crystals. Often, however, the surfaces of these crystals become occluded by a "snowing effect" of tiny crystallites from the surface of the solution due to small temperature gradients that promote rapid nucleation a t the evaporating surface. Moreover, small vibrations or disturbances sometimes induce t,he crystal to rotate or rock about the string axis causing torsional stress a t thestring-crystal contact which results in cracks and stress marks and often also initiates occluded crystalline gmwth a t the crystal surface. To avoid the above hazards, a novel flotation method is advocated that rapidly yields large single crystals using only a. minimal amount of saturated solution. I n the flotation method one simple technique is to cement a part of the crystal with molten paraffin wax to a flat polyethylene disc which then provides a. convenient floating canopy that supports the immersed and downward pointing crystal (see part A of figore). The relatively inert,, inexpensive, and readily available polyethylene with its low specific gravity of 0.92 is ideally suited for this use with aqueous or other solutions of higher specific gravity as is also the molten or softened paraffin which adheres st,rangly to both polyethylene and the crystal and is completely inert to all solvents.' Other adaptations of the above method include prying the crystd gently into a slotted opening of the polyethylene disc (see part B of figure) (thus eliminating the use of psraffin) or dternstively utilizing only a solid cake of paraffin (specific gravity 0.87-0.91)3 to provide the protective supporting platform. The farmer technique is necessary for growing crystals from solutions a t higher temperatures where pareffin softens or melts. The limiting high temperature is around 11.5'F where polyet,hylene begins to mften or d i s t o r t . V h e latter method using only paraffin is ideal for growing crystals a t 2 5 T or lower in extremely caustic organic or inorganic solutions of specific gravity greater than 0.92 where polyethylene may soften or decompose. Using the flotation method one can allow for rapid - .... .... ........ . . 1 tid.vi.r evaporation of the saturated solut,ion without worrying 1**.1111.," about occluding the suspended and shielded crystal with tiny smiace crystdlites, thus saving eonsidereble time when compared to the usually much slower conventional .0,ure,.* string-tied crystal technique. Nor does one need t o .~I"llOn .totw dirk pma c v s + m ~ worry about minor vibrations or disturbances causing stress or distortion to a crystal firmly imbedded in a freefloating platform, The author hss used this method successfully a t 26'C for growing large single acicrdar crystals of a variety of a-hydroxy and aralkoxy carboylate complexes with divalent trensition metals. The floating downward pointing needlelike crystdlites of maximum dimension of 2 mm grew to large 2-3 em sise within a 2-wk period by rapid evaporation from'mall elongated vials initially containing no more than 20 ml of saturated solution. I t took approximately 2 man using larger containers and considerably more saturated solution to grow the same size non-occluded crystals by slow evaporation using t,he conventional string-tied crystal technique where the needle dimension of the crvstal erew in an a ~ ~ m x i m a t ehorizontal lv direct,ion.
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' HOLDEN,A., AND SINGER,P.,
"Crystals and Crystal Growing," Doubleday Anchor Books, New York, 1960, 93-104. LANGE,N. A., "Handbook of Chemistry," McGraw Hill, New York, 1961, p. 849. ' WEAST,It. C., AND SELBY,S. M., "Handbook of Chemistry and Physies" Chemical Rubber Co., Cleveland, 1967-1968, F-I.
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Journol of Chemicol Educofion
