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Equipment and Methods Used for Single Crystal Growth

Crystal growing is an art, and there are as many variations to the basic crystal growing recipes as there are crystallographers. The techniques chosen will largely depend on the chemical properties of the compound of interest: Is the compound air sensitive, moisture sensitive? Is it hygroscopic? etc.

Czochralski Crystal Growth Method

The Czochralski crystal growth method is the method that is used to produce the majority of commercially available single crystals. This method yields fairly large crystals with low defect concentrations and is especially suited to semiconductor (e.g. SI and GaAs) and oxide (Nd: YAG, Nd:YVO4 and Ti: sapphire) crystal growth.

The units employ automatic diameter control (ADC) through the crystal weighing method. The stainless steel chambers allow atmosphere control with limited overpressure and vacuum capabilities. In addition to conventional seed rotation and translation capabilities, the systems also have crucible rotation and translation, coil translation and translation of an inner crucible or separator plate.

The Czochralski method (abbreviated as CZ in the technical literature) begins with powder that is 99.999999999% pure. A lot of that ridiculously pure powder is placed in a large crucible. The crucible is then put in a vacuum chamber, which is sealed and then filled with some conveniently inert gas, usually argon.

The chamber is then heated up to melt the powder. When the powder is nicely melted, a small seed crystal mounted on the end of a rotating shaft is slowly lowered until it just dips below the surface of the red-hot melt. The shaft rotates counterclockwise and the crucible rotates clockwise. The rotating rod is drawn upwards very slowly, allowing a roughly cylindrical boule to form by some technical magic as it does so. The boule can be as long as two metres, depending on how much melt there is in the crucible.

Optical Floating Zone

The Optical Floating Zone Unit uses ellipsoidal mirrors to focus the radiation to generate the temperatures necessary to melt the various materials of the feed and seed rods. The shells are gold plated, and they are elliptical in shape so that light rays generated from the halogen lamps on the ends of the shells are reflected to a focal point, which is then referred to as the "hot zone" which is ~1 cm in diameter.

The stainless steel rods that support the feed and seed rods rotate opposite each other thereby causing a shearing action to occur as the feed and the seed rods are joined and also during the duration of the growth. This counter-rotation generates convective flow to transport heat throughout the molten zone.

During the growth, the stainless steel rods both move down through the stationary hot-zone at a rate specified by the operator. The rate is usually very slow and is measured in millimeters per hour. This process melts the material from the feed-rod and solidifies it on the growing seed crystal.

Furnaces

Several different furnaces are used in crystal growing. The furnaces are used for sintering, preparing powders for crystal growth, developing charges prior to growth, and drying. There ia a range of ovens, box furnaces and tube furnaces that span the range from a few hundred degrees C up to 1800 degrees C.

Cold Isostatic Press

Cold Isostatic Press is used to press the powdered material for the feed and seed rods used for crystal growth in the Optical Floating Zone Units. Additional rod preparation methods which are used include arc-melting and hot isostatic pressing.

More information about crystal growth

Slow Evaporation. This is the simplest way to grow crystals and works best for compounds which are not sensitive to ambient conditions in the laboratory. Prepare a solution of the compound in a suitable solvent. The solution should be saturated or nearly saturated. Transfer the solution to a CLEAN crystal growing dish and cover. The covering for the container should not be air tight. Aluminium foil with some holes poked in it works well, or a flat piece of glass with microscope slides used as a spacer also will do the trick. Place the container in a quiet out of the way place and let it evaporate. This method works best where there is enough material to saturate at least a few milliliters of solvent.

Slow Cooling. This is good for solute-solvent systems which are less than moderately soluble and the solvent's boiling point is less than 100 deg C. Prepare a saturated solution of the compound where is the solvent is heated to just it's boiling point or a just below it. Transfer the solution to a CLEAN large test tube and stopper. Transfer the test tube to a Dewar flask in which hot water (heated to a temperature of a couple of degrees below the solvent boiling point). The water level should exceed the solvent level in the test tube, but should not exceed the height of the test tube. Stopper the Dewar flask with a cork stopper and let the vessel sit for a week. A more elaborate version of this involves a thermostated oven rather than a Dewar flask.

Variations on Slow Evaporation and Slow Cooling. If the above two techniques do yield suitable crystals from single solvent systems, one may expand these techniques to binary or tertiary solvent systems. The basic rationale for this is by varying the solvent composition one may inhibit growth of certain crystal faces and promote the growth of other faces, yielding crystals of suitable morphology and size. If you choose this route for growing crystals, it absolutely necessary to record the solvent composition you use! If crystal growing is an art, growing crystals from binary or tertiary solvent mixtures is that much more imprecise. Remember reproducibility is paramount in science.

Vapor Diffusion. (excerpted and paraphrased from Stout and Jensen p. 65). This method is good for milligram amounts of material. A solution of the substance is prepared using solvent S1 and placed in test tube T. A second solvent, S2, is placed in a closed beaker, B. S2 is chosen such that when mixed with S1 the solute will become less soluble. The test tube containing S1 is then placed in the beaker and the beaker is sealed. Slow diffusion of S2 into T and S1 out of T will cause crystals to form. If S2 is more volatile than S1 the solvent level will increase and prevent microcrystalline crusts from forming on the sides of T.

Solvent Diffusion (Layering Technique). This method also is good for milligram amounts of materials which are sensitive to ambient laboratory conditions (air, moisture). Dissolve the solute in S1 and place in a test tube. Slowly dribble S2 into the tube so that S1 and S2 form discreet layers. This will only be successful if 1) The density of S2 < S1 and 2) Care is exercised in creating the solvent layer. I have found that a syringe is the best way to add the second solvent. The narrower the tube , the easier it is to build up the layer. Five millimeter NMR tubes are excellent vessels to use for this crystal growing technique. CH2Cl2/Et2O is a good solvent combination to try this method (if your compound is insoluble in ether).

Reactant diffusion. This similar to the other diffusion methods except that solutions of the reactants are allowed to diffuse into one another. If the product of the reaction is insoluble, crystals of the product will form where the reactants mix. There is mention in the literature of this technique being used with diffusion in silica gels (see Acta. Cryst. 19, 483, (1965)).

Sublimation. Andrea Sella (a.sella@ucl.ac.uk) suggests a couple of ways to grow crystals of somewhat volatile air sensitive crystals. The first way is to simply seal a sample under vacuum into a glass tube and placing the tube into an oven for a few days or weeks. Larger crystals tend to grow at the expense of smaller ones. If it doesn't work raise the temperature of the oven or move to another hotter one. In some cases a tube furnace can be used. Andrea uses an all glass furnace which was originally designed by Prof. Geoff Cloke of Sussex University (email him for the details if you want to try this method). The tube can be placed close to one end of the furnace so that there is a mild temperature gradient.

The second method is to seal off a long piece of glass tubing at one end and put a joint with a vacuum adapter (or a Young's/Ace/Kontes valve) at the other. The sample is placed at the sealed end of the tube and is followed by a glass wool plug which cleans the tube as it is pushed along and serves to prevent bulk/crude material blowing along the tube itself. The tube is then evacuated and inserted into a length of copper pipe which will serve to produce a temperature gradient. The sealed end of the tube (where the sample is) and the copper jacket are then heated in an oil bath (vertical set-up) or in a tube furnace (either horizontal or vertical). The tube can be left under either dynamic or static vacuum (it's worth trying both).

Convection. One may attempt to grow crystals by convection by creating a thermal gradient in the crystal growing vessel. The idea behind this method is that the solution becomes more saturated in the warm part of the vessel and is transferred to the cooler region where nucleation and crystal growth occur. To create the convection one may use either local heating or local cooling. The velocity of the convection current is proportional to the thermal gradient across the vessel. Care must be taken to not make the gradient too large, or the convection will be too rapid and inhibit crystal growth.

To use local cooling, simply take a flat bottomed crystal growing dish (similar to what one would use for slow evaporation) and place one side of it against the heat sink. Normally, simply placing the vessel against a cold outside window will suffice. Notice that the heat sink needs to be against the side of the vessel. Laying the vessel on a cool surface will not induce convection.

A Thiele tube with a resistive heater attached to it makes a good apparatus to grow crystals by local heating convection. This method was contributed by Professor Clarence Pfluger (cpfluger@keene.edu) He writes,

First, use a solvent in which the solute has limited solubility at room or perhaps somewhat below room temperature, but of course increased solubility at elevated temperature. The Thiele tube is filled with this solvent and a small container with the compound in question is hung in the large part of the tube, slightly below the top "entrance" of the smaller 'side' tube. Wrap a section of the bottom of the small tube with nichrome wire to make a heating element (adjustable, using a Variac or such). When the bottom part of the smaller tube is heated, the solvent will begin to circulate, delivering heated solvent to the container with the compound which slowly builds up the solute concentration in the solvent. Since the tube has a temperature gradient ranging from the temperature of the solvent at the heater element to almost room temperature near the bottom of the 'large tube' crystals will grow, if the gods are with you, in this part of the tube. Obviously one can also wrap part of the Thiele tube with small plastic tubing with tap water flowing through it to enhance the temperature gradient, if needed.
Instructions for constructing a similar device may be found in the literature. See J. Appl. Cryst., 1971, 4, 333.

An inexpensive thermal gradient device can be constructed which can be used to obtain good crystals of "difficult to crystallize" compounds. This was suggested by Chuck Barnes (chemclb@mizzou1.missouri.edu) on the sci.techniques.xtallography newsgroup. According to Chuck, his advisor came up with this design after he had seen a rather elegant device described in the literature (Watkin, D.J., J. Appl. Cryst., 1972, 5, 250). To make the device do the following: Cut off the tip of a Pasteur pipet about one cm above the beginning of the narrowing part, and then heat seal the small end, you get a nice cheap thermal gradient vial. Place a slurry of your material in a relatively poor solvent in the vial and centrifuge to pack the undissolved material in the tip. After centrifuging, you have a pellet of solid (~5mm) in the tip, covered by clean "poor" solvent. Seal the vial with teflon tape and parafilm. I make the heater from the small cylinder shaped ceramic resistors which are usually available from the electronics shop. I found some green ones of 100 ohms which are a good size. Place just the tip of the vial with the solid pellet in the heater, with the vial at ~45 degrees from vertical. Apply voltage to get maybe 50 C at the resistor, and you have a nice thermal gradient up the vial. With luck, you get crystals growing up the vial. It seems important to make sure all the amorphous material is packed down...no powder where you want the crystals to grow. Chuck adds about this method, " This has given me excellent crystals at times, and has on occassion given superior crystals (smaller mosaic spread) even when crystals were available from evaporation."

Co-crystallants. If you are not getting suitable crystals from other crystallization methods, you can try to cocrystallize your compound with a "crystallization aid". In the literature (see J. Amer. Chem. Soc. 1988, 110, 639- 640) triphenylphosphine oxide (TPPO) has been used as a cocrystallant for organic molecules which are proton donors. Obviously, this method of crystallization is not for all compounds, but for those it is, the results may be rewarding. Essentially this method of crystal growing is the same as slow evaporation except that an equimolar amount of TPPO is added to the solution. I suppose that the other crystallization methods outlined above would also work with this method.

Counterions. If your compound is ionic, and is not giving suitable crystals with a given counterion, perform a metathesis reaction to change the counterion. Very often the crystal quality will improve when a different counterion used. Ions of similar size tend to pack together better and subsequently give better crystals. In addition, you might also get more desirable physical properties for your crystal. For example, the sodium salt of your compound might be hygroscopic, but the tetramethyl ammonium salt might not be or at least, less so. When selecting possible counterions, avoid ions which contain large non-rigid groups (like tetrabutyl ammonium). Use ions which have very limited conformational flexibility. A word of warning: Make sure that whatever ion you use for your metathesis does not react with your ion of interest.

Ionization of Neutral Compounds. Anthony Linden (alinden "at" oci.unizh.ch) suggests that if your compound is neutral and contains proton donor or acceptor groups, better crystals may grown by first protonating or deprotonating (as applicable) your compound. The ionic form of your compound could then take advantage of hydrogen bonding to yield better crystals. This will alter the electronic properties of your compound, but if a general confirmation of the success of a synthesis is what is needed from the structure determination, then this is not a problem.

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