Growing Crystals

Type of physical science: Condensed matter physics

Field of study: Solids

Crystals are grown to increase the understanding of solid structures and for many practical applications, especially in electronics.

Overview

Crystals support the basic idea that nature has an underlying order. Large, single crystals of diamond, salt, or sugar often impress the viewer with their flat surfaces, consistent angles, and clarity. The value of gems is associated with these qualities. Most solids, however, have a crystalline nature even if they fail to form single, large structures. Such materials with many small crystals are called polycrystalline. Metals are generally polycrystalline. The zinc coating on a newer trash can is a good example. A brass doorknob that has been etched by perspiration may also show the interlocking crystal structures. If there is no crystalline structure, the material is said to be amorphous, and it may even forfeit its claim to being a "true" solid.

Glass, asphalt, and certain plastics are common materials without crystal structure and can be thought of as materials that fall on the fuzzy boundary between liquids and solids.

Crystals are rigid bodies in which the constituent parts are arranged in a repeating pattern. The repeating pattern of unit cells determines many of the properties of the crystal. Such properties as hardness, density, conductivity, and, most obviously, shape depend on the nature of the repeating pattern.

Crystals are nonliving and grow by adding atoms, ions, or molecules from the outside.

Crystals first form a tiny nucleus of their own material or sometimes a foreign material (a seed crystal). As materials solidify either from the gaseous or from the liquid states, the molecular particles line up, layer by layer, in a three-dimensional array and the crystal becomes larger. Four basic steps can be identified: diffusion of the molecules to the surface of the growing crystal, diffusion to the appropriate location on the surface, bonding, and diffusion of the heat produced by crystallization away from the growth area. The rate of growth can be limited by any one of these steps.

The rate of growth can be increased by lowering the temperature of a solution and causing the liquid to be supersaturated. Such a solution is unstable because it contains more molecules of solute than are normal for a given temperature. If the solution is very far from stability, the growth pattern of the crystal may be affected. Treelike (dendritic) shapes, as in the case of snowflakes, may develop.

Another principle is that crystal growth is generally faster at relatively higher temperatures. Larger grains will also grow faster than smaller ones at higher temperatures. Larger faces of a crystal grow more slowly than smaller ones because of the relative amounts of material required. A small face may disappear as its edges become part of a larger face.

The pattern of growth is called the crystal's "habit." The atoms bond with one another to form basic three-dimensional structural patterns called unit cells. These unit cells repeat throughout the crystal. A two-dimensional example of a unit cell is the repeating design in wallpaper. Fundamentally, the shape and properties of the crystal depend on the atoms involved and the angles at which they bond with each other. In metals, however, fixed bond angles do not play a major role. Metals with dense packing of particles tend to have the most stable structure.

If one geometrically links the basic particles, the resulting imaginary network is called a lattice structure. Note that it may be normal for some intersections in a particular network (face-centered cubic) not to be occupied; the lattice is not the same as the actual structure. The concept of a lattice structure, however, assists thinking about the type of organization and allows classification.

Habit of growth varies with environmental conditions. While interfacial angles remain basically unaltered, variations occur. In nature, crystals are seldom formed under optimum conditions, and the perfect crystal is extremely rare. Many crystals grown in the laboratory are never found in nature, but even these are likely to be imperfect. Impurities, the time allowed, pressure, and temperature may all affect the rate of growth and the product. The resulting arrangement of the atoms may not be perfect.

Sometimes, as a crystal grows, an atom may fail to fill a usual station on the lattice structure. If many of these vacancies occur, the crystal will appear cloudy. Then again, an atom may be trapped in a space where no atom should be. This is called an interstitial defect.

An impurity may be included in the structure. This is usually not desired. In the case of ruby crystals, however, the beautiful red color results from an impurity. Dislocations in the crystal structure may result from deformation of the lattice structure. An extra layer of molecules may begin or disappear at a particular place.

In many solid materials, especially metal alloys, growth of the solid structure may also begin at many different locations and result in many small crystalline structures that finally meet but have different orientations. "Grain boundaries" are formed as crystal structures meet. These also affect the properties of the material.

Some crystals form differently under certain conditions. Two different minerals, graphite and diamond, are both made from different arrangements of identical carbon atoms. The carbon atoms in graphite use covalent bonds to form flat sheets, with weaker forces holding the sheets together. This allows the sheets of carbon to move. In contrast, the carbon atoms forming diamond use covalent bonds exclusively to form a rigid three-dimensional structure. The internal structure affects its properties. Hence, the graphite is slippery, flaky, and soft, while diamond is one of the hardest substances known. In 1955, after many years of effort, scientists succeeded in making diamonds in the laboratory by using extremely high pressure and temperature.

Certain alloys of metal change their structure easily within a given temperature range but then snap back to their rigid crystalline shape when the temperature changes. "Memory metals" are able to transform their shapes and many applications are likely as they become economical.

Molecules of water may be bound intimately with the atoms that form the crystal.

Nickel sulfate forms two different crystals depending on the amounts of water in the structure.

The formula of such a crystal is shown as a hydrate: in this case either NiSO₄∙6H₂O or NiSO₄∙7H₂O. A blue crystal of copper sulfate, CuSO₄∙24H₂O, will turn into a white powder if the water is allowed to evaporate. Hydrated crystals should be stored in a sealed jar to prevent this from occurring.

Many methods for growing crystals depend on the particular product desired and the preferences of the scientists. Crystals can be formed from a melt of the substance to be crystallized with no water present. In one procedure, growth occurs as the container cools with the hope that only one crystal will form. In a variation of this method, the seed crystal is gradually withdrawn from the surface of the melt but still maintains contact as it grows. A flux (foreign material) can be added to lower the melting point. Later, the mixture can be separated if the flux is soluble in a solvent such as water, while the desired crystal is not.

Zone melting is a technique that moves a series of hot zones along a relatively long mass. Impurities may be either distributed or moved to the end of the ingot by this method. The purest manufactured materials known have been produced by zone melting. Also, crystals of germanium, with only one foreign atom in 10 billion, have been produced by this method.

In the Verneuil method, rubies and sapphires can be made by feeding powdered aluminum oxide into an oxygen-hydrogen flame, which is directed at the support on which the crystal is to be grown.

Many large crystals have been grown from hot solutions where water is the solvent and the pressure is greater than normal. Autoclaves are often used in this process.

Some crystals can easily be produced using moderate temperatures and water as a solvent. Large sodium chloride (common table salt) crystals can be grown by allowing the water to evaporate from a saturated solution. This produces small crystals called seeds. One seed can then be selected to be suspended by a thread in a second saturated solution. A possible variation is to add more solute after the crystal grows.

Temperature can be changed as the crystal grows to decrease the solubility. With most substances (but not all) the solubility is decreased as the temperature is lowered. Hence, the crystal will gain more material as the temperature of the solution drops.

One often thinks of crystals as growing only from an aqueous solution or solidifying from a melt. Yet, crystals can be grown from the gaseous state. Crystals in this case are often, but not always, needlelike. Iodine crystals may be formed from vapor.

Chemical reactions where the resulting product of the reaction crystallizes are sometimes used. The production of ceramics for superconductors often depends on such chemical reactions.

Applications

Crystals can be grown with very basic equipment. The results will vary considerably.

One method is called growth from solution. To illustrate the growth of crystals, obtain sugar, salt, or borax, which are available at grocery stores. Rochelle salt can be obtained at some drug stores or high school chemistry laboratories. This basic recipe can be used with such kitchen chemicals. Prepare a saturated solution of the chemical in a peanut butter jar. Allow the jar to stand undisturbed. Small crystals of the salt should appear in the jar as the water evaporates.

Select a single crystal from these as a seed crystal and tie it to a thread. Save any extra seed crystals for further experiments.

Prepare a second saturated solution of the chemical. Be sure that the solution is at room temperature and then pour it into a peanut butter jar. Suspend the seed crystal into the center of the solution. If the solution is warmer than the crystal, the seed crystal may dissolve. Allow it to stand undisturbed. This is the easiest way to grow large crystals at home. It is important to allow the solvent to evaporate slowly.

There is considerable demand for various types of synthetic crystals. Most gems are single crystals. Transistors are crystalline materials with small amounts of impurities. An impurity in a crystal structure can change its electrical nature. Consider a crystal of silicon.

Silicon forms four bonds, one with each of its neighboring atoms. Nevertheless, if an impurity of arsenic, with a valence of five, is trapped in the lattice structure, four bonds will form with adjacent silicon atoms, while one electron remains free to travel. Such electron-rich material is called an n-type semiconductor. A reverse situation occurs if boron, which has three valence electrons, is "doped" into the silicon. A "hole" is produced. The hole may also move as electrons move in the structure. Crystals with holes are called p-type semiconductors. If n-type and p-type crystals are interfaced, the direction of current can be controlled (a rectifier), and if p-type material is placed between layers of n-type, current traveling from one end to the other can be amplified by varying the charge on the center.

Thousands of transistors can be set up together on one chip of material with other electronic components as an integrated circuit. The chip may be the size of a fingernail.

The properties of the n-p junction can be used to convert solar energy or radiation into electrical energy. A practical solar cell was developed at Bell Telephone Laboratories. Amorphous silicon is 5-8 percent efficient, while polycrystalline cells can reach 11 percent efficiency. Monocrystalline silicon is generally 14-16 percent efficient. Martin Green, an engineer at the University of New South Wales, patented a monocrystalline silicon cell, which has a laser-etched surface for minimum reflection. This cell is about 18 percent efficient.

Unfortunately, crystalline silicon cells tend to be brittle and tricky to assemble.

Crystals such as ruby have been used to produce laser beams. Also, a laser beam passing through some crystals can, after a short time, suddenly burst into a spray of light. This "photorefractive" effect is caused by a change in the arrangement of atoms in the crystal (and hence its ability to bend light). It may have possible computer information storage applications.

Experimental superconducting materials are being produced by chemical reactions.

X-ray interference patterns are then used to check if the crystalline structure has turned out as expected. Superconduction involves removal of electrical resistance from a conductor.

Superconduction has always required temperatures that approach -273 degrees Celsius--as cold as it can get. Practical conductors will need to operate at warmer temperatures. The search is focusing on certain ceramics. One new, breakthrough material (yttrium-barium-copper oxide) shows screw dislocations in its thin film when viewed with the scanning tunneling microscope.

Some scientists may think that this type of dislocation may be allowing superconduction of current without the high costs of supercooling the material. The material superconducts at -173 degrees Celsius.

Context

In 1669, Nicholas Steno observed that the angles between corresponding faces on different crystals of the same substance were identical. By the late eighteenth century, this first principle of crystallography included the idea that external appearances of crystals of the same material may vary with different environmental conditions. In 1784, Rene-Just Hauy suggested that crystal symmetry reflected their internal order. In 1912, Max von Laue showed that X rays had wave properties by causing their diffraction. Such diffraction patterns occur with water waves, sound, light, and other energy when the waves of the phenomena pass through openings that are near the size of their wavelength. Von Laue aimed a narrow beam of X rays at a crystal and recorded a diffraction pattern on film. William Henry Bragg and his son, Lawrence Bragg, turned the experiment on its head, establishing X rays as a powerful tool for the study of different crystal structures and developing mathematical rules that showed how the crystal pattern could be deducted from the interference pattern. They treated the diffraction as if it were caused by "reflections" off the layers of atoms inside the crystal. Waves reflected from the closest layer traveled a shorter distance than those reflected by the next layer. If the reflected waves reinforced each other, the returning waves were in phase, and the distance between the layers could be calculated in terms of the wavelength.

On this basis, it was crystal physics that led to deeper understanding of biology. In 1953, the structure of the deoxyribonucleic acid (DNA), the genetic code molecule found in all living things, was determined by Francis Crick and James D. Watson. Using excellent X-ray diffraction pictures taken by Rosalind Franklin, Watson and Crick determined the number of strands, the pitch, and the unit cell size in the DNA molecule.

Ironically, impurities in crystal structures turned out to be very important in electronics.

The invention of the transistor in 1947 by John Bardeen, Walter H. Brattain, and William Shockley at Bell Telephone Laboratories revolutionized the size of electronic circuits and their power requirements. Bell Labs also developed the first solar cells and was first to notice the photorefractive effect.

The development of the integrated circuit in 1958 by Jack Kilby at Texas Instruments continued the miniaturization of all electronic devices. The integrated circuit chip included many transistors wired together with other electronic devices on a small piece of semiconducting material. The revolution in solid-state electronics and modern computers has fundamentally depended on the work done in growing and understanding crystals.

In the late 1950's, General Electric commercialized producing diamonds from graphite.

This developed into a half-billion-dollar-per-year industry that produces diamond-coated cutting tools for many purposes. Laboratory production of gem-quality diamonds has been successful but not economical.

Many researchers believe that the behavior of superconducting ceramics can be explained by their crystal structure. Practical normal-temperature-range superconductors hold much promise for the design of electronics, computers, and transportation.

The study of crystalline growth with its insights into the properties of matter continues to be vital to science and technology.

Principal terms

CRYSTAL: generally, a transparent solid with flat surfaces; more technically, the orderly, rigid internal structure of a solid no matter what the external appearance

LATTICE STRUCTURE: a regular three-dimensional structure consisting of imaginary points at which atoms in a crystal are fixed; each atom may vibrate around its location but not normally move through the structure

SATURATED: a stable situation in which a solvent at a given temperature contains all the solute that it can hold in solution

SOLUTE: a chemical, such as salt, that has been dissolved in a liquid

SOLVENT: a liquid, such as water, which is used to dissolve a chemical

SUPERSATURATED: an unstable solution in which there is more solute dissolved in the solvent than it can hold at a given temperature

UNIT CELL: the basic unit of arrangement of atoms that repeats to form the crystal; the smallest unit with all the properties of the crystal

Bibliography

Amato, Ivan. "The Curling Crystal Club." SCIENCE NEWS 135 (February 25, 1989): 124-125. Scientists are not always sure about the various chemical and physical factors affecting crystal growth. This article describes how an organic compound produced a helical crystal, which coiled one way in the Northern Hemisphere but the opposite in the Southern Hemisphere. Some other crystals have also shown curling and uncurling during formation.

Augarten, Stan. BIT BY BIT: AN ILLUSTRATED HISTORY OF COMPUTERS. New York: Ticknor & Fields, 1984. A very readable account of how modern computers developed. Chapter 8 is of special interest because it tells the story of the necessary development of semiconductors and integrated circuits. Good illustrations. The fundamental work done at Bell Labs on crystal structures was a key to the revolution in electronics.

Cahn, R. W. "Transformations." In THE ENCYCLOPAEDIA OF IGNORANCE, edited by Ronald Duncan and Miranda Weston-Smith. New York: Simon & Schuster, 1977. Cahn muses in an interesting essay about the polymorphic forms of crystalline materials that have been found or manufactured. He discusses the attractiveness of the further study of steel, memory molecules, and liquid crystals.

Gleick, James. CHAOS: MAKING A NEW SCIENCE. New York: Viking, 1987. Gleick examines the new interest in the study of disorder in nature. Especially interesting is his discussion of why all snowflakes are different, beginning on page 308. The many variables in the formation of a snowflake as it falls are beautifully described.

Glusker, Jenny Pickworth, and Kenneth N. Trueblood. CRYSTAL STRUCTURE ANALYSIS: A PRIMER. New York: Oxford University Press, 1972. This book is intended for undergraduates with some previous chemistry and physics. Careful definitions, a glossary, many illustrations, and an extensive annotated bibliography make this a valuable source for thinking about structure. Although slightly technical; it is still a helpful reference for the novice.

Holden, Alan, and Phylis Singer. CRYSTALS AND CRYSTAL GROWING. Garden City, N.Y.: Doubleday, 1960. A valuable outcome of the efforts to improve science teaching in the late 1950's was a series of paperbacks by experts on fundamental topics in physics. The Physical Science Study Committee aimed the books at students and the general public. If the reader wishes to grow crystals, this book contains both theory and recipes from a chemist at Bell Telephone Laboratories.

O'Donoghue, Michael. THE ENCYCLOPEDIA OF MINERALS AND GEMSTONES. New York: G. P. Putnam's Sons, 1976. The first two chapters of this text are especially appropriate to understanding crystal growth. More than half of the book contains information and photographs covering more than one thousand minerals. Many outstanding examples of natural crystals are pictured in color.

Pepper, David M., Jack Feinberg, and Nicolai V. Kukhtarev. "The Photorefractive Effect." SCIENTIFIC AMERICAN 263 (October, 1990): 62-74. Shows that the study of crystals and their effects continues to have promise for the future. Laser light is believed to modify the crystal structure by moving electrons. Diagrams and illustrations are typical of the high quality that this magazine has generally maintained.

Chemical Bond Angles and Lengths