What Is the Link Between Purity & Crystal Growth?
Crystals grown from a chemical dissolved in solvent or from a melt possess a definite geometry. Crystal geometry is categorized into standard types, including rhombic, monoclinic, triclinic, etc. Trueness to form requires purity of material.
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Trueness to Form
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Whenever a pure substance crystallizes under the same conditions, the same crystalline form results. Although there is no such thing as an absolutely perfect crystal, the closer to perfect a crystal is, the purer the starting material was from which it grew.
Atomic Sizes and Bond Lengths
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Molecules have definite bond lengths. As a crystal forms, if traces of other substances are present, they can co-deposit with the desired substance. Different atoms have different sizes, and different compounds have different bond lengths. In short, the presence of impurities interferes with crystal perfection.
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Categorization of Crystal Defects
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Defects in crystal growth are categorized into point defects, linear defects, and planar defects.
Point and Linear Defects
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Point defects, especially voids, are common--especially at elevated temperatures. Not all voids mean contamination. If a matrix is drawn of 5 rows (x) and 5 columns (y), and a sixth row is inserted in the first three rows, from top to bottom the rows would contain: 6, 6, 6, 5, and 5 elements, respectively. Add a third dimension (z)--that of depth--and the imperfections change from points to lines.
Planar Defects
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Point and linear defects are associated with single crystals. Except in a semiconductor device laboratory, in which Herculean efforts are taken to assure single crystal growth, polycrystalline growth will result, with some crystals attached to each other.
Conclusion
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Because a crystal isn't perfect doesn't mean a crystallized substance isn't pure. A crystal that is near-perfect, however, does have pure contents.
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- Photo Credit Image by Flickr.com, courtesy of Brenda Clarke
