Crystal Defects And Growth Of Crystals
The growth and size of a crystal depends on the conditions of its formation. Temperature, pressure, the presence of impurities, etc., will affect the size and perfection of a crystal. As a crystal grows, different imperfections may occur, which can be classified as either point defects, line defects (or dislocations), and plane defects.
Point defects occur: a) if a particle site is unoccupied (a Schottky defect); b) if a particle is not in its proper site (which is vacant) but is in a space or hole (a Frenkel defect); or c) if an extra particle exists in a space or hole, with no corresponding vacancy (an anti-Schottky defect). Line defects occur: a) if an incomplete layer of particles occurs between other, complete layers (an edge dislocation); or b) if a layer of particles is not planar, but is out of alignment with itself so that the crystal grows in a spiral manner (a screw dislocation). Plane defects occur; a) if two crystallites join to form a larger crystal in which the rows and planes of the two crystallites are mismatched (a grain boundary); or b) if a layer in an ABCABC pattern occurs out of sequence (a stacking fault).
Sometimes, imperfections are introduced to crystals intentionally. For example, the conductivity of silicon and germanium can be increased by the intentional addition of arsenic or antimony impurities. This procedure is called "doping," and is used in materials, called semi-conductors, that do not conduct as well as metals under normal conditions. The additional electrons provided by arsenic or antimony impurities (they have one more electrons in their outermost shells than do silicon or germanium) are the source of increased conductivity.
Experiments in decreased gravity conditions aboard the space shuttles and in Spacelab I demonstrated that proteins formed crystals rapidly, and with fewer imperfections, than is possible under regular gravitational conditions. This is important because macromolecules are difficult to crystallize, and usually will form only crystallites whose structures are difficult to analyze. Protein analysis is important because many diseases (including Acquired Immunity Deficiency Syndrome, AIDS) involve enzymes, which are the highly specialized protein catalysts of chemical reactions in living organisms. The analysis of other biomolecules may also benefit from these experiments. It is interesting that similar advantages in crystal growth and degree of perfection have also been noted with crystals grown under high gravity conditions.
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