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A
single crystal is required to have a three-dimensional array of atoms or
molecules that exhibit translational symmetry. The crystal structure is
described in terms of two components, the Bravais lattice and the basis
that is placed on each lattice point with the same composition and orientation.
The smallest unit of a single crystal to show the full symmetry and chemistry
of the crystal structure is the Unit cell.
A
single crystal need not have an external morphology that shows the symmetry
of the crystal structure. Crystals grown from the liquid phase of the material
may have a shape determined by the container in which growth takes place.
This is exploited in the production of single crystal gas turbine blades
(top photograph) to a near net shape. Crystals grown from solution tend
to have low surface energy faces bounding them as shown in the lower photograph. |
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