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A lattice constant or lattice parameter is one of the physical dimensions and angles that determine the geometry of the unit cells in a crystal lattice. Lattices in three dimensions generally have six lattice constants: the lengths a, b, and c of the three cell edges meeting at a vertex, and the angles α, β, and γ between those edges.
The crystal lattice parameters a, b, and c have the dimension of length. Their SI unit is the meter, and they are traditionally specified in angstroms ; an angstrom being 0.1 nanometer , or 100 picometres. Typical values start at a few angstroms. The angles α, β, and γ are usually specified in degrees.
A chemical substance in the solid state may form crystals in which the atoms, molecules, or ions are arranged in space according to one of a small finite number of possible crystal systems , each with fairly well defined set of lattice parameters that are characteristic of the substance. These parameters typically depend on the temperature, pressure , electric and magnetic fields, and its isotopic composition. The lattice is usually distorted near impurities, crystal defects, and the crystal's surface. Parameter values quoted in manuals should specify those environment variables, and are usually averages affected by measurement errors.
Depending on the crystal system, some or all of the lengths may be equal, and some of the angles may have fixed values. In those systems, only some of the six parameters need to be specified. For example, in the cubic system, all of the lengths are equal and all the angles are 90°, so only the a length needs to be given. This is the case of diamond, which has a = 3.57 Å = 357 pm at 300 K. Similarly, in hexagonal system, the a and b constants are equal, and the angles are 60°, 90°, and 90°, so the geometry is determined by the a and c constants alone.