ABSTRACT
The crystal structure of diamond is face-centered cubic (fcc) with a basis of two carbon atoms with each lattice point. The length a of the diamond unit cell is 3.56683 Å. There are eight atoms in a unit cell with a tetrahedral bond arrangement of carbon atoms. Artificial diamonds are produced using high pressure (1 × 105 kg/cm2) and high temperature (>2300 K) in a process developed in 1955 at Research Laboratory, General Electric Company, Schenectady, New York.
The crystal structure is corundum. The chemical formula is Al2O3. The crystal class is hexagonal. The lattice constants are a = 4.875 Å and c = 12.991 Å. A vertical-pulling technique is used for the growth of highly perfect sapphire single crystals. Pull rates of 6–50 mm/hr have been used. A seeded vertical-gradient-freeze (VGF) method was developed at MIT Lincoln Laboratory for the growth of titanium-doped laser-quality crystals with relatively low infrared absorption. With a He atmosphere, the temperature gradient of the VGF method is about 2 mm/hr. Because the Ti3+ concentration in Ti:Al2O3 crystals grown by the VGF method varies with distance from the seed, laser rods are cut perpendicular to the growth axis in order to minimize the variation in the Ti3+ concentration along their lengths.
