In synthetic diamond grown by the high-pressure high-temperature synthesis or chemical vapor deposition, defects with symmetry lower than tetrahedral align to the direction of the growth. However, introducing any defect (even "very symmetrical", such as N-N substitutional pair) breaks the crystal symmetry resulting in defect-induced infrared absorption, which is the most common tool to measure the defect concentrations in diamond. For example, one-phonon (infrared) absorption in pure diamond lattice is forbidden because the lattice has an inversion center. The defect symmetry allows predicting many optical properties. In diamond, only defects of the following symmetries have been observed thus far: tetrahedral (T d), tetragonal (D 2d), trigonal (D 3d, C 3v), rhombic (C 2v), monoclinic (C 2h, C 1h, C 2) and triclinic (C 1 or C S). They differ from the space groups describing the symmetry of crystals by absence of translations, and thus are much fewer in number. The symmetry of defects in crystals is described by the point groups. In particular, there is no clear distinction between the meaning of labels GR (general radiation), R (radiation) and TR (type-II radiation). observed after irradiation and heating), many are not. observed in natural diamond) or H3 (H for heated, i.e.
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