Cristallographic defects in diamond.

Imperfections in the crystal lattice of diamond are common. Such crystallographic defects in diamond may be the result of lattice irregularities or extrinsic substitutional or interstitial impurities, introduced during or after the diamond growth. They affect the material properties of diamond and determine to which type a diamond is assigned; the most dramatic effects are on the diamond color and electrical conductivity, as explained by the band theory.

The defects can be detected by different types of spectroscopy, including electron paramagnetic resonance (EPR), luminescence induced by light (photoluminescence, PL) or electron beam (cathodoluminescence, CL), and absorption of light in the infrared (IR), visible and UV parts of the spectrum. Absorption spectrum is used not only to identify the defects, but also to estimate their concentration; it can also distinguish natural from synthetic or enhanced diamonds.[1]

The number of defects in diamond whose microscopic structure has been reliably identified is rather large (many dozens), and only the major ones are briefly mentioned in this article.

Labeling of diamond centers:

There is a tradition in diamond spectroscopy to label a defect-induced spectrum by a numbered acronym (e.g. GR1). This tradition has been followed in general with some notable deviations, such as A, B and C centers. Many acronyms are confusing though:

• Some symbols are too similar (e.g., 3H and H3).
• Accidentally, same labels were given to different centers detected by EPR and optical techniques (e.g., N3 EPR center and N3 optical center have no relation).
• Whereas some acronyms are logical, such as N3 (N for natural, i.e. observed in natural diamond) or H3 (H for heated, i.e. observed after irradiation and heating), many are not. In particular, there is no clear distinction between the meaning of labels GR (general radiation), R (radiation) and TR (type-II radiation).

Defect symmetry:

The symmetry of defects in crystals is described by the point groups. They differ from the space groups describing the symmetry of crystals by absence of translations, and thus are much fewer in number. In diamond, only defects of the following symmetries have been observed thus far: tetrahedral (Td), tetragonal (D2d), trigonal (D3d,C3v), rhombic (C2v), monoclinic (C2h, C1h, C2) and triclinic (C1 or CS).

The defect symmetry allows predicting many optical properties. For example, one-phonon (infrared) absorption in pure diamond lattice is forbidden because the lattice has an inversion center. 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.

An interesting symmetry related phenomenon has been observed that when diamond is produced by the high-pressure high-temperature synthesis, non-tetrahedral defects align to the direction of the growth. Such alignment has been also been observed in gallium arsenide and thus is not unique to diamond.

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