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Crystallographic defects in diamond

2007 Schools Wikipedia Selection. Related subjects: Materials science

   Crystallographic defects in the crystal lattice of diamond are common;
   they may be the result of extrinsic substitutional impurities, or
   intrinsic (interstitial and structural) anomalies. All diamonds possess
   crystal lattice defects of some sort; the defects themselves may be
   either anthropogenic or natural, epigenetic or syngenetic. The material
   properties of diamond are affected by these defects and determine to
   which type a diamond is assigned; the most dramatic effects are on a
   diamond's colour and semiconductivity, as explained by the band theory.

   The defects can be detected by different types of spectroscopy,
   including ESR, photoluminescence in ultraviolet light, and absorption
   of infrared light. The resulting absorption spectrum can then be
   analyzed, identified, and used to separate natural from synthetic or
   enhanced diamonds.

Extrinsic defects

   Infrared spectrum of Type IaB diamond. (1) region of nitrogen
   impurities absorption, (2) B2 peak, (3) self absorption of diamond
   lattice, (4) hydrogen peaks
   Enlarge
   Infrared spectrum of Type IaB diamond. (1) region of nitrogen
   impurities absorption, (2) B2 peak, (3) self absorption of diamond
   lattice, (4) hydrogen peaks

   The burning of diamonds in a vacuum and the analysis of resultant gases
   and remnant matter has shown that diamonds can contain many elements
   present as substitutional (i.e., replacing carbon atoms in the lattice)
   impurities: nitrogen, boron, hydrogen, oxygen, sulfur, nickel, cobalt,
   and iron have all been thus detected.

Nitrogen

   The most common impurity in diamond is nitrogen, which can comprise up
   to 1 % of a diamond by mass. Nitrogen as a diamond impurity was first
   identified in 1959 by Kaiser and Bond of Bell Telephone (Kaiser and
   Bond 1959). Previously, all lattice defects in diamond were thought to
   be the result of structural anomalies; later research revealed nitrogen
   to be present in most diamonds and in many different configurations.

   The light absorption and other material properties of diamond are
   highly dependent upon nitrogen content and aggregation state. Although
   all aggregate configurations cause absorption in the infrared and
   ultraviolet, diamonds with high levels of nitrogen are usually
   colorless. It is the interactions between different aggregate
   configurations which cause colour rather than the aggregates themselves
   (Anderson et. al. 1998, p. 215).

Main nitrogenous defects

   There are more than 50 forms of nitrogenous defects that occur in
   diamonds, and the three main forms observed in visible and infrared
   spectra are as follows:

   C centre
          C centre defects consist of single substitutional nitrogen atoms
          in the diamond lattice that are spacially isolated. These are
          easily seen in ESR spectra (in which they are called P1
          centers). C form defects impart a deep yellow to brown colour;
          these diamonds are classed Type Ib and are commonly known as
          canary diamonds, which are rare in gem form. In most cases
          synthetic diamonds contain a high level of nitrogen in the C
          form because nitrogen from the atmosphere is difficult to
          exclude from the synthesis process; as little as one nitrogen
          atom per 100,000 carbon atoms will produce a deep yellow (Nassau
          1980, p. 191). Because the nitrogen atoms have five available
          electrons (one more than the carbon atoms they replace), they
          act as deep donors; that is, each substituting nitrogen has an
          extra electron to donate, thereby forming a donator energy level
          within the band gap. Light with energy above ca. 2.5 eV and
          above can excite the donor electrons into the conduction band,
          thereby allowing light absorption (Nassau, p. 332).

   A centre
          The A center is probably the most common defect in natural
          diamonds. The structure of this form remains a topic of debate:
          first researchers supposed that it consisted of nitrogen, but
          later the conclusion was reached that the A center was due to
          microscopic platelets (now platelets connected with B2 peaks).
          This theory remained for twenty years, until E. V. Sobolev
          offered the theory of two nitrogen atoms (bonded strongly
          together as a molecular pair) replacing carbon in the diamond
          lattice. Recent research has shown the accuracy of this model.
          The A centre does not cause discoloration on its own; these
          diamonds are classed as Type IaA.

   B1 centre
          The structure of B1 defects is not yet clear. The most popular
          explanation involves four nitrogen atoms surrounding a vacancy.
          These diamonds are classed as Type IaB; most gem diamonds
          contain a mixture of A center and B centre defects, together
          with N3 centers, the combination producing the yellow-brown Cape
          series. As with A center defects, B1 centre defects do not cause
          discoloration by themselves (Anderson et. al., p. 215).

Minor nitrogenous defects

   N3 centre
          The N3 centre consists of three nitrogen atoms surrounding a
          vacancy in a flat configuration. It can occur along with other
          aggregate forms, with which it produces strong
          colors—particularly with A and B1 centers (Anderson et. al., p.
          215). The N3 centre is paramagnetic so its structure is
          well-developed by the ESR method. In ultraviolet fluorescence
          spectra, this defect produces a characteristic absorption line
          in the far violet at 415.5 nm, termed the N3 line (O'Donoghue
          2002, p. 52). A closely related aggregate is the N2 centre,
          which produces a line at 478 nm (Reinitz 2005).

Boron

   Diamonds containing boron as a substitutional impurity are termed Type
   IIb. Only one percent of diamonds are of this type, and most are blue
   to grey (O'Donoghue 2002, p. 52). The boron acts as an acceptor; that
   is, because the substituting boron atoms have one less available
   electron than the carbon atoms they replace, each boron atom creates an
   electron hole in the band gap that can accept an electron from the
   valence band. This allows red light absorption, and due to the small
   energy (c. 0.4 eV) needed for the electron to leave the valence band,
   holes are created in the latter even via thermal heat at room
   temperatures. These holes can move in an electric field and render the
   diamond electrically conductive (i.e., a p-type semiconductor). Very
   little substitutional boron is required for this to happen—a typical
   ratio is one boron atom per 1,000,000 carbon atoms (Nassau, p. 333).

   Type IIb diamonds transmit in the ultraviolet down to c. 250 nm but do
   not absorb in the visible region apart from the far red (hence the blue
   colour); they may phosphoresce blue after exposure to shortwave
   ultraviolet. Synthetic diamonds containing boron are blue and either
   Type IIb or a mixture of IIb and IIa material (O'Donoghue 2002, p. 52,
   46).

Intrinsic defects

   Every natural diamond crystal contains typical intrinsic or
   self-defects: vacancies, dislocations, and interstitial atoms.

Vacancies

   A vacancy is an empty position in a diamond's lattice. Vacancies may be
   affected or created by radiation damage—high-energy subatomic particles
   knock carbon atoms out of the lattice. This may be the result of
   natural or artificial radiation (see Diamond enhancement -
   Irradiation). The vacancies interact with interstitial atoms ("extra"
   atoms, most commonly nitrogen, which occupy space between carbon atoms
   rather than substituting for them) and act as colour centers by
   absorbing visible light, thus producing green or blue colors in Type I,
   and brown colors in Type IIa diamond. Radiation-induced vacancies can
   be detected by ultraviolet fluorescence, as well as by a characteristic
   absorption line at 741.2 nm, termed the GR 1 (General Radiation) line.
   This line is destroyed if the diamond is annealed above 400°C, after
   which a number of additional lines (e.g. 575, 595, 503 [H3 center],
   497, 1935 [H1c center], and 2924 [H1b centre] nm) are formed (Gemlab
   2002b).

   The annealing process (or the heat of the earth over geological
   timescales) also allows carbon atoms neighboring a vacancy to jump into
   a vacant place and leave an empty position in the diamond lattice; by
   this process a vacancy can migrate through the diamond, and can form
   compound defects with other vacancies, interstitial atoms (forming
   Frenkel pairs), or nitrogenous defects (NV centers). The newly-formed
   compound defects are optically active, producing strong yellows, pinks,
   and reds, the precise colour dependent on the annealing time and type
   of pre-existing defects present. Vacancies can also be created or
   modified by HTHP treatment.

Dislocations

   The purest diamonds, which contain little if any extrinsic impurities
   (Type IIa), may have their colour modified by structural dislocations
   or plastic deformations, which are breaks in the translational symmetry
   of the lattice. There are two important types of dislocations in
   diamond: the glide set, in which bonds break between layers of atoms
   with different indices (those not lying directly above each other); and
   the shuffle set, in which the breaks occur between atoms of the same
   index. The dislocations produce dangling bonds which introduce energy
   levels into the band gap, enabling the absorption of light (Kolodzie
   and Bleloch).

   These defects are thus believed to cause brown, pink, or purple
   coloration. Like boron-containing Type IIb diamonds, Type IIa diamonds
   transmit in the ultraviolet down to 250 nm. If treated with high
   temperatures and high pressures, the dislocations can be "healed" and
   the colour removed (see next section).

   B2 centre
          Some diamonds contain platelets in the 100 plane visible by
          microscope. This intriguing defect causes a sharp peak at 1600
          cm^-1 in IR spectra.

Effects of HTHP on defects

   Experiments with synthetic and natural diamonds treated at high
   temperatures (1700–2800°C) and high pressures (6–8 GPa; HTHP) have
   shown that, with time, lattice defects can be altered or repaired. In
   Type IIa diamonds with structural dislocations, a small number of NV
   centers are created—as indicated by absorption peaks at 637 nm (NV^-),
   575 (NV^0), and 3760 cm^-1—the lattice is realigned and ruptured bonds
   repaired, and much of the original brown color is removed (O'Donoghue
   and Joyner 2003, p. 35; Gemlab 2002a). Sometimes, a pink color is
   induced instead; some are left with a yellowish cast due to the NV
   centers. The occasional brown Type IIb diamonds subjected to HTHP will
   turn pure blue due to their boron content. HTHP can also be used to
   remove color from brown Type IaB diamonds colored by grainig planes
   which contain amorphous carbon. No single nitrogen is introduced in
   this case; however, N3 centers sometimes are, and impart a light
   yellow-grey colour (Deljanin and Fritsch 2000).

   Pale yellow Type IaA/B Cape series diamonds can have their A- and
   B-centers converted (broken up) to C-centers via HTHP above 1960°C,
   thereby creating intense canary-type colors in shades of yellow,
   brownish-yellow, olive, or green. The strongest colors are produced at
   the highest temperatures, which also produces tell-tale green
   transmission fluorescence under visible and ultraviolet light (Gemlab
   2002b). The green fluorescence is attributed to the H3 center (a
   vacancy trapped at an A-center aggregate) and produces a line at 503
   nm. Green HTHP-treated diamonds also exhibit a line at 985 (or 986) nm,
   known as the H2 centre, that is also the result of a vacancy-nitrogen
   aggregate complex (Reinitz 2005). The H1c and H1b centers common to
   irradiated and annealed diamonds may also be present.

   Diamonds with C form aggregates can be converted to the A form. This
   process is called aggregation of nitrogen because nitrogen atoms tend
   to assemble to the aggregate locations with lower energy. The next step
   of this process is conversion of A form to B1 form of nitrogen with
   attendant constitution of platelets (B2 centre). Possibly when most of
   the nitrogen is in B1 form the platelets disintegrate with the
   formation of micro-voids. Conversion of A form into B1 form takes place
   at noticeably higher temperatures and/or longer treatment times.
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