![]() ![]() 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. This allows red light absorption, and due to the small energy (c. 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. Only one percent of diamonds are of this type, and most are blue to grey (O'Donoghue 2002, p. A closely related aggregate is the N2 centre, which produces a line at 478 nm (Reinitz 2005).ĭiamonds containing boron as a substitutional impurity are termed Type IIb. ![]() 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. The N3 centre is paramagnetic so its structure is well-developed by the ESR method. It can occur along with other aggregate forms, with which it produces strong colors-particularly with A and B1 centers (Anderson et. Minor nitrogenous defects N3 centre The N3 centre consists of three nitrogen atoms surrounding a vacancy in a flat configuration. As with A center defects, B1 centre defects do not cause discoloration by themselves (Anderson et. 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. The most popular explanation involves four nitrogen atoms surrounding a vacancy. B1 centre The structure of B1 defects is not yet clear. The A centre does not cause discoloration on its own these diamonds are classed as Type IaA. Recent research has shown the accuracy of this model. Sobolev offered the theory of two nitrogen atoms (bonded strongly together as a molecular pair) replacing carbon in the diamond lattice. This theory remained for twenty years, until E. 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). A centre The A center is probably the most common defect in natural diamonds. 2.5 eV and above can excite the donor electrons into the conduction band, thereby allowing light absorption (Nassau, p. 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. ![]() 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. 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. These are easily seen in ESR spectra (in which they are called P1 centers). 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. It is the interactions between different aggregate configurations which cause colour rather than the aggregates themselves (Anderson et. Although all aggregate configurations cause absorption in the infrared and ultraviolet, diamonds with high levels of nitrogen are usually colorless. The light absorption and other material properties of diamond are highly dependent upon nitrogen content and aggregation state. 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. Nitrogen as a diamond impurity was first identified in 1959 by Kaiser and Bond of Bell Telephone (Kaiser and Bond 1959). The most common impurity in diamond is nitrogen, which can comprise up to 1 % of a diamond by mass. 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. (1) region of nitrogen impurities absorption, (2) B2 peak, (3) self absorption of diamond lattice, (4) hydrogen peaks ![]()
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