Platelets and the infrared absorption of type Ia diamonds

Abstract

The B' localized mode, infrared absorption peak (which is due to {100} platelets) and its relations with other features in the infrared absorption spectrum of type la diamonds have been investigated. It is recognized that it is possible, on the basis of their one-phonon infrared absorption spectra, to separate type la diamonds into two categories. In ‘regular’ diamonds, there is a strict proportionality between the strength of the B' peak and the strength of the lattice absorption due to B nitrogen aggregates; this is interpreted to be indicative of an epoch of smooth, undisturbed conversion of A nitrogen aggregates (or pairs of N atoms) to B aggregates, with the concurrent nucleation and growth of platelets. For ‘irregular’ diamonds this proportionality does not hold, and such specimens are recognized as crystals in which a catastrophic degradation of some or all of the platelets has occurred. It is argued that the proportionality observed for the regular specimens implies that nitrogen cannot be the dominant atomic species in the platelets, and a mechanism is described for platelet production by the formation and aggregation of carbon interstitials. Platelets are here seen as a necessary, but incidental, product of the nitrogen aggregation sequence. The B' peak is ascribed to carbon-carbon bond vibrations of atoms within the platelet. The presence of N atoms as a minority species in the platelets is discussed, and it is suggested that other localized mode absorptions seen in type I a spectra may be attributed to vibrations of N-C and N-N bonds of atoms in the platelets. The B' peak strength is also found to be linearly proportional to the recently-discovered D component of the lattice absorption spectrum, for both regular and irregular specimens. The D absorption is thus attributed to platelet-stimulated lattice vibrational modes. Among regular specimens, the strength of the N3 optical absorption is proportional to the B', and hence also B, absorption strengths. The N3 centres are considered to result from minor side reactions occurring during the epoch of conversion of A centres to B centres.