Aggregation of nitrogen in diamond, including platelet formation

Abstract

Nitrogen occurs in most natural diamonds in concentrations of up to 0.3 at. %. In about 0.1% of diamonds, the nitrogen occurs as single substitutional atoms but in most diamonds aggregates of two or more nitrogen atoms occur. The various types of aggregate have been identified by detailed study of optical absorption spectra and by other methods. This paper describes experiments in which synthetic diamonds, containing substantial quantities of nitrogen in the form of single substitutional atoms, have been heated to cause aggregation. All the aggregates found in natural diamonds have been produced under controlled conditions of temperature and pressure. Of particular interest has been the formation of platelets which are found in type 1 natural diamonds in the {100} planes. Several experiments suggest strongly that these platelets are large aggregates of nitrogen atoms. To obtain aggregation in a reasonable time, the mobility of the nitrogen atoms was increased by irradiating the diamonds with 2 MeV electrons before heating. This irradiation produced numerous vacancies and interstitial atoms in the diamond lattice. Even so, temperatures of up to 2200°C were necessary to produce the required rapid motion of the nitrogen atoms. To prevent graphitization of the diamond at these temperatures, the specimens were subjected to a pressure of 8.5 GPa during the heating. The experiments establish the sequence of the aggregation of nitrogen atoms in diamond and are relevant to the understanding of the conditions under which aggregation occurred in natural diamonds. It seems probable that, for some diamonds at least, the interval between their formation and their ejection to the surface of the earth was very short on a geological time-scale.