Electron irradiation and the formation of vacancy–interstitial pairs in diamond

A large number (>60) of nitrogen-containing type Ia natural diamonds, and nitrogen-free type IIa natural and synthetic diamonds, have been electron irradiated and their optical absorption spectra have been analysed in detail. The analysis reveals that the concentrations of vacancies [V] and interstitials [I0 ] in type IaA diamonds increase linearly with the concentration [NA ] of the A-form of nitrogen. This increase of [V] and [I0] with [NA ] is attributed to the trapping of vacancies and interstitials in the strain field of nitrogen. The trapping produces 'strained' vacancies and partly immobilizes interstitials, thus reducing the vacancy?interstitial recombination during electron irradiation, thereby increasing [V] and [I0 ]. Those strained vacancies and interstitials recombine upon annealing at temperatures ~390?420??C. In type IaB diamonds no correlation has been found between [V] and the concentration [NB ] of the B-form of nitrogen; furthermore [I0] was found to decrease linearly with [NB ]. In both type IaA and type IaB diamonds the width of the GR1 zero-phonon line, associated with the neutral vacancies [V0 ], increases with nitrogen concentration. A new, more appropriate and accurate formula to calculate [V0] is proposed based on measuring the height H of the GR1 absorption band at 2.0?eV.