ENDOR and high-temperature EPR of the N3 centre in natural type Ib diamonds

The N3 is a single paramagnetic electron centre observed in type Ib diamonds. It has monoclinic symmetry below 200 degrees C and becomes axially symmetric at higher temperatures. It displays an unusual hyperfine structure which is shown to be from a single 14 N nucleus. The spin-Hamiltonian parameters required at higher temperatures have been determined at 550 degrees C from conventional EPR measurements, whereas the low-symmetry parameters have been determined at room temperature from ENDOR measurements. Some unusual ENDOR and EPR line intensities are explained. The model proposed for the defect involves a substitutional nitrogen as well as a substitutional oxygen.