The identity of the AB1 and NE4 electron paramagnetic resonance spectra in high-pressure-high-temperature diamond

Similar electron paramagnetic resonance (EPR) spectra caused by the trigonal centres NE4 and AB1 have been detected in nitrogen-rich diamond synthesized with Ni catalyst by two different groups. The NE4 centre is commonly considered to be the basic structure of several nickel-nitrogen centres that are formed upon the annealing of this type of diamond. However, its EPR spectrum is observed so far only in diamond crystals grown in Novosibirsk, whereas in similar crystals grown in the NIRIM the AB1 centre could be detected. This discrepancy in the EPR detection could be solved by a re-analysis of the published EPR data of the NE4 Centre and through EPR investigations of synthetic diamonds from Novosibirsk and the NIRIM. The study revealed that the EPR spectrum of the NE4 Centre corresponds in fact to the spectrum of the trigonal AB1 centre, which is described with S = 1/2 and the g-values g(parallel to =) 2.0027 +/- 0.0002 and g(perpendicular to) = 2.0923 +/- 0.0002. Furthermore, we study whether the experimental g-values can be successfully explained, within the framework of crystal-field theory, by the previously proposed models for the AB1 (NE4) centre.