On the basis of 11B and 14N hyperfine interactions, as determined by using the electron paramagnetic resonance technique, the NIRIM-4 center in electron-irradiated B-doped synthetic diamond crystals was identified as being a <100>-split [B-N]+ interstitialcy. The C2v symmetry of the spectrum required that the N and B atoms lay on the same [100] axis. The contributions to the wave function of unpaired electrons from, the atomic orbitals of N and B, were almost entirely p-like, with the directions of p-orbitals along [011] and [0¯11], respectively.

J.Isoya, H.Kanda, Y.Morita: Physical Review B, 1997, 56[11], 6392-5