Structure and creation conditions of complex nitrogen-nickel defects in synthetic diamonds

The mechanisms of nickel incorporation into the lattice of synthetic diamond, obtained on a seed by the temperature gradient method in the FeNiC system at 1700 K and 5.5 GPa, as grown or additionally annealed, have been studied using electron spin resonance (ESR) and optical spectroscopy. In the as-grown diamonds, in addition to the interstitial Ni+ a new paramagnetic Ni+ center with C3v symmetry environment and g∥ = 2.0227, g⊥ = 2.0988 (NE4), which correlates with the 882 nm vibronic system, was observed. In diamonds annealed at 2100 K and 5.5 GPa, containing NE1–NE3 centers, three types of photochromic paramagnetic defect (NE5–NE7) were found, which are also due to Ni incorporation into the diamond lattice. They are characterized by g1 = 2.0903; g2 = 2.039 and g3 = 2.0044 and hyperfine splitting (HFS) from two magnetically equivalent nitrogen atoms with A∥ = 12.25 G and A⊥ = 9.5 G (NE5); g1 = 1.995, g2 = 2.0207 and g3 = 2.0109 (NE6). For NE7, g > 2 and HFS from one nitrogen is observed while a detailed study of the ESR angular dependence of the ESR spectrum was hampered by superposition from the other systems. The NE6 and NE7 photochromic and thermal properties are typical for shallow electron traps and determined by a Ni2+-to-Ni+ charge transfer by ionization of the donor nitrogen, while in the case of NE5 they can be explained in terms of internal transformation processes.