Photochromic nickel centers in diamons

Abstract

Natural and synthetic (grown in the Fe−Ni−C system) diamonds were investigated by ESR and optical spectroscopy; five new photochromic nickel centers were found in addition to the interstitial sites and NE1-NE4 centers. Upon P, T-treatment at 2100 K and 55 kb, the interstitial nickel is partly transformed into nonparamagnetic defects; these may become ESR-active as a result of transformation of nickel to the Ni+ state by photoexcitation at λ<360 nm. Paramagnetic photochromic nickel-containing defects of three types were found: NE5 with g1=2.0903, g2=2.039, and g3=2.0044 and with hfs from two magnetically equivalent nitrogen atoms, A‖=12.25 G and A⊥=9.5 G; NE6 without hfs and with g1=1.995, g2=2.0207, and g3=2.0109, and finally, NE7 with hfs from one nitrogen atom in the range of 6–10 G and g-factors significantly larger than 2.0024. In the optical obsorption spectra, two lines (at 546 and 552 nm) are induced by light in the UV-visible range on irradiation, but no correlation with the paramagnetic centers was found. The samples were subjected to annealing and photobleaching combined with thermoluminescence measurements. For NE5 in the ESR spectra as well as for the 546 and 552 nm lines in the optical spectra, the photochromic processes are due to the internal rearrangement of the electronic configuration of the defects; for NE6 and NE7, they are caused by recharge of small electron traps.