PHOTOCHROMIC NICKEL CENTERS IN DIAMONDS

Natural and synthetic (grown in the Fe-Ni-C system) diamonds were inve stigated 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 nic kel is partly transformed into nonparamagnetic defects; these may beco me ESR-active as a result of transformation of nickel to the Ni+ slate by photoexcitation at lambda < 360 nm. Paramagnetic photochromic nick el-containing defects of three types were found: NE5 with g(1) = 20903 , g(2) = 2.039, and g(3) = 2.0044 and with hfs from two magnetically e quivalent nitrogen atoms, A parallel to = 12.25 G and A perpendicular to = 9.5 G; NE6 without hfs and with g(1) = 1.995, g(2) = 2.0207, and g(3) = 20109; 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 absorption spectra, two lines (at 546 and 552 nm) are induced by light in the UV-visible range on irradiation, but no correlation w ith the paramagnetic centers was found. The samples were subjected to annealing and photobleaching combined with thermoluminescence measurem ents. For NE5 in the ESR spectra as well as for the 546 and 552 nm fin es in the optical spectra, the photochromic processes are due to the i nfernal rearrangement of the electronic configuration of the defects; for NE6 and NE7, they are caused by recharge of small electron traps.