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Paramagnetic defects in amber-colored superhard c-BN crystalline powders

Two samples of large-grained commercial superabrasive cubic boron nitride crystalline powders in different shades of amber coloration have been investigated by low frequency X (9.4 GHz)-band electron paramagnetic resonance (EPR) spectroscopy at room and low (T < 15 K) temperatures. Both samples contain three spectra component lines, A1, A2 and A3, in the same proportion, and the total concentration of the corresponding paramagnetic centers being proportional to the intensity of the amber coloration. After in situ broad-band UV illumination at low temperature, the intensity of the A1 component line becomes dominant. The observed effects are attributed to electron trapping at EPR silent A1+ precursor centers, which seem to coexist with the A1 centers and to the eventual ionization of the paramagnetic A3 centers responsible for the A3 component line.