EPR studies of interstitial Ni centers in synthetic diamond crystals

Two new electron-paramagnetic-resonance (EPR) spectra, tentatively labeled NIRIM-1 and NIRIM-2, have been studied using synthetic diamond crystals grown from the Ni solvent to which various amounts of nitrogen getters (Ti, Zr) and/or boron were added. The NIRIM-1 spectrum (g=2.0112) having the effective spin S=1/2, which has been determined from the microwave pulse-width dependence of the two-pulse echo intensity, is assigned to be isolated interstitial Ni⁺ with electronic configuration 3d⁹. The anisotropic spectra of powderlike line shape at 4 K might be ascribed to an intermediate Jahn-Teller effect coupled predominantly with trigonal distortions. The NIRIM-2 spectrum having an axially symmetric g tensor (S=1/2, g?=2.3285, g_⊥{}0) arises from a 3d⁹ ion in a crystal field of trigonally distorted octahedron. It is proposed that the center is interstitial Ni⁺ associated with a vacancy or with local charge compensation.