Talk abstract

When Ni alloys are used as solvent-catalysts in the synthesis of diamond under high-pressure and high-temperature conditions (HPHT), Ni becomes incorporated as a dispersed impurity and produces various defects. Beside the isolated substitutional Ni_s- (W8) [1] which is the dominant paramagnetic Ni-defect in such diamond samples, many Ni-related defects were observed by electron paramagnetic resonance (EPR) [2-5]. Recently, five new nickel-related paramagnetic centres (AB1-AB5) have been detected in HPHT diamond samples annealed at 1600°C [4,5]. In this work we present for the first time photo-EPR investigations on the AB5 centre together with new photo-EPR results on the W8 and the P1 (substitutional N°) centres as well as latest findings concerning the formation conditions of the AB5 centre. Among the AB centres, only AB5 is an S = 1 centre. It was tentatively assigned to a nickel-nitrogen pair in the negative charge state [4]. Our EPR study of a set of as-grown and annealed diamonds grown using Ni (or a Ni alloy) as solvent/catalyst reveals that AB5 is always present in samples grown without a nitrogen getter. The photo-EPR investigations were performed in the Q-band (34GHz). In order to avoid any unwanted infrared irradiation of the sample, we used a bath cryostat. In samples with a strong W8 signal and a relatively weak P1 spectrum, we measured the W8 EPR signal intensity behaviour for different photoexcitation energies, and we observed two complementary photoionisation transitions (PTs) involving the W8 energy level: a photoinduced quenching at about 2.5 eV in agreement with [6] and a photoinduced enhancement at 3.0 eV. In order to determine the nature of these two PTs, we also investigated the behaviour of the deep nitrogen donor N°. We found a recharging of this donor for photon energies higher than 2.5 eV. This indicates directly that the photoionisation transition of the Ni_s- (W8) centre at 2.5 eV is caused by the excitation of electrons to the conduction band. Relating this recharging processes to the observed photoinduced changes of the AB5 EPR signal intensity we determine the level position of this centre to be E = Ec - 1.88 ± 0.03 eV.

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