J. M. Baker

A study of 13C hyperfine structure in the EPR of nickel-nitrogen-containing centres in diamond and correlation with their optical properties

Electron paramagnetic resonance (EPR) and optical spectroscopy have been used to determine the structure and electronic state of nickel-nitrogen centres in natural diamonds and in synthetic diamonds enriched in 13C. The latter were grown in an Fe-Ni-C solvent/catalyst system at 1750 K, under stabilizing pressure, by the temperature gradient method and afterwards treated at high temperature and pressure. The parameters and directions of the 13C hyperfine structure (HFS) tensors for the NE1 centre were found to confirm the previously proposed model for this defect, with a nickel ion at the centre of a double semivacancy as the basic structural unit. In this unit the nickel atom has six atoms in its coordination shell. The NE1 centre has C2h symmetry, and the two equivalent nitrogen atoms in the coordination shell lie in the symmetry plane. New data on the HFS of 14N and 13C for the NE5 centre, also with C2h symmetry, indicated the same structural unit, but the two equivalent nitrogen atoms (and two equivalent carbon atoms) lie out of the symmetry plane and are related to one another by reflection in it. A new paramagnetic centre was found, labelled NE8, also with C2h symmetry, with four equivalent nitrogen atoms in the coordination shell all lying out of the symmetry plane. This centre is responsible for the 793.6 nm vibronic system in absorption and luminescence spectra. The new data have allowed a reinterpretation of the HFS tensors for the NE2 centre, which has C1 symmetry, suggesting that it has the same structure as NE1 but with one additional nitrogen atom in the coordination shell. The electronic states of these nickel-containing centres are discussed using the approach of Ludwig and Woodbury to transition metal ions in covalent crystals.

Effect of HPHT annealing on the photoluminescence of synthetic diamonds grown in the Fe-Ni-C system

Abstract The photoluminescence (PL) spectra of synthetic diamonds (SD) containing high concentrations of nitrogen and nickel impurities have been examined in both as-grown samples and the ones annealed at 1950 or 2200 K. A large number of zero-phonon lines (ZPLs), including two tens of vibronic PL systems, have been identified and their behaviour on annealing has been studied, one-third of documented lines have also been observed in natural diamonds. An analysis of their behaviour on annealing allows their division into three main groups, namely: (I) The systems present in as-grown SD, but which disappear after annealing at 1950 K; (II) The systems, which appear after annealing at approximately 1950 K, but their intensity is lower or they disappear completely after annealing at 2200 K; (III) The system, whose intensity increases after annealing at 1950 K and does not decrease after further annealing at 2200 K. A combined optical and ESR study allowed one to identify the systems of group I with individual impurities, whereas the items within group III were associated with nickel–nitrogen complexes containing a single nickel ion in the divacancy position surrounded by a few (n≥2) nitrogen atoms. The systems within group II are related to a relaxed single nickel defect (nickel ion in divacancy position) and simple nickel–nitrogen complexes, containing a single nitrogen atom. The PL excitation spectra were measured for 15 most intense PL systems. A broad absorption band in the annealed SD, with a peak approximately 450 nm and a superimposed fine structure, was decomposed into individual components, related to the NE1, NE2 and NE3 paramagnetic centres. The lower excited states of these three centres are split with ΔE=125 meV, 62 meV and 221 meV, respectively, while the spin-allowed electronic transitions take place only to the upper sublevel.

Recombination-enhanced diffusion of self-interstitial atoms and vacancy–interstitial recombination in diamond

Abstract In this paper we report the results of EPR and optical studies on vacancies and interstitials produced in type IIa diamond irradiated with 2-MeV electrons at temperatures between 90 and 900 K. Post-irradiation annealing studies at temperatures up to 1900 K were also performed. Data showing recombination-enhanced diffusion of self-interstitial atoms in diamond is presented and our analysis of the phenomena leads us to the conclusion that a highly mobile interstitial I* (possibly a charged/neutral T d interstitial) is produced by electronic excitation and/or charge transfer. We estimate that the migration energy for I* is 0.3 eV. No evidence for vacancy recombination-enhanced diffusion was observed. The experimental data confirms that there is a significant barrier to vacancy–interstitial recombination in diamond.

13C, 14N and 15N ENDOR measurements on the single substitutional nitrogen centre (P1) in diamond

New ENDOR measurements on the single substitutional nitrogen centre in diamond are reported. The CW-ENDOR mechanism utilizes cross relaxation, and measurements have been made on both 14N and 15N, as well as the first detailed 13C ENDOR study on the isotope at the natural abundance of 1.1%. The change in EPR intensity induced by driving the 13C nuclear transitions was approximately equal to 100% of the EPR intensity of the appropriate 13C hyperfine satellite. The assignment of 13C hyperfine coupling matrices with specific sites is considered, and predictions made about the signs of some of the 13C hyperfine interactions.

14N ENDOR of the OK1 centre in natural type Ib diamond

An ENDOR investigation has confirmed that the OK1 centre is a low-symmetry ( sigma h) centre, incorporating a single nitrogen atom. The 14N hyperfine and quadrupole coupling matrices have been determined by fitting the data to an exact solution of the energy matrix. Using this and other new experimental data the authors propose a model for this defect.

EPR study of the peculiarities of incorporating transition metal ions into the diamond structure

In this paper previously obtained data is reviewed and new data is discussed about nickel-containing centers in diamonds. These data are used to suggest interpretation of new data about cobalt-containing centers and to understand the influence of iron on the defects in diamonds grown in the iron system. A newly discovered nickel-nitrogen center has three nitrogen atoms in the first neighbor sphere around the double semi-vacancy and looks like the N3 (P2) center. In diamonds grown in the cobalt system two new types of cobalt-containing centers were found (NLO2 and NWO1). Both centers have electron spinS=1/2 and hyperfine structure from one cobalt ion (I=7/2 with natural abundance 100%). A case can be made for a double semi-vacancy structure for these defects. Special growth of diamond in the system enriched in15N decreased the line width down to 0.6 G, but gave no direct evidence of the existence of nitrogen in the defect structure. Asymmetrical shapes of the lines in the electron paramagnetic resonance (EPR) spectra of cobalt-containing centers with opposite signs in low and high magnetic field parts of spectra are due to very sensitive spin-Hamiltonian parameters of these defects to the lattice distortions. Annealing of cobalt-containing crystals at 2600 K produces the disappearance of all cobalt-containing EPR spectra, probably due to the capture of an additional nitrogen atom and the creation of a 3d6 diamagnetic state. In diamonds grown in the iron system with a high content of nitrogen there is evidence of an influence of ferromagnetic inclusion on the exchange interaction between substitutional nitrogen as an additional channel of indirect exchange interaction.

EPR and optical imaging of the growth-sector dependence of radiation-damage defect production in synthetic diamond

Abstract EPR imaging data are presented for the distribution of single substitutional nitrogen (P1) in a synthetic diamond of mixed IIa/Ib character, and compared with the distribution of the di-〈001〉-split interstitial (R1) produced during an electron irradiation. Since the defects are localised in different parts of the sample, it seems that the formation of R1 is somehow inhibited in the Ib-type material. The formation of isolated interstitials (R2) seems, however, to be the same in both types.

ENDOR studies on the N1 di-nitrogen centre in diamond

New ENDOR measurements on the N1 centre confirm the N-C-N+ model for the defect. The N+ is in a substitutional site with approximately tetrahedral symmetry. The N-C fragment of the centre resembles the P1 centre, with slightly larger unpaired electron density on the nitrogen, and a smaller quadrupole interaction.

An electron paramagnetic resonance investigation of paramagnetic defects in diamond films grown by chemical vapour deposition

Defects in free-standing diamond films grown by microwave-plasma-assisted chemical vapour deposition have been studied by electron paramagnetic resonance (EPR). The EPR spectra observed for the as-grown material each consisted of two distinguishable Lorentzian lines at g = 2.0028(2), along with weak satellites centred on g = 2.0028 and separated from each other by 1.15 - 1.35 mT. Comparison of the local concentration (up to 500 ppm) determined by lineshape analysis and the bulk concentration (0.3 - 8 ppm) determined from the total EPR absorption revealed that the defects were inhomogeneously distributed in the diamond film. Multi-frequency EPR measurements showed that the satellite separation depended on the microwave frequency. It is proposed that the satellite lines originate from a pair of coupled electron spins which form a biradical centre. This appears to be the only model which is consistent with the observed microwave frequency dependence of the satellite separation.

Do isolated interstitial nickel atoms occur in diamond? A re-examination of the electron paramagnetic resonance defects NIRIM-1 and NIRIM-2

All of the electron paramagnetic resonance (EPR) and optical data relating to some of the basic nickel-related defects in diamond have been compared, as well as previously little considered information about the effects of spin–orbit coupling and the magnitude of the crystal field. New models are proposed for the EPR defects, which are found in synthetic diamond grown at high pressure and high temperature with getters to reduce the content of nitrogen impurity: NIRIM-1 and NIRIM-2, which some authors have attributed to isolated interstitial nickel at an undistorted, or distorted, Td site, respectively. It is suggested that NIRIM-1 is more likely to be substitutional Nis+, 3d5, at a Td site, than the previously suggested interstitial Nii+, 3d9, and that NIRIM-2 is Nii+, 3d9, pinned at a site 0.308 nm along from a Bs− impurity, beyond one of its nearest C neighbours. This supports the suggestion that isolated interstitial Nii is mobile.