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This article presents evidence that cobalt forms a series of optically active defect centers in diamond grown by high-temperature, high-pressure synthesis. Photoluminescence (PL) studies reveal that the newly observed vibronic systems with zero-phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367, and 2.590 eV appear only in samples grown using a cobalt-containing solvent–catalyst. Results of an annealing study, carried out in the temperature range 1500 to 1800 °C, establish that many of the new bands appear during the temperature regime of nitrogen aggregation. It is therefore proposed that nitrogen forms complexes with cobalt to produce optically active centers, in a manner analogous to that of nickel point defects in diamond. Detailed radiative decay time measurements and temperature dependence measurements show that all but one of the bands which are here associated with nitrogen–cobalt complexes have long radiative decay times (∼100 μs), and this again is a characteristic of the PL centers arising from nickel–nitrogen complexes. All of the vibronic bands observed by PL may also be produced by electron-beam excitation (cathodoluminescence). In this case it is necessary to use a low beam current density (≤10 mA cm−2), otherwise the spectra are dominated by emission from optical centers with much shorter decay times (∼20 ns). Only one vibronic band, with a zero-phonon line at 1.852 eV, has been detected in absorption measurements, and the center responsible for this system does not give rise to luminescence.

An extensive examination of the cathodoluminescent emissions from natural diamonds has been performed with due regard to their inhomogeneity, correlating cathodoluminescence properties point-by-point with the local crystal lattice texture and imperfection content as revealed by other topographic techniques (in particular X-ray topography). Some dozens of crystals have been examined, mainly prepared in the form of cut and polished sections but in some cases as whole stones in their natural state. The cathodoluminescence observations have been made by visual microscopy, by photomicrography, and by 'spectrum topography’ with spatial resolution down to 5 pm. Particular attention was devoted to those crystals, not uncommon, whose growth stratigraphy included 2ones of type II (ultraviolet-transmitting) diamond intercalated within regions of the more usual type la (ultraviolet-absorbing) diamond. These type II zones prove to be particularly rich in fine structure within their patterns of cathodoluminescent emission, and in the spectral variety of their emissions. Joint cathodoluminescence topographic and X-ray topographic examinations were made on all specimens. Where feasible, the specimens were also characterized by ultraviolet transmission topographs, and by topographic recording of the anomalous spike diffuse X-ray reflexions. Many cathodoluminescence emissions (including both well-known and littleknown spectral systems) were discovered to have clearly defined topographically localized sources, e.g. dislocation lines or regions which had sustained natural a-particle irradiation. Some findings among many of this nature which are set out in detail concern the emission system (known as H3) which has zero phonon line at 2.46 eV and strong coupling to phonons of 40 meV energy. Its sources include curvilinear growth bands in regions where crystal growth has been of non-faceted, ‘cuboid’ habit rather than of the usual {111} faceted habit, slip traces and individual dislocation lines in matrices of type II character, and occasionally, in similar matrices, (lOO)-orientation platelets ranging from ca. 1 pm to several tens of micrometres in diameter. The H3 system emission from the platelets is more than 90 % linearly polarized with E vector in the platelet plane. (These platelets also emit in the near infrared, at energies of ca. 1.25 eV.) Another emission system with zero phonon line at 2.46 eV, but with only weak phonon coupling (dominant phonon energy ca. 66 meV), was found solely in emissions from the natural radiation-damaged rinds of diamonds, or from patches of natural radiation damage on their external surfaces. Noteworthy is the occurrence of dislocations blue-emitting and of dislocations emitting the H3 system in close juxtaposition within type II matrices. The deep blue broad-band spectral emission from dislocations is strongly polarized with E vector parallel to the dislocation line. The H3 system emission from dislocations is unpolarized. In dislocation-rich type II crystals possessing a mosaic texture the blue emission from dislocations is the dominant source of visible cathodoluminescence at room temperature. Evidence bearing upon the relation of the visible {100} platelets to the submicrometre size {100} platelets which give rise to the anomalous ‘spike’ diffuse X-ray reflexions is examined: as far as their X-ray diffracting properties show, they are indistinguishable.

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