J. A. Van Wyk

Infrared absorption by the single nitrogen and A defect centres in diamond

Abstract The infrared absorption spectra of a suite of twenty-one type Ib synthetic diamonds have been recorded, and the concentrations of nitrogen in the form of single substitutional N atoms in these specimens determined by electron spin resonance measurements. It is found that an absorption coefficient, at 1130 wave-numbers, of 1 cm−1 is produced by 250 ±2at. p.p.m. of nitrogen in this form, a. result identical to that of Chrenko, Strong and Tuft and consistent also with that of Woods, van Wyk and Collins. Seventeen of these specimens were heated at temperatures between 1800 and 2050°C, under a stabilizing pressure, to bring about an appreciable degree of aggregation of the single N defects into A centres, or nitrogen pairs. For each specimen the residual, unaggregated single N defect concentration was then determined by electron spin resonance. The difference between this and the original value gave the concentration of nitrogen in the form of A defects. The infrared absorption strength of the A centr...

Expression of the Osteogenic Phenotype in Porous Hydroxyapatite Implanted Extraskeletally in Baboons

A porous hydroxyapatite was used as a morphogenetic matrix to study early tissue formation preceding the morphogenesis of bone in extraskeletal sites of the baboon (Papio ursinus). Porous hydroxyapatites, obtained by hydrothermal conversion of the calcium carbonate exoskeleton of coral, were implanted extraskeletally in 16 baboons. Specimens were harvested at days 30, 60 and 90, and processed to obtain decalcified sections for histomorphometry, and undecalcified sections for enzyme histochemical demonstration of alkaline phosphatase, immunohistochemical demonstration of laminin and type I collagen, and for comparative histologic analysis. At day 30, the tissue that invaded the porous spaces showed mesenchymal condensations at the hydroxyapatite interface, and prominent vascular penetration. Collagen type I staining was localized within mesenchymal condensations. Bone had not formed in any specimen harvested at day 30. At days 30 and 60, alkaline phosphatase staining was initially localized in the invading vasculature, and subsequently found in cellular condensations prior to their transformation into bone, and in capillaries close to cellular condensations. Laminin staining was localized around invading capillaries adjacent to and within mesenchymal condensations, and in capillaries in direct contact with the hydroxyapatite. Bone had formed by day 60; cartilage, however, was never observed. By day 90, bone formation within the porous spaces was often extensive. Goldners trichrome stain and fluorescence microscopy of tetracycline-labeled specimens demonstrated nascent mineralization within condensations during initial bone morphogenesis. Coating the hydroxyapatite with collagen type I prepared from baboon bone did not increase the amount of bone formation. In this hydroxyapatite-induced osteogenesis model in primates, vascular invasion and bone differentiation appear to be accompanied by a specific temporal sequence of alkaline phosphatase expression. The differentiation of osteogenic cells in direct apposition to the hydroxyapatite suggests that this substratum may act as a solid state matrix for adsorption and controlled release of endogenously-produced bone morphogenetic proteins. The porous hydroxyapatite, as used in this bioassay in primates, may be an appropriate delivery system for bone morphogenetic proteins for the controlled initiation of therapeutic osteogenesis.

The dependences of ESR line widths and spin - spin relaxation times of single nitrogen defects on the concentration of nitrogen defects in diamond

Line widths and spin - spin relaxation times of P1 centres in synthetic Ib and natural Ia diamonds with concentrations of P1 and P2 centres covering the range 0.03 - 400 atomic parts per million have been measured. At concentrations higher than about ten atomic parts per million the line width is linearly dependent on the concentration. At lower concentrations the electron - dipolar contribution to the line width dominates and the width of the line remains constant. Since the pulse sequence employed for measurements eliminates the effects of inhomogeneous line broadening, of the line remains linearly dependent on the total paramagnetic impurity concentration, even at very low paramagnetic impurity concentrations.

The nitrogen content of type Ib synthetic diamond

Abstract The concentration of single nitrogen atoms in type Ib synthetic diamond has been measured, by electron paramagnetic resonance and by inert-gas fusion, as a function of the strength of the infrared absorption produced by these centres. The two methods yield, in good agreement, values of 22·0 ±.1·1 at. p.p.m. and 20·6±1·5 at. p.p.m. respectively, for an absorption coefficient of 1cm −1 at 1130 wavenumbers. These values agree tolerably well with an earlier value of 25 at. p.p.m./cm−1 given by others, so calling into question the generally accepted data pertaining to the strength of the infrared absorption produced by the A centres, or nitrogen pairs, found in type Ia (natural) diamond.

Correlation between optical absorption and EPR in high-pressure diamond grown from a nickel solvent catalyst

Abstract There is a general tendency for the magnitude of the W8 electron paramagnetic resonance, attributed to substitutional negatively charged nickel, Nis−, to increase in sympathy with the strengths of the 1.883 eV and 2.51 eV absorption bands in high-pressure synthetic diamond. The ratio of the intensities of the 1.883–2.51 eV bands is not constant, and is generally lower in diamonds with a low Nis− concentration; a better correlation is obtained between the Nis− concentration and the 2.51 eV absorption than with the 1.883 eV absorption. Concentrations of Nis− up to 70 ppm are detected in some diamonds, and the relatively weak absorption in the 2.51 eV system suggests that either the oscillator strength is very low or that the optical transition occurs at a minor defect with a concentration which increases approximately in direct proportion to the concentration of W8 centres. The 1332 cm−1 absorption induced by nickel, but attributed to N+, does not correlate with Nis− in diamonds with low nitrogen concentrations, in which the concentration of Nis− is also small, and the charge compensation mechanism proposed in earlier investigations needs to be reconsidered for such specimens.

Temperature dependence of spin-spin and spin-lattice relaxation times of paramagnetic nitrogen defects in diamond

Spin-lattice relaxation times of P1 centers in a suite of two natural type Ib, two synthetic type Ib, and one natural type Ia diamonds were measured at 9.6 GHz as a function of temperature in the range 300 K>T>4.2 K. An analysis of the results revealed that for three of the diamonds (two synthetic type Ib and the natural type Ia) spin-orbit phonon-induced tunneling is the main relaxation mechanism. In the case of the Ia diamond cross-relaxation takes place between P1 and P2 centers. In the natural type Ib samples a much more effective relaxation mechanism dominates at lower temperatures. Electron spin resonance spectra of the latter samples revealed the presence of N3 centers. It seems that the more effective relaxation mechanism is associated with the N3 centers and that the P1 centers relax via the N3 centers to the lattice at these temperatures.

ENDOR and high-temperature EPR of the N3 centre in natural type Ib diamonds

The N3 is a single paramagnetic electron centre observed in type Ib diamonds. It has monoclinic symmetry below 200 degrees C and becomes axially symmetric at higher temperatures. It displays an unusual hyperfine structure which is shown to be from a single 14N nucleus. The spin-Hamiltonian parameters required at higher temperatures have been determined at 550 degrees C from conventional EPR measurements, whereas the low-symmetry parameters have been determined at room temperature from ENDOR measurements. Some unusual ENDOR and EPR line intensities are explained. The model proposed for the defect involves a substitutional nitrogen as well as a substitutional oxygen.

Carbon-12 hyperfine interaction of the unique carbon of the P2 (ESR) or N3 (optical) centre in diamond

The latest model proposed for the P2 centre shows that the paramagnetic electron is associated mainly with one carbon atom. The observation of the carbon-13 hyperfine interaction with this carbon is reported, and the appropriate hyperfine parameters for this interaction are given.

ENDOR of the P2 centre in type-Ia diamonds

The P2 centre was the first defect observed in diamond with electron spin resonance. Because of a very complicated ESR spectrum many years elapsed before the correct model was determined. Accurate spin Hamiltonian parameters have been determined from extensive ENDOR measurements at room temperature. These parameters are consistent with a model in which the defect consists of three nitrogens plus a vacancy. The observed ENDOR transitions do not follow the normal selection rules, and it will be explained how it can be predicted what frequencies will present in the ENDOR spectra.

ESR and Mössbauer studies on detoxified crocidolite: Mechanism of reduced toxicity

Abstract A process for the detoxification of crocidolite fibers was previously reported in the literature. The fibers of this mineral asbestiform were treated with ferric oxide salts to form a metal-micelle polymer surface coating which prevented physiological reactions with the mineral. In the present study, detoxified crocidolite was tested for its ability to generate hydroxyl radicals in the presence of hydrogen peroxide; the intensity of the electron spin resonance signal was less than that produced by the native toxic crocidolite fibers. Similar experiments showed that the ability of the detoxified crocidolite to reduce oxygen was also decreased compared with the native crocidolite. The availability of ferrous iron present in the two crocidolite fibers to catalyze the above reactions was investigated with the chelating agent ferrozine. The results indicate that ferrozine was able to mobilize fewer ferrous ions from detoxified crocidolite compared with the native crocidolite. Moreover, Mossbauer-effect spectroscopy studies have shown that the detoxification process results in both bulk and surface changes of the crystal-chemistry of the detoxified sample. This detoxification process also introduces a surface coating comprising ferric ions which shield near-surface ferrous irons and consequently reduces the Fenton-type reactivity of the fibers. It is therefore inferred from this combination of techniques that the ability of the crocidolite fibers to generate oxygen-centered radicals is dependent on the iron redox state and its chelation to different molecules. This in turn, may have an important effect on the ability of the fibers to exert their toxicity.