I. Kiflawi

The relationship between infrared absorption and the A defect concentration in diamond

Abstract The nitrogen concentrations of a suite of thirteen purely type IaA diamonds have been determined by a destructive method involving combustion of the specimens and measurement, by capacitan...

Infrared absorption by the B nitrogen aggregate in diamond

Abstract Two approaches have been adopted in attempting to determine the strength of the one-phonon infrared absorption by the B nitrogen aggregate in diamond. On the one hand, we have heat treated purely type IaA specimens with known infrared absorption strengths (and thus also known nitrogen concentrations) to bring about partial aggregation of the A centres to B centres, monitoring changes in the strengths of the A, and resultant B, one-phonon components by decomposition of the ensuing infrared spectra. Secondly, we have made direct chemical assays of nitrogen in diamonds that show dominant B absorption features. This approach is complicated because diamonds in which the B aggregate is the only point defect also contain extended defects that may or may not involve nitrogen. These defects may or may not, in turn, contribute to absorption in the one-phonon region. Nevertheless, the nitrogen concentrations have been measured for two distinct groups of diamonds that both uncritically might at first glance ...

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...

Spectroscopic study of cobalt‐related optical centers in synthetic diamond

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 fr...

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 aggregation sequence and the formation of voidites in diamond

Abstract High-temperature annealing of type IaB diamonds containing platelets caused the conversion of the platelets to dislocation loops and the formation of octahedral shaped defects [T. Evans, I. Kiflawi, W. Luyten, G. Van Tendeloo, G.S. Woods, Proc. R. Soc. Lond. A 449 (1995) 295]. The chemical composition of these octahedral defects was investigated, and they were found to contain molecular nitrogen, thus showing them to be the same as the voidite defects observed in natural diamonds. This demonstrates the last stage of the sequence of the aggregation of nitrogen in diamonds. The amount of nitrogen in the ‘man-made’ voidites was found to be similar to the amount of nitrogen in the platelets before their conversion to dislocation loops. Also, a possible mechanism for the formation of B centres from A centres is discussed.

The effect of nickel and the kinetics of the aggregation of nitrogen in diamond

Abstract Using infra-red absorption topographic spectroscopy the aggregation of nitrogen in synthetic diamonds grown with a nickel catalyst was investigated. It was found: (a) that the rate of aggregation is a function of the nickel concentration, showing conclusively that the presence of nickel is responsible the enhancement of the nitrogen aggregation in diamond: and (b) that the aggregation process in the specimens containing nickel, does not follow second order kinetics. Two possible mechanisms for the involvement of nickel in the nitrogen aggregation process are presented.

The aggregation of nitrogen and the formation of A centres in diamonds

Abstract Topographic infra-red spectroscopy has been used to investigate the inhomogeneity of the aggregation of nitrogen in {111} growth sectors of synthetic diamonds. The aggregation rate constant was established to be different even within the same {111} growth sector. In particular, changes in the growth temperature, most likely changing the impurity content, are shown to result in changes in the aggregation rate constant; namely an increase in the growth temperature, in diamonds grown with a Co catalyst is found to reduce the nitrogen aggregation rate constant, and vice versa. These results indicate that the presence of Co impurities in the {111} growth sector influences the aggregation rate. It is found that the aggregation process follows second order kinetics as first established by Chrenko et al. [Nature, London, 270 (1977) 141]. The activation energy of the aggregation process in {111} growth sectors is measured as 5.5 ± 0.7 eV. It is also found that increasing the applied pressure during the annealing process reduces the aggregation rate constant, such that there is a significant difference between annealing at ambient pressure and annealing at 10 GPa.

The creation of the 3107 cm−1 hydrogen absorption peak in synthetic diamond single crystals

Abstract The 3107 cm −1 hydrogen related local mode was produced in HPHT grown diamonds after annealing at temperatures above 2100°C. A correlation was found between the intensity of the peak and the concentration of nitrogen at different locations of the same specimen. The peak position did not shift in 15 N doped samples.

‘Natural’ and ‘man-made’ platelets in type-Ia diamonds

Abstract ‘Natural’ platelets are planar defects in {001} planes found in natural type-IaA/B diamonds. ‘Man-made’ platelets are platelets formed in the laboratory by annealing type-IaA diamonds at temperatures over 2500°C. Careful study shows that the infrared (IR) spectra of the ‘man-made’ platelets are different from the IR spectra of ‘natural’ platelets. High-temperature (T ≥ 2000°C) annealing of platelets containing type-IaA/B diamonds modifies the IR absorption spectrum owing to the ‘natural’ platelets and makes it similar to the IR spectrum of the ‘man-made’ platelets. It is suggested that such high-temperature annealing changes the structure of the ‘natural’ platelets. The changes are too subtle to be detected by electron microscopy techniques. Topographic electron-energy-loss spectroscopy shows that platelets contain nitrogen at an average density of 0.7 atoms per a 2 0; however, high-temperature annealing does not seem to affect the concentration of the nitrogen in the platelets.