Brown Diamonds Research Articles

Defect Aggregation and Dissociation in Brown Type-Ia Diamonds by Annealing at High Pressure and High Temperature (HPHT)

Defect Aggregation and Dissociation in Brown Type-Ia Diamonds by Annealing at High Pressure and High Temperature (HPHT)

Dislocation Structures in Diamond: Density-Functional Based Modelling and High-Resolution Electron Microscopy

The core structures of perfect 60 and edge dislocations in diamond are investigated atomistically in a density-functional based tight-binding approach, and their dissociation is discussed both in terms of structure and energy. Furthermore, high resolution electron microscopy is performed on dislocation cores in high-temperature, high-pressure annealed natural brown diamond, and HRTEM image simulation allows a comparison of theoretically predicted and experimentally observed structures.

Vacancy-type defects in brown diamonds investigated by positron annihilation

Five brown natural type IIa diamonds were found to contain concentrations of trapped monovacancy-type defects and large vacancy clusters that significantly exceed the vacancy content in colourless natural or synthetic type IIa diamonds. A proprietary heat-treatment reduced the vacancy cluster content by about 50% and removed the brown colour. The origin for the brown colour is associated with highly distorted regions.

Dislocations in diamond: Dissociation into partials and their glide motion

The dissociation of $60\ifmmode^\circ\else\textdegree\fi{}$ and screw dislocations in diamond is modeled in an approach combining isotropic elasticity theory with ab initio--based tight-binding total-energy calculations. Both dislocations are found to dissociate with a substantial lowering of their line energies. For the $60\ifmmode^\circ\else\textdegree\fi{}$ dislocation, however, an energy barrier to dissociation is found. We investigate the core structure of a screw dislocation distinguishing ``shuffle,'' ``mixed,'' and ``glide'' cores. The latter is found to be the most stable undissociated screw dislocation. Further, the glide motion of $90\ifmmode^\circ\else\textdegree\fi{}$ and $30\ifmmode^\circ\else\textdegree\fi{}$ partials is discussed in terms of a process involving the thermal formation and subsequent migration of kinks along the dislocation line. The calculated activation barriers to dislocation motion show that the $30\ifmmode^\circ\else\textdegree\fi{}$ partial is less mobile than the $90\ifmmode^\circ\else\textdegree\fi{}$ partial. Finally, high-resolution electron microscopy is performed on high-temperature, high-pressure annealed natural brown diamond, allowing the core regions of $60\ifmmode^\circ\else\textdegree\fi{}$ dislocations to be imaged. The majority of dislocations are found to be dissociated. However, in some cases, undissociated $60\ifmmode^\circ\else\textdegree\fi{}$ dislocations were also observed.

A model of HPHT color enhancement mechanism in natural gray diamonds

In the present paper HPHT-processing of natural gray graphite containing diamonds is studied and a new model of the color enhancement mechanism is proposed. It is proposed that there occurs a polymorphous transformation of graphite inclusions into diamond during HPHT-processing. During this transformation hydrogen, the content of which depends to a great extent on the number of graphite inclusions, is transferred from graphite to diamond getting localized on the nearest structural defects with uncompensated negative charges. In the optical spectra this is accompanied by an anomalous increase in the intensity of the hydrogen-related lines in the IR spectrum. It has been observed that the intensity increases in the 2.988 eV line attributed to N3 centers after HPHT-processing at an unusually low temperature of 1800 °C. In natural brown diamonds the intensity increase of the 2.988 eV line takes place at higher HPHT temperatures (over 2000 °C).

Gem-Quality Synthetic Diamonds Grown by a Chemical Vapor Deposition (CVD) Method

chusetts, has succeeded in growing facetable, single-crystal type IIa synthetic diamonds using a patented chemical vapor deposition (CVD) technique (Linares and Doering, 1999, 2003). For characterization of this material, the company has provided a number of brown-to-gray and near-colorless gem-quality crystals and faceted samples, which represent what is intended to become a commercial product for use in high-technology applications as well as for jewelry purposes (figure 1). Because of the growth conditions and mechanisms used, the gemological properties of these CVD-produced synthetic diamonds differ from those of both natural diamonds and synthetic diamonds grown at high pressures and temperatures. For the same reasons, the brown coloration of CVD synthetic diamonds may not react in the same way, or as efficiently, as most natural type IIa brown diamonds, which can be decolorized at high pressure and high temperature (see box A). Preliminary notes on GIA’s examination of some of these Apollo samples were published by Wang et al. (2003) and appeared in the August 8, 2003 issue of the GIA Insider (GIA’s electronic newsletter: http://www.gia.edu/newsroom/issue/2798/1842/ insider_newsletter_details.cfm#3). Spectroscopic analysis of an Apollo CVD synthetic diamond also was performed recently by other researchers (Deljanin et al., 2003). The purpose of the present article is to provide a more complete description of this material and its identifying features.

An infrared investigation of inclusion-bearing diamonds from the Venetia kimberlite, Northern Province, South Africa : implications for diamonds from craton-margin settings

The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal–Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite, relative to diamonds from the Argyle lamproite, indicate that deformation was at a significantly lower level. The comparatively low value of diamonds from Argyle (at approximately US$8/carat) as opposed to Venetia (US$90/carat) is in large part because of the very high abundance of brown diamonds at Argyle. Therefore, it is apparent that deformational history of the mantle in which the diamonds were resident prior to or during sampling by the host may have an important role to play in the profitability of a primary diamond deposit. The apparently consistent association of diamonds with unusually aggregated nitrogen with kimberlites, or lamproites intruded into craton margin or mobile belt settings suggests that it may be possible to recognise such contributory sources in alluvial diamond deposits, through the study of the infrared characteristics of the diamonds. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00410-002-0385-2

Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment

Abstract Absorption and luminescence spectra have been measured for natural brown diamonds before and after high-pressure, high-temperature (HPHT) treatment at 1700–1800°C, and after HPHT treatment at 2025°C. The reduction of the brown colour noted in diamonds with low nitrogen concentration is attributed to the annealing of plastic deformation. In nitrogen-containing diamonds the vacancies released during this annealing are trapped to form N–V–N centres. In addition, the photoluminescence band with a zero-phonon line (ZPL) at 2.526 eV (490.7 nm), characteristic of decorated slip traces, is reduced in intensity during this process, confirming that changes are taking place at the slip traces which have resulted from the plastic deformation. At the lower annealing temperatures the N–V–N centres are in the neutral charge state, giving rise to the H3 absorption band, with a ZPL at 2.463 eV (503.2 nm) and resulting in a yellow colour. At the higher annealing temperature some nitrogen aggregates decompose, producing single substitutional nitrogen which is an electrical donor. Consequently some N–V–N centres are in the negative charge state, and give rise to the H2 absorption band with a ZPL at 1.257 eV (986.9 nm). The combination of absorption in the H2 and H3 bands gives the diamonds a yellow-green or green colour. Annealing at the highest temperature, at pressures very close to the diamond/graphite transition, seems to be effective at reducing the non-radiative recombination channels in the diamonds, and, in white light, many specimens exhibit bright green luminescence, produced by absorbed energy being re-emitted in the H3 band. To the eye these processed diamonds appear similar to the very rare, naturally-occurring, ‘green transmitters’. However, because the latter have resulted from much lower geological temperatures, acting over much longer periods of time, they contain negligible concentrations of single substitutional nitrogen and consequently have low H2 absorption.

Measurement of the lattice displacement across {100} platelets in diamond by large-angle convergent-beam electron diffraction

Abstract Large-angle convergent-beam electron diffraction is used to measure the displacement across {100} ⟨R00⟩ platelets in an Argyle brown diamond. By examining the intersection of a series of two-beam diffraction contours g with an inclined platelet, it is shown that integral and fractional components of g · R can be separately determined. The integral part of g · R is determined by applying standard Cherns-Preston rules to the image of the bounding partial dislocation. The fractional part f of g · R is determined by comparing the rocking curve in the faulted region with two-beam dynamical simulations. It is shown that the rocking-curve asymmetry at small deviation parameters provides a simple measurement of f which is relatively unaffected by background inelastic scattering and a finite probe size. Higher-order reflections are used to improve sensitivity, giving R =0.40 ± 0.01. This result is compared with measurements obtained by other techniques and its significance for understanding the structure of diamond platelets is discussed.

Photoconductivity of natural diamonds

Abstract The photoconductivity of natural brown diamonds was studied between 77 K and room temperature. Centres were revealed, whose presence is hidden in the absorption spectrum by the structureless broad absorption that extends into the visible spectral region in this type of diamond. Apart from ND1, the centres where photoconductivity originates have not been identified in other types of diamond.

Cathodoluminescence of brown diamonds as observed by transmission electron microscopy

Abstract Cathodoluminescing features in brown diamonds correlate with the crystalline microstructure as observed by transmission electron microscopy. We identified distinctive emissions at 415.3, 425, 491.3 and 525 nm. The first two correlate with the so-called N3 and band-A emissions, while the last correlates with what is called a strain-broadened H3 band. The N3 emission is associated with stacking faults on {111} planes. H3 emission is correlated with straight dislocations and dislocation tangles. The 491.3 nm and H3 emissions suggest that the diamonds have undergone severe plastic deformation, which may have also contributed to the apparent high concentrations of the N3 defects.

Dynamical processes of the 2.818 eV centre in diamond

In natural Si-rich brown diamonds a luminescence centre with a zero-phonon line at 2.818 eV and a room temperature decay time of 1.7 ms is often found. The vibronic coupling and the decay characteristics of the band are analysed. The luminescence originates mainly in two sublevels of a triplet electronic state that decays to a singlet ground state. The third triplet sublevel reaches thermal equilibrium with the other two above 40 K. The possibility of stimulated tunable emission from this centre is discussed.

Spin forbidden transitions in diamond

The vibronic analysis of centres with zero phonon lines at 2.082 and 2.052 eV in natural brown diamonds is presented. Time-resolved spectra and its temperature dependence reveal a thermal population of higher energy emitting levels with a much higher transition probability (ca. 104) to the ground state, similar to other centres found in this type of diamond.

One-phonon absorption by type IIa diamonds

Brown type IIa diamonds have been found to exhibit a very weak one-phonon infrared absorption that begins near the Raman energy at 1332cm -1, and increases, with decreasing wavenumber, to a maximum at 1016cm -1 with a shoulder on the high-wavenumber side at about 1050cm -1. A colourless type IIa specimen showed an even weaker absorption beginning again near 1332cm -1 and increasing to show two maxima near 1100cm -1 and 1000cm -1. These specimen also showed birefringence patterns between crossed polars that are indicative of plastic deformation. It is suggested that the infrared absorptions are caused by dislocations.

Cross-relaxation effects in the 2.818-eV zero-phonon emission in brown diamond.

In brown diamond, the long-lived emission at 2.818 eV is known to be due to a localized center in the photoexcited triplet state. In this paper, the emission intensity of the 2.818-eV center is studied as a function of the strength of an externally applied magnetic field. The cross-relaxation (CR) with N-V centers, ${\mathit{P}}_{1}$ centers, and g=2 doublet spins of unknown origin, as well as level anticrossing (LAC), is found to affect the phosphorescence intensity for certain magnetic-field strengths and orientations. The CR dynamics is studied optically by measuring the phosphorescence transients induced by microwave pulses resonant with a spin transition in the photoexcited triplet state. Under CR conditions, spin population relaxation in the excited triplet state becomes faster and deviates from a single-exponential recovery behavior. The triplet-state kinetics including CR effects is studied theoretically, and a double-exponential recovery is predicted. Based on an analysis of the kinetics, the experimental microwave recovery transients under the CR conditions are satisfactorily simulated. The CR rate is estimated to be (5.0\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the N-V center and (8.0\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}${10}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the g=2 doublet species.

Optical detection of magnetic resonance in the photoexcited triplet state of a deep center in diamond.

From an optically detected magnetic resonance study of the 2.818-eV zero-phonon emission in brown diamond, direct evidence for the existence of a photoexcited phosphorescent triplet state in diamond has been obtained. The emission is attributed to a deep-center triplet state with spin-Hamiltonian parameters \ensuremath{\Vert}D\ensuremath{\Vert}=924\ifmmode\pm\else\textpm\fi{}2 MHz, \ensuremath{\Vert}E\ensuremath{\Vert}=198\ifmmode\pm\else\textpm\fi{}2 MHz, and g=2.00. The magnetic main axes are along [100], [011], and [011\ifmmode\bar\else\textasciimacron\fi{}]. Time-resolved microwave recovery and microwave-induced delayed phosphorescence experiments at 1.4 K yield lifetimes of the radiative sublevels of 0.5 and 1.8 ms, whereas the nonradiative sublevel has a lifetime of 23 ms.

Trace-element analysis of argyle diamonds using instrumental neutron activation analysis

Thirty four elements were determined by instrumental neutron activation analysis in colourless, brown, and pink diamonds, with and without inclusions. These were compared with data obtained for similar elements in the host lamproite rock. The natural radioactivity of these samples was measured by instrumental techniques, and found to be negligible.

Implanted 3He, 4He, and Xe in further studies of diamonds from Western Australia

In measurements of the noble gases in additional samples of diamonds from the Argyle and Ellendale lamproites in Western Australia we have failed to encounter any neon-rich stones such as showed solar-like isotopic compositions in earlier work. No neon was detected above the relatively high blank levels in our glass apparatus. White and brown diamonds showed no differences in noble gas content, nor did samples segregated by the color of long-wave UF fluorescence. The rare gas patterns in the 1.2 Ga Argyle pipe are largely consistent with implanted 3He, 4He, and fissiogenic Xe from U/Th in the matrix rock in which the diamonds have been stored for so long. These implanted species are absent in diamonds from the much younger (~20 Ma) Ellendale pipe. We give implantation formulae for several models of inhomogeneously distributed U/Th. Differences in 3He content between pipe and alluvial Argyle samples are consistent with expected cosmogenic production in the latter. An expanded data base for helium and carbon isotopic data on the same samples supports a negative [ 4He]-δ 13C correlation seen earlier in work from our group, but if the Argyle samples, which contain light carbon, are corrected for implanted 4He-δ 13C, the correlation is considerably weakened. We no longer see an earlier 3He 4He-δ 13 C correlation.

Models for the di-nitrogen centres found in brown diamond

No new experiments are reported in this paper: the paper comprises a re-interpretation of published EPR data of the various di-nitrogen centres in brown diamond, including the new ENDOR results of the authors' previous paper on the W7 centre (see ibid., vol.3, p.3591, 1991). The new evidence about W7 provided by these measurements, particularly the identification of the site symmetry as C1, together with other data about the centre, has enabled them to propose a definitive model for W7: a puckered ring structure (comprising four C atoms, one N atom and N+ ion). This model for the W7 centre is shown to lead, by comparison of the properties of the two centres, to a second neighbour, N-C-N+, model for the N1 centre. This model for N1 is shown, in the appendix, to allow new information to be drawn from 3C hyperfine structure in the EPR spectrum of P1, the single substitutional nitrogen. A re-examination of published experimental EPR data of Welbourn (1978) for the N4 centre, showing that the two nitrogen atoms are equivalent, is used to propose a collinear N-C-+C-N model for N4. The occurrence of only some of the possible orientations of W7 and N4 centres allowed by the symmetry operations of the N4 centre, are explained in terms of a model of the formation of these two centres by plastic deformation.

ダイヤモンド原石の色のバイト摩耗への影響

Even for diamond tool, the tool wear increases with cutting distance after on diamond turning of an Al-Mg alloy. According to diamond tool manufacturers, it is said that brown diamonds are stronger and tougher than yellow ones. It has not been made clear, however, what coloured diamond is preferable for cutting tools. On the other hand, Ikawa has reported that microfracture strength can be estimated by IR absorption coefficient on diamond crystals. In this paper, the correlation between the colour of diamond and the tool wear was examined on turning experiments of an Al-Mg alloy. Yellow, light yellow, white, light brown and brown crystals of natural diamond were tested as cutting tools. These tools have same crystallographic orientation and same ratio, RA, of IR absorption coefficient α1 at wave length λ=7.3 μm relative to the value α2 at λ=7.8 μm. The following results were obtained. (1) Microfracture strength on the brown tools is approximately 1.2 times larger than that on the yellow tools, according to the estimation with α1 and α2 measured. The wear rates of both cutting tools are, however, almost same in the stationary state after some primary tool wear. (2) The surface roughness using the brown tools can not be distinguished from that using the yellow tools at any cutting distance.