Natural Type Ib Diamonds Research Articles

Spectroscopic Study of the 3107 cm−1 and 3143 cm−1 H-Related Defects in Type Ib Diamonds

Hydrogen-related infrared absorption bands in natural diamonds have been extensively investigated and widely used to identify natural, treated, and synthetic diamonds grown by high pressure and high temperature (HPHT) and chemical vapor deposition (CVD) techniques. However, the evolutional behavior of the hydrogen-related defects and the relationship between the hydrogen-related and nitrogen-related defects in natural and HPHT-treated Ib diamonds are unclear. In this article, the hydrogen-related defects, particularly the infrared absorption bands of 3107 cm−1 and 3143 cm−1 in natural type Ib diamonds and HPHT-treated natural diamonds, were systematically investigated using spectroscopic techniques. It was found that the 1405 cm−1 absorption intensity was directly proportional to the 3107 cm−1 absorption intensity; the 3143 cm−1 absorption intensity increased with the increase in the 3107 cm−1 absorption intensity, but there was no strict linear relationship between them. The 3143 cm−1 band was not only related to the intensity of the 3107 cm−1 but also related to the value of NC/NA in natural diamonds. When the value of NC/NA was less than one, the 3143 cm−1 band was more pronounced. After high-temperature annealing, the absorption intensities of the 3107 cm−1 and 3143 cm−1 in natural type Ib diamonds became stronger. However, in HPHT synthetic diamonds, only a 3107 cm−1 defect was introduced with the increase in the A centers in the diamonds. The difference and the detectability of the 3143 cm−1 and 3107 cm−1 bands investigated could be efficiently used to identify natural type Ib diamonds from their counterparts, including the synthetic diamonds and the HPHT-treated diamonds.

The origin of color in natural C center bearing diamonds

The properties of 152 natural diamonds with C centers – detectable by the absorptions at about 1344 and/or 2688cm−1 in the infrared spectra – were analyzed in order to better understand their origin of color. While such diamonds are generally thought to be yellow, type Ib natural diamonds are usually not so, but mainly orange-yellow, orange, brown, ‘olive’ (a mixture of yellow with brown and/or gray with always a greenish component) and mixtures thereof. The only natural diamonds found to be of pure yellow coloration were – with very few exceptions – type IaA diamonds with a very minor Ib component, of cuboid–octahedral growth, often so-called re-entrant cubes. This was verified by the analysis of over 70,000 bright yellow and over 20,000 yellow-orange melee diamonds (i.e. diamonds weighing less than 0.20cts) submitted for testing at the laboratory.In natural type Ib diamonds of octahedral growth the color is strongly influenced by vacancy-related defects that originate mainly from plastic deformation; natural type Ib diamonds of regular octahedral growth generally show distinct deformation-related strain and often some associated color zoning or ‘colored graining’ along octahedral planes. None of the nickel-rich, C-center-containing natural diamonds included in this study showed any specific Ni-related absorption band in the visible range spectrum that had an influence on color.The “olive” to brown color in type Ib diamonds was found to be caused by a combination of continuum absorption with increased absorbance from the NIR to about 480nm plus distinct NV− center absorption.

Luminescence spectroscopy and microscopy applied to study gem materials: a case study of C centre containing diamonds

The methods of luminescence spectroscopy and microscopy are widely used for the analysis of gem materials. This paper gives an overview of the most important applications of the analysis of laser and UV excited luminescence by spectroscopy and visually by microscopy with emphasis on diamond, and specifically natural type Ib diamond, little studied so far. Luminescence based techniques are paramount to the gemmological analysis of diamond, in order to determine whether it is natural, treated or synthetic. The great sensitivity of luminescence helps detect some emitting centres that are undetectable by any other analytical method. Hence, especially for diamond, luminescence is an enabling technology, as illustrated by its pioneering use of imagery for the separation of natural and synthetic diamond, and of spectroscopy for the detection of High Pressure–High Temperature treatment. For all other gemstones the applications are at the moment less numerous, but nevertheless they remain highly important. They provide quickly information on the identification of a gem material, and its treatment. Besides the study of broad band emissions caused by various colour centres, the typical PL-causing trace elements (amongst others) are chromium, manganese, uranium and rare earth elements. In pearls the study of broad band luminescence can be useful, and particularly the study of pink to red porphyrin luminescence in pearls from certain species such as Pinctada and Pteria and others can help identify the pearl-producing mollusc, or if a pearl has been dyed or not. Type Ib diamonds are representative of the importance and complexity of the analysis of luminescence by microscopy and spectroscopy. They show a wide range of sometimes very complex emissions that result in luminescence colours from green to yellow to orange or red. These emissions show generally very inhomogeneous distribution. They are caused by a range of defects, however only a few of them are well characterized.

A new defect center in type Ib diamond inducing one phonon infrared absorption: The Y center

The infrared spectra of 68 natural and synthetic diamonds with detectable C centers at about 1130 cm − 1 and 1344 cm − 1 were recorded. After correction and normalization of the spectra there was an attempt to determine the A, B and C center nitrogen content of the samples using the spectral fitting spreadsheet supplied by David Fisher from the DTC. It became clear that for the majority of our samples the results were erroneous for the C center content. In fact, the calculated C center content based on the 1130 cm − 1 absorption was significantly higher than what the 1344 cm − 1 absorption height indicated; in the fitted traces proposed by the spreadsheet the 1344 cm − 1 absorption was always at least 50% higher than in the original spectrum. The overestimated C center content and the residual trace after spectral fitting indicated that some absorption bands were underlying the well-known, single-substitutional nitrogen absorption at 1130 cm − 1 . For this reason all spectra were decomposed again by progressive subtraction of the A, B, C and X center absorptions, in order to visualize any residual absorption not attributed to any of those four, classically-recognized, nitrogen-related one-phonon absorption systems. In the spectra of 38 diamonds the decomposition work resulted in a consistent residual absorption feature with a relatively broad apparent maximum centered at about 1145 to 1150 cm − 1 . For the sake of convenience we use “1145 cm − 1 ” to describe this feature. Two additional samples did not exhibit any 1130 cm − 1 absorption but instead precisely the same one-phonon absorption features as the newly-found residual. Consequently several hundred nominally type Ib diamonds were screened by infrared spectroscopy, and 22 had a one phonon absorption with a dominant 1145 cm − 1 absorption. This confirms that this absorption system was not an artifact of the original spectral decomposition work. Further, this new system is apparently a characteristic component of many natural type Ib diamonds. Based on the fact that this newly described center is clearly related to single substitutional nitrogen and that the last such one-phonon IR absorption had been named “X center” [1] (positively charged single nitrogen), we propose to call this defect “Y center”.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. III. Dynamic polarization of nuclear spin system by nuclear orientation via electron spin locking

AbstractDynamic nuclear polarization of nuclei by means of paramagnetic electron spin locking (Hartmann‐Hahn cross‐polarization between paramagnetic electrons and nuclei, or NOVEL) is discussed. The theory is demonstrated by experiments executed at 2.4 and 9.6 GHz on a natural type Ib diamond. It is shown that the 13C polarization rate is independent of the microwave frequency, in agreement with theory. NOVEL polarization takes place only while the spin‐locking pulse is on. The rate at which the nuclei are polarized is proportional to the electron polarization in the rotating frame. Therefore, the length of the spin‐locking pulse is limited by the value of T1ρ(e), and because T1ρ(e) ≪ T1(e) for diamond the effective NOVEL polarization rate of 13C nuclei is usually relatively low. A comparison between the relative effectiveness of 13C polarization rates between NOVEL and the solid‐state effect is made for high and low paramagnetic impurity concentrations. The dependence of the 13C polarization rate on the paramagnetic impurity concentration has been determined for a suite of natural diamonds. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 44–49, 2003.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. II. Dynamic polarization of nuclear spin system by thermal mixing and solid‐state effect

AbstractDynamic nuclear polarization of nuclear spins via the solid‐state and thermal mixing effects is discussed. Continuous‐wave S‐ and X‐band microwave radiation have been employed to measure 13C signal enhancements and polarization times for 13C nuclei in a natural type Ib diamond as a function of magnetic field. It was found that thermal mixing plays an important role in the 13C signal enhancement because the central electron spin resonance (ESR) line width HL ≃ H0γC/γe, resulting in flip‐flip and flip‐flop forbidden transitions taking place simultaneously. On the other hand, the 13C spin‐lattice relaxation rate is determined to a large extent by the solid‐state effect (forbidden transitions). 13C polarization rates have also been measured for a suite of natural diamonds. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration, in agreement with the theory. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 36–43, 2003.

EPR data on the self-interstitial complex O3 in diamond

A previously unreported defect, which is labeled $O3,$ has been observed in the EPR spectrum of synthetic type-IIa diamonds irradiated at 100 K with 2 MeV electrons. This defect was not observed in identical diamonds whose temperature during electron irradiation was \ensuremath{\geqslant}300 K. This center has also been observed in neutron irradiated natural type-Ib diamonds, but only after isochronal annealing at about 650 K: it subsequently annealed out at about 720 K. Analysis of the angular variation of the EPR line positions has determined the zero-field interaction $(\underset{\ifmmode\bar\else\textasciimacron\fi{}}{\mathbf{D}}),$ showing that the center has a triplet $S=1$ ground state and ${C}_{2}$ symmetry, with a $〈100〉$ rotation axis. The use of a synthetic type-IIa diamond with 5% ${}^{13}\mathrm{C}$ isotopic enrichment allowed observation of ${}^{13}\mathrm{C}$ hyperfine satellites. Analysis of the ${}^{13}\mathrm{C}$ hyperfine couplings by a simple molecular orbital calculation shows that 76% of the unpaired electronic wave function is localized in two nonbonding $2p$ orbitals, on different carbon atoms. Interpretation of the parameter $\underset{\ifmmode\bar\else\textasciimacron\fi{}}{\mathbf{D}}$ as arising primarily from dipole-dipole interaction between these two orbitals indicates that they are separated by 0.32(2) nm. The nonbonding $2p$ orbitals indicate the involvement of $〈100〉$-split interstitials $({\mathbf{I}}_{100})$ in the structure. Two possible models are proposed, one involving two and one three parallel ${\mathbf{I}}_{100}$ at next-nearest-neighbor positions, of which the latter gives the better fit to the data.

Kinetics of Ib to IaA nitrogen aggregation in diamond

Diamonds containing nitrogen impurities in the form of singly substituted C centres (type Ib diamond) are rare in nature because lb nitrogen atoms aggregate to form A centres (type IaA diamond) during the long-term (∼0.5 to 3 Gyr) upper mantle residence period experienced by most diamonds. Preservation of N-rich diamond with a type Ib component requires a young age of crystallization, or unusually cool conditions of mantle storage, or both. Quantitative constraints on the origin of Ib diamonds are possible if the kinetics of nitrogen aggregation are accurately known. However, previous experimental studies have produced significantly different values for the activation energy of Ib → IaA conversion (Ea) which is at least partly due to sector dependence of the aggregation rate in the starting materials. We report a series of high-pressure (P), temperature (T) experiments on synthetic and natural type Ib diamond in which we have used infrared (IR) microspectrometry techniques to unravel the sector dependency. The results show that cube sectors have an Ea of 6.0 ± 0.2 eV, which is significantly greater than the Ea of 4.4 ± 0.3 eV determined for the octahedral sectors. Both values are in excellent agreement with a recent theoretical study in which the sector dependency is ascribed to different mechanisms of nitrogen migration. The higher Ea is most relevant to natural Ib diamond which are invariably of cube or cuboid habit. Application to the Kokchetav microdiamonds of metamorphic origin indicate that their aggregation state is consistent with peak temperatures of 950°C and a burial to exhumation history of ∼17 Myr. Application to yellow cubes and “coat” from Yakutia having ∼20% Ib component, indicate this diamond generation grew_ˇMyr before kimberlite eruption for mantle T_˘950°C but did not grow directly from the transporting kimberlite magma.

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.

Luminescence decay time of the 1.945 eV centre in type Ib diamond

The luminescence decay time of the 1.945 eV centre has been determined as 13+or-0.5 ns for natural type Ib diamonds. For synthetic diamonds the value is slightly lower, probably because of competitive non-radiative recombination in these relatively impure crystals. The similarity of the decay time to that for the H3 centre further supports the model proposed for the vacancy-enhanced aggregation of nitrogen in diamond. No temperature dependence of the 1.945 eV decay time has been detected in the range 77-700K, and this result is not inconsistent with the energy level scheme used to interpret ESR data for the 1.945 eV centre.

Defects in natural type IB diamond

NATURAL type IB diamond has an anomalously low thermal conductivity. To investigate this we have carried out birefringence, infrared and ultraviolet absorption spectroscopy, electron spin resonance and transmission electron microscopy (TEM) experiments. We suggest here that the low IB thermal conductivity is due to the presence of previously unknown defects in this type of diamond. It is also suggested that these defects are related to the more common ‘platelets’ or planar faults which are found in type IA diamond.

The effect of nitrogen impurity on the annealing of radiation damage in diamond

Impurity nitrogen in type Ia diamond is known to trap radiation damage centres during annealing. Most of the nitrogen is also known to be present in two forms, labelled A and B from the absorption peaks produced by them in the range 1000-1332 cm-1. In this paper, the effect of these different forms of nitrogen on the annealing of radiation damage is considered. The visible absorption spectra of 41 annealed diamonds have been compared with their infrared spectra. The results suggest that the A form of nitrogen produces the 2.463 eV centre, while the B form gives the 2.499 eV centre, by trapping a radiation damage centre during annealing. The trapping centres compete with each other for the damage centres. In natural type Ib diamond (containing paramagnetic nitrogen), the 640nm (1.945 eV) centre is produced by the annealing, in agreement with du Preez (1965).

Analysis of an Electron Spin Resonance Spectrum in Natural Diamonds

An anisotropic electron spin resonance spectrum was observed in three natural Type Ib diamonds. The diamonds which exhibit the spectrum also show the spectrum from substitutional nitrogen donors previously observed by others. The spectrum consists of three anisotropic groups of lines. The spectrum is interpreted in terms of a model in which each defect center is characterized by a spin Hamiltonian of the form H=βS·g·H+A·I·S, where S = ½ and I = 1, and in which the tensors g and A each have one component g1 and A1 oriented in a 〈110〉 direction. The spin-one nucleus is believed to be nitrogen. The principal components of g and A have the values g1 = 2.0031±0.0003, g2 = 2.0019±0.0003, g3 = 2.0025±0.0003, A1 = 5.303±0.005×10−4 cm−1, A2 = 7.164±0.005×10−4 cm−1, and A3 = 5.293±0.005×10−4 cm−1. The components g2 and A2 make angles Ψ = 45.2°±0.3° and α = 22.4°±0.1°, respectively, with 〈110〉 directions. The theoretical angular dependencies of the spectra when the magnetic field H is rotated in {100}, {110}, and {111} crystalline planes were calculated on the basis of the above model and are in excellent agreement with the experimentally observed angular behaviors.