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Observation of the H2 defect in gem-quality type Ia diamond
Diamond and Related Materials 8 (1999) 1061–1066
Observation of the H2 defect in gem-quality type Ia diamond k Peter R. Buerki a, *, Ilene M. Reinitz b, Sam Muhlmeister a, Shane Elen a a Gemological Institute of America (GIA), GIA Research, 5345 Armada Drive, Carlsbad, CA 92008, USA b Gemological Institute of America (GIA), Gem Trade Laboratory, 580 Fifth Avenue, New York, NY 10036, USA Received 27 July 1998; accepted 4 January 1999
Abstract The H2 defect with its zero-phonon line at 986.1 nm (1.257 eV, 10141 cm−1) was first described in 1956 and characterized in detail in 1990–91. In this paper, the observation of strong H2 bands in greenish-yellow natural type Ia gem-quality diamonds is reported. These ‘H2 diamonds’ were apparently subjected to a treatment possibly involving irradiation and clearly involving high temperature annealing. Besides burns on the surface and in fractures, they typically showed strong green luminescence associated with brownish-yellow graining, strong H3/H4 bands, weak to strong H2 bands, and no H1b/H1c lines. Some stones displayed a weak 637 nm line as well. A model explaining the formation of the H2 defect in type Ia diamond is proposed based on the stability and interaction of defects in diamond as a function of temperature and time. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: H2 defect; Vacancy complexes; Annealing; Type Ia diamond; VIS–NIR spectrometry; Fancy color diamonds
1. Introduction The H2 absorption band with its zero-phonon line at 986.1 nm (1.257 eV, 10141 cm−1) was first reported in 1956. It was detected in a natural type Ia diamond that had been irradiated with 1.0 MeV electrons at a flux of 20 to 50 mA/cm2 and subsequently annealed at 900°C for 10 h [1]. A much stronger H2 band was created in 1988 by irradiating type Ib diamond with electrons of higher energy followed by a high temperature anneal. The acceleration voltage was 3 to 4 MeV and the dose 1×1018 to 5×1019 electrons/cm2. The diamonds were annealed at temperatures from 1600 to 1800°C for 5 to 30 h with a stabilizing pressure of 3.0 to 5.0 GPa [2]. Others were annealed in vacuum at 1550 to 1700°C for 20 h [2]. A detailed characterization followed in 1990–91: the results of Lawson et al. [3] and Mita et al. [4] provided evidence that the H2 center is a negatively charged state of the H3 center. To our knowledge, no further work has been published on the occurrence of the H2 band in type Ia diamond since the work of Clark et al. [1]. k
Presented at the Diamond ’98 Conference, Crete, Greece, 13–18 September, 1998. * Corresponding author. Fax: +1-760-603-4021. E-mail address: [email protected] (P.R. Buerki)
GIA Research has been carrying out a systematic ongoing study of the origin of color in fancy-color gem diamonds since 1986, which currently includes data on over 3000 diamonds of various colors. This project supports the work of the GIA Gem Trade Laboratory, which issues reports specifying whether the color of a fancy color diamond arises naturally or is due to laboratory treatment. Prior to 1996, we documented the properties of 27 type Ia diamonds containing H2 centers, ranging in color from greenish-yellow to brownishyellow. 16 of these displayed features indicating laboratory irradiation and annealing, but the other 11 showed characteristics that — to the best of our knowledge — indicated natural origin of color. Thus, prior to 1996, we regarded the occurrence of an H2 absorption band in a type Ia diamond as something of a curiosity with little diagnostic value for determining the origin of a greenish color component. Our opinion changed dramatically that autumn, when we received about 50 greenish-yellow to yellow–green faceted diamonds over a 1 month period that showed moderate to intense H2 bands. Since then, we have seen approximately 100 ‘H2 diamonds’. Other gemological laboratories around the world have also reported H2 diamonds. This prompted both the GIA Gem Trade Laboratory and the Diamond High Council (HRD) in Antwerp to issue warnings to the gem trade about a
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suspected new treatment process [5–7]. This larger group included diamonds with a brownish-orange– yellow color. Intensely colored yellow diamonds with strong green luminescence to visible light (called ‘green transmitters’ by gemologists) are rare among the colors found naturally in gem diamonds. The prices paid for such diamonds typically exceed $10 000 per carat, whereas the market price for a known treated color diamond of similar appearance would be one-fifth to one-tenth as much. As reliable information on the origin of color typically does not follow a colored diamond through the trade, both consumer confidence and the stability of the colored diamond market as a whole depend on the ability of independent gemological laboratories to identify such treated colors properly. We have observed three types of green transmitter relevant to this study: diamonds with an H2 band that appear to be naturally colored; diamonds with an H2 band displaying other properties that indicate laboratory treatment; diamonds without an H2 band that also show these other properties. In the following discussion we shall refer to this last group as ‘non-H2 green transmitters’. The H2 defect appears to occur naturally, in rare cases, when type Ia diamonds are subjected to natural irradiation with simultaneous or subsequent exposure to temperatures above 1000°C. In nature diamonds are generally exposed to much lower radiation doses than they are in the laboratory. Thus natural H2 bands are likely to be weak or below the detection limits of VIS– NIR spectrophotometers. This is supported by our observation of only 11 possibly natural untreated H2 diamonds out of a total of 510 yellow diamonds with green luminescence that were examined in our laboratory during the last 12 years.
2. Experimental Faceted H2 diamonds and non-H2 green transmitters were subjected to non-destructive testing consisting of a variety of visual observations and spectroscopic methods. For the visual inspection, a gemological Mark VII Gemolite Ultima B (GIA GEM Instruments, Carlsbad, CA) microscope with dark field and fiber optic illumination capable of magnifications between 10× and 70× was used. The UV fluorescence was tested in a dark room using an LW/SW UV fluorescent lamp (GIA GEM Instruments) emitting a broad band centered at 356 nm with a sharp superimposed peak at 365 nm for LWUV and 254 nm for SWUV. VIS–NIR spectra at liquidnitrogen temperature between 400 and 1000 nm were obtained with a Hitachi U-4000 spectrophotometer. Slit widths of 0.5 nm and 5.0 nm were used for the VIS range (400 to 850 nm) and for the NIR range (850 to 1000 nm) respectively; a sampling interval of 0.2 nm was
used for both. FTIR spectra in the range 400 to 6000 cm−1 were recorded with a Nicolet Magna-IR spectrometer 550 equipped with a 6× beam condenser from SpectraTech.
3. Properties of H2 diamonds Typical treated-color H2 diamonds show a range of characteristic features including: a saturated yellow (occasionally brownish-orange–yellow) bodycolor, a strong green luminescence to visible light that produces an intense greenish-yellow to yellow–green color under standard color grading conditions [8], and a strong, chalky, greenish-yellow fluorescence to LWUV and SWUV followed by a greenish-yellow afterglow, moderate to very strong initially, then weakening but continuing for at least 30 s after the UV lamp is switched off. The bodycolor of these diamonds is unevenly distributed, showing obvious strong, brownish-yellow graining (i.e. octahedral slip planes) in the microscope. Green luminescence, which was generally restricted to the graining planes, was visible using a high-intensity fiber optic light source. Many of these diamonds displayed burn marks, e.g. pitted and frosted surfaces on facet junctions, surface-reaching fractures, and naturals (i.e. remnants of the original crystal faces). The remnants of facets were present on some of the frosted girdle surfaces. The initial group of H2 diamonds, received in the autumn of 1996, had a poor finish, including misaligned facet junctions due to overcutting and heavily bearded girdles (i.e. with numerous small radial fractures). H2 diamonds received later had a better finish. The absorption spectra in the visible and near-infrared range showed a weak N3 system at 415 nm, a moderate to very strong H3 band at 503 nm, a 637 nm line (in 47% of the cases) and a weak to very strong H2 absorption band with a zero-phonon line at 986 nm ( Fig. 1). A few diamonds had a weak absorption at 535 nm as well. With one exception, all of these diamonds were of type Ia with no detectable Ib component. In most cases, the concentration of B aggregates was higher than that of A aggregates ( Fig. 2). The presence of a strong H3 band, which is due to (N–V–N )0 defects, indicates that the diamonds had been exposed to both ionizing radiation and high temperatures. The absence of other radiation defects, such as the GR1 band at 741 nm, the 595 nm line, and — in most cases — the H1b and H1c lines at 4941 cm−1 and 5171 cm−1, indicates an annealing temperature of at least 1400°C, possibly performed under a stabilizing pressure of ~5 GPa [2,3]. Annealing at this temperature for 4 h causes the disappearance of the H1b and H1c lines [9], whereas the H3 band is hardly affected [10]. Annealing in air would account for the extensive surface burns evident in these diamonds (even when done with boric acid, which is typically used as a protective coating
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Fig. 1. Low temperature visible–near-infrared spectra of two natural type Ia diamonds with well-developed H2 bands; (a) 0.63 ct, treated-color greenish-yellow round brilliant cut diamond #334; (b) 0.56 ct, treated-color greenish-yellow round brilliant cut diamond #320. The step and noise in the range 850 to 900 nm is due to a detector change and noise from the PbS detector. The feature at 670 nm in spectrum (b) is an artifact due to a polarization mismatch between sample and reference beam.
during gem diamond cutting), which can subsequently be removed by repolishing. We can only speculate as to the nature of the starting material that will possess this color and spectral features after treatment, but likely candidates are diamonds whose slightly brownish to brownish-yellow color is considered undesirable in the jewelry market. In agreement with other groups [11], we believe that the H2 center can be created in diamonds by both natural and laboratory irradiation. It is known that diamonds may experience temperatures of up to 1250°C during their formation and transport to the earth’s crust. Although the spectral features related to laboratory annealing can also be produced naturally, albeit at lower temperatures and significantly longer time scales [12,13], we expect naturally colored diamonds with an H2 band to be rare.
4. Formation of the H2 center in type Ia diamond Based on the properties of the H2 diamonds examined, we propose the following reaction mechanism as
the cause for the formation of the H2 center in type Ia diamond. (1) The irradiation of diamond with high-energy electrons of 0.3 MeV or higher creates lattice defects consisting of interstitial–vacancy ( I–V ) pairs [14]. For a uniform creation of lattice defects throughout the diamond lattice an accelerating voltage of at least 2 MeV (typically 3 to 4 MeV ) is required [2]. The neutral single vacancy V0 is the lattice defect producing the wellknown GR optical absorption bands consisting of the GR1 band at 741 nm, and the GR2–GR8 lines in the wavelength region of 430 to 412 nm. The simultaneously formed carbon interstitial is optically inactive [14]. (2) Annealing the diamond after irradiation causes the vacancies to diffuse through the crystal lattice until they are trapped by other defects. The most likely candidates in type I diamond are nitrogen impurities. In type Ib diamonds, V0 vacancies are trapped by single substitutional nitrogen atoms forming (N–V )0 centers with an absorption line at 637 nm [15]. In type Ia diamond, the nitrogen is aggregated. Therefore, the vacancies created by irradiation in type Ia diamond are
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Fig. 2. Corresponding room temperature FTIR spectra for the H2 diamond spectra shown in Fig. 1; (a) 0.63 ct, treated-color greenish-yellow diamond #334; (b) 0.56 ct, treated-color greenish-yellow diamond #320. The spectra were obtained from round brilliant cut diamonds. Wavelengthdependent multiple reflections and scattering within the diamonds cause distortions of the two-phonon bands.
most likely to form (N–V–N )0 defects during annealing by combining with nitrogen pairs (N–N )0, i.e. A aggregates. This defect corresponds to the H3 center with a zero-phonon line at 503 nm and phonon sidebands between 500 and 420 nm [16 ]. If the vacancy is trapped by a B aggregate (group of four N atoms surrounding an empty carbon site), the H4 center is formed with a zero-phonon line at 496 nm and phonon sidebands between 490 and 420 nm [16 ]. Single substitutional nitrogen was detected in just one of the H2 diamonds. Temperatures above 650°C are required for the transformation of GR centers into H3 and H4 centers, destroying the GR absorption bands in the process. At 800°C this transformation is completed in about 12 h [10,13,17]. (3) Annealing an irradiated diamond at 900°C for 10 h causes the transfer of electrons from single substitutional nitrogen atoms to H3 centers to create an additional defect, the H2 center [Eq. (1)]: (N–V–N )0+N0(N–V–N )−+N+ i.e. H3+N0H2+N+
(1)
The absorption band for this defect covers the wavelength range between 992 and 661 nm and represents the phonon side bands of the H2 center. The first report of this absorption band [1] included the phonon sidebands of the H2 center, but did not include the H2 zerophonon line at 986 nm. Subsequent studies on the H2 center were performed using type Ib diamond [2,3]. The authors concluded that the H2 defect corresponds to a negatively charged H3 center (H3−), and that a much stronger H2 band with an intense zero-phonon line can be produced by annealing type Ib diamonds at temperatures above 1400°C [2,3]. To date, only one source for the formation of single substitutional nitrogen in type Ia diamond has been reported: the A aggregate. However, the breaking up of A aggregates into individual nitrogen atoms requires a temperature of at least 1960°C [18]. Two other possible sources for single substitutional nitrogen may be the H3 and H4 centers. Collins [10] postulated that the H3 center can dissociate into a vacancy and an A aggregate according to the reaction in Eq. (2). He also postulated that the H4 center can break up into an H3 center and two nitrogen
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atoms according to the reaction in Eq. (3), and that the H3 centers created in this reaction tend to break up and the nitrogen atoms to rearrange to form B aggregates according to the reaction given in Eq. (4) [10]. (N–V–N )0V0+N–N i.e. H3V0+A
(2)
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which coincides with an absorption line from the B aggregate. Consequently, the two lines cannot be distinguished from each other unless the diamond is a ‘pure’ type IaA. Indeed, one of the three type IaA H2 diamonds we examined showed a weak absorption band at 1332 cm−1, but more samples are needed for a definitive conclusion.
(N–N–V–N–N )0(N–V–N )0+2N0 i.e. H4H3+2N0
(3) 5. Stability of the H2 center in type Ia diamond
(N–V–N )0+2N0V0+N–N–N–N i.e. H3+2N0V0+B
(4)
In addition to the processes suggested by Collins, we propose that H3 centers — including those formed by the dissociation of H4 centers — break up into (N–V )0 centers and single nitrogen atoms according to the reaction (N–V–N )0(N–V )0+N0
i.e. H3(N–V )0+N0 (5)
Collins’ model does not explain our observation of a 637 nm line (i.e. the (N–V )0 center that is typical for diamonds containing single substitutional nitrogen) in 47% of the H2 diamonds that we examined. His hypothesis is based on the examination of one type IaA and one type IaB diamond. He did not observe the development of new absorption bands after annealing the H3 center of the IaA diamond. During our own study, we recorded visible spectra for five type IaA diamonds and none of these had a 637 nm line. No spectra in the visible range were recorded for the three other type IaA diamonds that came through our laboratory. Our results seem to support the Collins model. Nevertheless, the examination of just six type IaA diamonds is insufficient to eliminate the reaction in Eq. (5) as a possibility. The intensity of the 637 nm line may be below our current detection limits, as it may well have been for Collins 20 years ago. We are currently upgrading our equipment with new detectors in order to increase our sensitivity to the H2 and 637 nm lines. Collins observed the creation of new absorption lines, including the 637 nm line, after annealing the H4 center in the IaB diamond [10] (we examined three ‘pure’ type IaB diamonds in this study, none of which showed either an H2 band or a 637 nm line). Collins does not suggest any mechanism that may cause the formation of (N–V )0 centers during this process. The most plausible explanation is, again, the destruction of the H3 centers according to the reaction in Eq. (5). The combination of reactions in Eqs. (1), (4), and (5) may explain the formation of H2 centers in type Ia diamond. The simultaneously formed positively charged single substitutional nitrogen atom N+ has only one absorption peak at the Raman energy of 1332 cm−1,
The existence of non-H2 green transmitters, which have apparently been treated in a similar manner to that of the H2 diamonds, raises questions about the stability and the detectability of the H2 center in type Ia diamonds and whether there is a method for removing it. Perhaps, the H2 center can be destroyed by annealing at much higher temperatures (~2200°C ?) under a stabilizing pressure [19]; however, under such extreme conditions, the H3 and H4 centers would anneal out within minutes and a considerable fraction of the A and B centers broken up into measurable amounts of single substitutional nitrogen, which contradicts our findings. Mita et al. [4] reported that irradiating type Ib H2 diamonds with 514.5 nm Ar+ laser light causes the temporary ionization of the H2 centers and thus their transformation into H3 centers. They concluded that the N+ center is one of the dominant traps for the released electrons. This raises the question of what happens if H2 diamonds are photoionized at elevated temperatures. None of the 90 non-H2 green transmitters, for which we have reliable and complete data, had a 637 nm line. We can think of only two plausible explanations for this observation: the H2 and 637 nm bands are present, but at a level below the detection limits of our current instrumentation, or there exists a method for permanently reversing the formation of the H2 center. Such a method is not known to us; however, it would include the neutralization of single substitutional nitrogen atoms by the H2 centers followed by the recombination of the neutralized nitrogen atoms with the (N–V )0 centers. As a result, two additional H3 centers would be formed. This process may be summarized as shown in the reaction (N–V–N )−+N++(N–V )02(N–V–N )0 i.e. H2+N++(N–V )02H3
(6)
A quantitative determination of the relative strengths of the absorption bands of interest may provide us with important information about the irradiation and annealing history of H2 diamonds. However, a quantitative determination of absorption coefficients in faceted diamonds is impossible owing to multiple reflections, diver-
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gent paths that are wavelength dependent, and scattering of the sample beam.
6. Conclusions The H2 absorption band at 986 nm was observed in a number of type Ia gem-quality diamonds. It probably is the result of a treatment process developed for changing off-color diamonds into intense greenish-yellowcolored diamonds. The suspected treatment process is believed to involve irradiation with a large dose of high energy electrons (3 to 4 MeV ) followed by high temperature annealing in air at temperatures around 1400°C. A model that explains the formation of the H2 defect in type Ia diamond was proposed. The limited data currently available and the many unanswered questions attest to the strong practical need for further research on the formation, stability and interaction of defects in type Ia diamond, and the H2 defect in particular. We are currently upgrading our equipment to improve the detection limits for the H2 and related bands as well as acquiring a set of suitable H2 diamonds in an attempt to duplicate the treatment process and to test our hypotheses.
References [1] C.D. Clark, R.W. Ditchburn, H.B. Dyer, Proc. R. Soc. London Ser. A: 237 (1956) 75–89. [2] S. Satoh, European Patent no. EP 0 324 179, 1992 (in the past, this patent was repeatedly quoted under its EP application number 88 121 859.8, application year 1988). [3] S.C. Lawson, G. Davies, A.T. Collins, A. Mainwood, J. Phys. Condens. Matter 4 (1992) 3439–3452. [4] Y. Mita, Y. Ohno, Y. Adachi, H. Kanehara, Y. Nisida, T. Nakashima, Diamond Relat. Mater. 2 (1993) 768–772. [5] Diamantaire 53 (1997) 3–4. [6 ] Diamond International 48 (1997) 22. [7] I.M. Reinitz, T.M. Moses, Gems Gemology 33 (2) (1997) 136. [8] J.M. King, T.M. Moses, J.E. Shigley, Y. Liu, Gems Gemology 30 (4) (1994) 220. [9] G.S. Woods, Philos. Mag. B 50 (6) (1984) 673–688. [10] A.T. Collins, Inst. Phys. Conf. Ser. ( UK ), No. 46, 1979, p. 327. [11] C. Welbourn, personal communication. [12] A.T. Collins, J. Gemm. 18 (1) (1982) 37–75. [13] A.T. Collins, J. Gemm. 20 (2) (1986) 75–82. [14] D.W. Palmer, in: G. Davies ( Ed.), Properties and Growth of Diamond, IEE/INSPEC, London, 1994, pp. 143–152. [15] G. Davies, N.B. Manson, in: G. Davies ( Ed.), Properties and Growth of Diamond, IEE/INSPEC, London, 1994, pp. 173–176. [16 ] G. Davies, J. Phys. C: Solid State Phys. 5 (1972) 2534–2542. [17] A.T. Collins, G. Davies, G.S. Woods, J. Phys. C: Solid State Phys. 19 (1986) 3933–3944. [18] M.R. Brozel, T. Evans, R.F. Stephenson, Proc.R. Soc. Lond. Ser. A: 361 (1978) 109–127. [19] A.T. Collins, personal communication.
Observation of the H2 defect in gem-quality type Ia diamond k Peter R. Buerki a, *, Ilene M. Reinitz b, Sam Muhlmeister a, Shane Elen a a Gemological Institute of America (GIA), GIA Research, 5345 Armada Drive, Carlsbad, CA 92008, USA b Gemological Institute of America (GIA), Gem Trade Laboratory, 580 Fifth Avenue, New York, NY 10036, USA Received 27 July 1998; accepted 4 January 1999
Abstract The H2 defect with its zero-phonon line at 986.1 nm (1.257 eV, 10141 cm−1) was first described in 1956 and characterized in detail in 1990–91. In this paper, the observation of strong H2 bands in greenish-yellow natural type Ia gem-quality diamonds is reported. These ‘H2 diamonds’ were apparently subjected to a treatment possibly involving irradiation and clearly involving high temperature annealing. Besides burns on the surface and in fractures, they typically showed strong green luminescence associated with brownish-yellow graining, strong H3/H4 bands, weak to strong H2 bands, and no H1b/H1c lines. Some stones displayed a weak 637 nm line as well. A model explaining the formation of the H2 defect in type Ia diamond is proposed based on the stability and interaction of defects in diamond as a function of temperature and time. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: H2 defect; Vacancy complexes; Annealing; Type Ia diamond; VIS–NIR spectrometry; Fancy color diamonds
1. Introduction The H2 absorption band with its zero-phonon line at 986.1 nm (1.257 eV, 10141 cm−1) was first reported in 1956. It was detected in a natural type Ia diamond that had been irradiated with 1.0 MeV electrons at a flux of 20 to 50 mA/cm2 and subsequently annealed at 900°C for 10 h [1]. A much stronger H2 band was created in 1988 by irradiating type Ib diamond with electrons of higher energy followed by a high temperature anneal. The acceleration voltage was 3 to 4 MeV and the dose 1×1018 to 5×1019 electrons/cm2. The diamonds were annealed at temperatures from 1600 to 1800°C for 5 to 30 h with a stabilizing pressure of 3.0 to 5.0 GPa [2]. Others were annealed in vacuum at 1550 to 1700°C for 20 h [2]. A detailed characterization followed in 1990–91: the results of Lawson et al. [3] and Mita et al. [4] provided evidence that the H2 center is a negatively charged state of the H3 center. To our knowledge, no further work has been published on the occurrence of the H2 band in type Ia diamond since the work of Clark et al. [1]. k
Presented at the Diamond ’98 Conference, Crete, Greece, 13–18 September, 1998. * Corresponding author. Fax: +1-760-603-4021. E-mail address: [email protected] (P.R. Buerki)
GIA Research has been carrying out a systematic ongoing study of the origin of color in fancy-color gem diamonds since 1986, which currently includes data on over 3000 diamonds of various colors. This project supports the work of the GIA Gem Trade Laboratory, which issues reports specifying whether the color of a fancy color diamond arises naturally or is due to laboratory treatment. Prior to 1996, we documented the properties of 27 type Ia diamonds containing H2 centers, ranging in color from greenish-yellow to brownishyellow. 16 of these displayed features indicating laboratory irradiation and annealing, but the other 11 showed characteristics that — to the best of our knowledge — indicated natural origin of color. Thus, prior to 1996, we regarded the occurrence of an H2 absorption band in a type Ia diamond as something of a curiosity with little diagnostic value for determining the origin of a greenish color component. Our opinion changed dramatically that autumn, when we received about 50 greenish-yellow to yellow–green faceted diamonds over a 1 month period that showed moderate to intense H2 bands. Since then, we have seen approximately 100 ‘H2 diamonds’. Other gemological laboratories around the world have also reported H2 diamonds. This prompted both the GIA Gem Trade Laboratory and the Diamond High Council (HRD) in Antwerp to issue warnings to the gem trade about a
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suspected new treatment process [5–7]. This larger group included diamonds with a brownish-orange– yellow color. Intensely colored yellow diamonds with strong green luminescence to visible light (called ‘green transmitters’ by gemologists) are rare among the colors found naturally in gem diamonds. The prices paid for such diamonds typically exceed $10 000 per carat, whereas the market price for a known treated color diamond of similar appearance would be one-fifth to one-tenth as much. As reliable information on the origin of color typically does not follow a colored diamond through the trade, both consumer confidence and the stability of the colored diamond market as a whole depend on the ability of independent gemological laboratories to identify such treated colors properly. We have observed three types of green transmitter relevant to this study: diamonds with an H2 band that appear to be naturally colored; diamonds with an H2 band displaying other properties that indicate laboratory treatment; diamonds without an H2 band that also show these other properties. In the following discussion we shall refer to this last group as ‘non-H2 green transmitters’. The H2 defect appears to occur naturally, in rare cases, when type Ia diamonds are subjected to natural irradiation with simultaneous or subsequent exposure to temperatures above 1000°C. In nature diamonds are generally exposed to much lower radiation doses than they are in the laboratory. Thus natural H2 bands are likely to be weak or below the detection limits of VIS– NIR spectrophotometers. This is supported by our observation of only 11 possibly natural untreated H2 diamonds out of a total of 510 yellow diamonds with green luminescence that were examined in our laboratory during the last 12 years.
2. Experimental Faceted H2 diamonds and non-H2 green transmitters were subjected to non-destructive testing consisting of a variety of visual observations and spectroscopic methods. For the visual inspection, a gemological Mark VII Gemolite Ultima B (GIA GEM Instruments, Carlsbad, CA) microscope with dark field and fiber optic illumination capable of magnifications between 10× and 70× was used. The UV fluorescence was tested in a dark room using an LW/SW UV fluorescent lamp (GIA GEM Instruments) emitting a broad band centered at 356 nm with a sharp superimposed peak at 365 nm for LWUV and 254 nm for SWUV. VIS–NIR spectra at liquidnitrogen temperature between 400 and 1000 nm were obtained with a Hitachi U-4000 spectrophotometer. Slit widths of 0.5 nm and 5.0 nm were used for the VIS range (400 to 850 nm) and for the NIR range (850 to 1000 nm) respectively; a sampling interval of 0.2 nm was
used for both. FTIR spectra in the range 400 to 6000 cm−1 were recorded with a Nicolet Magna-IR spectrometer 550 equipped with a 6× beam condenser from SpectraTech.
3. Properties of H2 diamonds Typical treated-color H2 diamonds show a range of characteristic features including: a saturated yellow (occasionally brownish-orange–yellow) bodycolor, a strong green luminescence to visible light that produces an intense greenish-yellow to yellow–green color under standard color grading conditions [8], and a strong, chalky, greenish-yellow fluorescence to LWUV and SWUV followed by a greenish-yellow afterglow, moderate to very strong initially, then weakening but continuing for at least 30 s after the UV lamp is switched off. The bodycolor of these diamonds is unevenly distributed, showing obvious strong, brownish-yellow graining (i.e. octahedral slip planes) in the microscope. Green luminescence, which was generally restricted to the graining planes, was visible using a high-intensity fiber optic light source. Many of these diamonds displayed burn marks, e.g. pitted and frosted surfaces on facet junctions, surface-reaching fractures, and naturals (i.e. remnants of the original crystal faces). The remnants of facets were present on some of the frosted girdle surfaces. The initial group of H2 diamonds, received in the autumn of 1996, had a poor finish, including misaligned facet junctions due to overcutting and heavily bearded girdles (i.e. with numerous small radial fractures). H2 diamonds received later had a better finish. The absorption spectra in the visible and near-infrared range showed a weak N3 system at 415 nm, a moderate to very strong H3 band at 503 nm, a 637 nm line (in 47% of the cases) and a weak to very strong H2 absorption band with a zero-phonon line at 986 nm ( Fig. 1). A few diamonds had a weak absorption at 535 nm as well. With one exception, all of these diamonds were of type Ia with no detectable Ib component. In most cases, the concentration of B aggregates was higher than that of A aggregates ( Fig. 2). The presence of a strong H3 band, which is due to (N–V–N )0 defects, indicates that the diamonds had been exposed to both ionizing radiation and high temperatures. The absence of other radiation defects, such as the GR1 band at 741 nm, the 595 nm line, and — in most cases — the H1b and H1c lines at 4941 cm−1 and 5171 cm−1, indicates an annealing temperature of at least 1400°C, possibly performed under a stabilizing pressure of ~5 GPa [2,3]. Annealing at this temperature for 4 h causes the disappearance of the H1b and H1c lines [9], whereas the H3 band is hardly affected [10]. Annealing in air would account for the extensive surface burns evident in these diamonds (even when done with boric acid, which is typically used as a protective coating
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Fig. 1. Low temperature visible–near-infrared spectra of two natural type Ia diamonds with well-developed H2 bands; (a) 0.63 ct, treated-color greenish-yellow round brilliant cut diamond #334; (b) 0.56 ct, treated-color greenish-yellow round brilliant cut diamond #320. The step and noise in the range 850 to 900 nm is due to a detector change and noise from the PbS detector. The feature at 670 nm in spectrum (b) is an artifact due to a polarization mismatch between sample and reference beam.
during gem diamond cutting), which can subsequently be removed by repolishing. We can only speculate as to the nature of the starting material that will possess this color and spectral features after treatment, but likely candidates are diamonds whose slightly brownish to brownish-yellow color is considered undesirable in the jewelry market. In agreement with other groups [11], we believe that the H2 center can be created in diamonds by both natural and laboratory irradiation. It is known that diamonds may experience temperatures of up to 1250°C during their formation and transport to the earth’s crust. Although the spectral features related to laboratory annealing can also be produced naturally, albeit at lower temperatures and significantly longer time scales [12,13], we expect naturally colored diamonds with an H2 band to be rare.
4. Formation of the H2 center in type Ia diamond Based on the properties of the H2 diamonds examined, we propose the following reaction mechanism as
the cause for the formation of the H2 center in type Ia diamond. (1) The irradiation of diamond with high-energy electrons of 0.3 MeV or higher creates lattice defects consisting of interstitial–vacancy ( I–V ) pairs [14]. For a uniform creation of lattice defects throughout the diamond lattice an accelerating voltage of at least 2 MeV (typically 3 to 4 MeV ) is required [2]. The neutral single vacancy V0 is the lattice defect producing the wellknown GR optical absorption bands consisting of the GR1 band at 741 nm, and the GR2–GR8 lines in the wavelength region of 430 to 412 nm. The simultaneously formed carbon interstitial is optically inactive [14]. (2) Annealing the diamond after irradiation causes the vacancies to diffuse through the crystal lattice until they are trapped by other defects. The most likely candidates in type I diamond are nitrogen impurities. In type Ib diamonds, V0 vacancies are trapped by single substitutional nitrogen atoms forming (N–V )0 centers with an absorption line at 637 nm [15]. In type Ia diamond, the nitrogen is aggregated. Therefore, the vacancies created by irradiation in type Ia diamond are
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Fig. 2. Corresponding room temperature FTIR spectra for the H2 diamond spectra shown in Fig. 1; (a) 0.63 ct, treated-color greenish-yellow diamond #334; (b) 0.56 ct, treated-color greenish-yellow diamond #320. The spectra were obtained from round brilliant cut diamonds. Wavelengthdependent multiple reflections and scattering within the diamonds cause distortions of the two-phonon bands.
most likely to form (N–V–N )0 defects during annealing by combining with nitrogen pairs (N–N )0, i.e. A aggregates. This defect corresponds to the H3 center with a zero-phonon line at 503 nm and phonon sidebands between 500 and 420 nm [16 ]. If the vacancy is trapped by a B aggregate (group of four N atoms surrounding an empty carbon site), the H4 center is formed with a zero-phonon line at 496 nm and phonon sidebands between 490 and 420 nm [16 ]. Single substitutional nitrogen was detected in just one of the H2 diamonds. Temperatures above 650°C are required for the transformation of GR centers into H3 and H4 centers, destroying the GR absorption bands in the process. At 800°C this transformation is completed in about 12 h [10,13,17]. (3) Annealing an irradiated diamond at 900°C for 10 h causes the transfer of electrons from single substitutional nitrogen atoms to H3 centers to create an additional defect, the H2 center [Eq. (1)]: (N–V–N )0+N0(N–V–N )−+N+ i.e. H3+N0H2+N+
(1)
The absorption band for this defect covers the wavelength range between 992 and 661 nm and represents the phonon side bands of the H2 center. The first report of this absorption band [1] included the phonon sidebands of the H2 center, but did not include the H2 zerophonon line at 986 nm. Subsequent studies on the H2 center were performed using type Ib diamond [2,3]. The authors concluded that the H2 defect corresponds to a negatively charged H3 center (H3−), and that a much stronger H2 band with an intense zero-phonon line can be produced by annealing type Ib diamonds at temperatures above 1400°C [2,3]. To date, only one source for the formation of single substitutional nitrogen in type Ia diamond has been reported: the A aggregate. However, the breaking up of A aggregates into individual nitrogen atoms requires a temperature of at least 1960°C [18]. Two other possible sources for single substitutional nitrogen may be the H3 and H4 centers. Collins [10] postulated that the H3 center can dissociate into a vacancy and an A aggregate according to the reaction in Eq. (2). He also postulated that the H4 center can break up into an H3 center and two nitrogen
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atoms according to the reaction in Eq. (3), and that the H3 centers created in this reaction tend to break up and the nitrogen atoms to rearrange to form B aggregates according to the reaction given in Eq. (4) [10]. (N–V–N )0V0+N–N i.e. H3V0+A
(2)
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which coincides with an absorption line from the B aggregate. Consequently, the two lines cannot be distinguished from each other unless the diamond is a ‘pure’ type IaA. Indeed, one of the three type IaA H2 diamonds we examined showed a weak absorption band at 1332 cm−1, but more samples are needed for a definitive conclusion.
(N–N–V–N–N )0(N–V–N )0+2N0 i.e. H4H3+2N0
(3) 5. Stability of the H2 center in type Ia diamond
(N–V–N )0+2N0V0+N–N–N–N i.e. H3+2N0V0+B
(4)
In addition to the processes suggested by Collins, we propose that H3 centers — including those formed by the dissociation of H4 centers — break up into (N–V )0 centers and single nitrogen atoms according to the reaction (N–V–N )0(N–V )0+N0
i.e. H3(N–V )0+N0 (5)
Collins’ model does not explain our observation of a 637 nm line (i.e. the (N–V )0 center that is typical for diamonds containing single substitutional nitrogen) in 47% of the H2 diamonds that we examined. His hypothesis is based on the examination of one type IaA and one type IaB diamond. He did not observe the development of new absorption bands after annealing the H3 center of the IaA diamond. During our own study, we recorded visible spectra for five type IaA diamonds and none of these had a 637 nm line. No spectra in the visible range were recorded for the three other type IaA diamonds that came through our laboratory. Our results seem to support the Collins model. Nevertheless, the examination of just six type IaA diamonds is insufficient to eliminate the reaction in Eq. (5) as a possibility. The intensity of the 637 nm line may be below our current detection limits, as it may well have been for Collins 20 years ago. We are currently upgrading our equipment with new detectors in order to increase our sensitivity to the H2 and 637 nm lines. Collins observed the creation of new absorption lines, including the 637 nm line, after annealing the H4 center in the IaB diamond [10] (we examined three ‘pure’ type IaB diamonds in this study, none of which showed either an H2 band or a 637 nm line). Collins does not suggest any mechanism that may cause the formation of (N–V )0 centers during this process. The most plausible explanation is, again, the destruction of the H3 centers according to the reaction in Eq. (5). The combination of reactions in Eqs. (1), (4), and (5) may explain the formation of H2 centers in type Ia diamond. The simultaneously formed positively charged single substitutional nitrogen atom N+ has only one absorption peak at the Raman energy of 1332 cm−1,
The existence of non-H2 green transmitters, which have apparently been treated in a similar manner to that of the H2 diamonds, raises questions about the stability and the detectability of the H2 center in type Ia diamonds and whether there is a method for removing it. Perhaps, the H2 center can be destroyed by annealing at much higher temperatures (~2200°C ?) under a stabilizing pressure [19]; however, under such extreme conditions, the H3 and H4 centers would anneal out within minutes and a considerable fraction of the A and B centers broken up into measurable amounts of single substitutional nitrogen, which contradicts our findings. Mita et al. [4] reported that irradiating type Ib H2 diamonds with 514.5 nm Ar+ laser light causes the temporary ionization of the H2 centers and thus their transformation into H3 centers. They concluded that the N+ center is one of the dominant traps for the released electrons. This raises the question of what happens if H2 diamonds are photoionized at elevated temperatures. None of the 90 non-H2 green transmitters, for which we have reliable and complete data, had a 637 nm line. We can think of only two plausible explanations for this observation: the H2 and 637 nm bands are present, but at a level below the detection limits of our current instrumentation, or there exists a method for permanently reversing the formation of the H2 center. Such a method is not known to us; however, it would include the neutralization of single substitutional nitrogen atoms by the H2 centers followed by the recombination of the neutralized nitrogen atoms with the (N–V )0 centers. As a result, two additional H3 centers would be formed. This process may be summarized as shown in the reaction (N–V–N )−+N++(N–V )02(N–V–N )0 i.e. H2+N++(N–V )02H3
(6)
A quantitative determination of the relative strengths of the absorption bands of interest may provide us with important information about the irradiation and annealing history of H2 diamonds. However, a quantitative determination of absorption coefficients in faceted diamonds is impossible owing to multiple reflections, diver-
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gent paths that are wavelength dependent, and scattering of the sample beam.
6. Conclusions The H2 absorption band at 986 nm was observed in a number of type Ia gem-quality diamonds. It probably is the result of a treatment process developed for changing off-color diamonds into intense greenish-yellowcolored diamonds. The suspected treatment process is believed to involve irradiation with a large dose of high energy electrons (3 to 4 MeV ) followed by high temperature annealing in air at temperatures around 1400°C. A model that explains the formation of the H2 defect in type Ia diamond was proposed. The limited data currently available and the many unanswered questions attest to the strong practical need for further research on the formation, stability and interaction of defects in type Ia diamond, and the H2 defect in particular. We are currently upgrading our equipment to improve the detection limits for the H2 and related bands as well as acquiring a set of suitable H2 diamonds in an attempt to duplicate the treatment process and to test our hypotheses.
References [1] C.D. Clark, R.W. Ditchburn, H.B. Dyer, Proc. R. Soc. London Ser. A: 237 (1956) 75–89. [2] S. Satoh, European Patent no. EP 0 324 179, 1992 (in the past, this patent was repeatedly quoted under its EP application number 88 121 859.8, application year 1988). [3] S.C. Lawson, G. Davies, A.T. Collins, A. Mainwood, J. Phys. Condens. Matter 4 (1992) 3439–3452. [4] Y. Mita, Y. Ohno, Y. Adachi, H. Kanehara, Y. Nisida, T. Nakashima, Diamond Relat. Mater. 2 (1993) 768–772. [5] Diamantaire 53 (1997) 3–4. [6 ] Diamond International 48 (1997) 22. [7] I.M. Reinitz, T.M. Moses, Gems Gemology 33 (2) (1997) 136. [8] J.M. King, T.M. Moses, J.E. Shigley, Y. Liu, Gems Gemology 30 (4) (1994) 220. [9] G.S. Woods, Philos. Mag. B 50 (6) (1984) 673–688. [10] A.T. Collins, Inst. Phys. Conf. Ser. ( UK ), No. 46, 1979, p. 327. [11] C. Welbourn, personal communication. [12] A.T. Collins, J. Gemm. 18 (1) (1982) 37–75. [13] A.T. Collins, J. Gemm. 20 (2) (1986) 75–82. [14] D.W. Palmer, in: G. Davies ( Ed.), Properties and Growth of Diamond, IEE/INSPEC, London, 1994, pp. 143–152. [15] G. Davies, N.B. Manson, in: G. Davies ( Ed.), Properties and Growth of Diamond, IEE/INSPEC, London, 1994, pp. 173–176. [16 ] G. Davies, J. Phys. C: Solid State Phys. 5 (1972) 2534–2542. [17] A.T. Collins, G. Davies, G.S. Woods, J. Phys. C: Solid State Phys. 19 (1986) 3933–3944. [18] M.R. Brozel, T. Evans, R.F. Stephenson, Proc.R. Soc. Lond. Ser. A: 361 (1978) 109–127. [19] A.T. Collins, personal communication.