Is dispersed nickel in natural diamonds associated with cuboid growth sectors in diamonds that exhibit a history of mixed-habit growth?

ARTICLE IN PRESS

Journal of Crystal Growth 263 (2004) 575–589

Is dispersed nickel in natural diamonds associated with cuboid growth sectors in diamonds that exhibit a history of mixed-habit growth? A.R. Langa,*, A.P. Yelisseyevb, N.P. Pokhilenkob, J.W. Steedsa, A. Wotherspoona b

a H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK Institute of Mineralogy and Petrography SB RAS, 3 Koptyug Avenue, Novosibirsk 630090, Russia

Received 22 August 2003; accepted 28 November 2003 Communicated by D.T.J. Hurle

Abstract Nickel atomically dispersed in the form of Ni–N-vacancy complexes is detected in selected natural diamonds and its location within them determined. The complexes belong to the ‘NE’ family of EPR-active centres studied in hightemperature annealed HPHT-grown diamonds synthesised in presence of Ni and N. Some NE centres exhibit characteristic photoluminescence (PL) spectra and by these emissions their presence is identified in natural specimens that are found to possess zones of mixed-habit growth where normal {1 1 1}-faceted growth has been accompanied by non-faceted ‘cuboid’ growth on B{1 0 0} surfaces. Some prior evidence that the Ni-containing centres reside in cuboid growth sectors is strongly confirmed by PL probing with confocal microspectrography using excitation by 325, 488 and 514.5 nm laser sources. Other spatially resolving techniques applied included FTIR absorption microscopy, X-ray and cathodoluminescence topography, and PL emission pattern microphotography. Growth structures, lattice defects, nickel occurrence, and nitrogen impurity concentration and aggregation state within polished near-central sections of two contrasting specimens are analysed, one specimen originating from the Snap Lake kimberlite intrusion, NW Territories, Canada, the other from an alluvial source in Sierra Leone. r 2003 Elsevier B.V. All rights reserved. PACS: 61.72; 61.72.H; 61.72.S; 81.10.A; 78.55 Keywords: A1. Crystal morphology; A1. Defects; A1. Impurities; A2. Natural crystal growth; B1. Diamond

1. Introduction This paper brings together two topics concerned with the growth and perfection of diamond single *Corresponding author. Tel.: +44-117-928-8954; fax: +44117-925-5624. E-mail address: [email protected] (A.R. Lang).

crystals. One is the occasional finding within natural diamonds of certain atomic-scale defects involving nickel and nitrogen. These have been extensively studied in synthetic diamonds grown by high-pressure, high-temperature (HPHT) methods and subsequently annealed at high temperature. The second is the detection of epochs of mixed-habit growth that have occurred in some

0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.116

ARTICLE IN PRESS 576

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

natural diamonds: during these epochs normal faceted octahedral growth has been accompanied by growth on curved surfaces in mean cube orientation, the latter growth being termed ‘cuboid’. The link between these two topics comes from looking into whether natural diamonds whose lattices harbour the Ni-containing defects referred to above exhibit particular identifiable characteristics in their morphological evolution during growth, lattice defect content, etc. Some previous reports pointed towards cuboid growth sectors as being the crystal volumes concerned. The experiments here reported firmly support the answer ‘yes’ to the question posed in this paper’s title. Key evidence was produced by point-bypoint photoluminescence (PL) probing by confocal microspectrography. Other crystal assessment methods applied were optical microscopy, FTIR absorption microscopy, X-ray and cathodoluminescence topography, and the microphotography of patterns of PL emission. Two very different natural diamonds have been subjected to detailed examination. Specimen SL-00/47 had been extracted from a recently discovered intrusive source in Canada. Specimen GDO2/1 came from an alluvial deposit in Sierra Leone.

2. Nickel–nitrogen complexes in diamond Because of the small interatomic spacing and strong bonding in diamond, few foreign elements can be incorporated atomically into its crystal lattice. Of these, nitrogen is the most common impurity element in both natural and synthetic diamonds. Of heavier elements, nickel is outstanding in its ability to enter the diamond structure and occupy sites of several types. In diamond, different impurity sites can be recognised, and sometimes positively identified crystallographically, on evidence from electron paramagnetic resonance (EPR) and the properties of optical vibronic bands and their characteristic zero phonon line (ZPL) energies [1,2]. The many ZPLs of interest range from IR to UV wavelengths and may be observed by absorption, PL or cathodoluminescence (CL) spectroscopy, as appropriate. Whereas the present work is concerned

with nickel–nitrogen complexes in natural diamonds, all detailed knowledge of these complexes derives from studies of large synthetic diamonds grown at HPHT [3,4]. Their chief impurity is N, atomically dispersed, singly substituting for C atoms. In this state the N produces a characteristic EPR signal and infrared absorption spectrum, together with absorption of visible wavelengths below 500 nm. In pioneering CL spectroscopic studies of natural and synthetic diamonds Dean [5] discovered a near-IR, 1.40 eV (884 nm) emission from synthetic diamond grit, which he attributed to N impurity. Further work made clear that the 1.40 eV centre, together with some other optical centres, arose from incorporation of Ni, not N [6– 9]. Many optical and EPR studies have been performed on N and Ni-containing synthetic diamonds grown at higher temperatures (B1800 K) and, in some experiments, annealed at temperatures up to 2500 K while kept stable by the necessary containing pressure. Whether or not Ni is present, holding above B2000 K reduces the concentration of N in singly substitutional, EPRactive form, causing it to aggregate into non-EPRactive states, first into A defects (nearest-neighbour substitutional N pairs) and, at highest T, additionally into B defects (believed to be four substitutional N surrounding a vacancy). In the great majority of natural diamonds N impurity exists principally in the form of A and B defects, see Clark et al. [10] on the optical properties of N in diamond, and Evans [11] concerning N aggregation in diamond. When synthetic diamonds containing Ni are subjected to HPHT annealing treatments the EPR and optical properties indicating presence of isolated Ni ions progressively weaken. Concurrently, new EPR centres appear, and those specimens in which the EPR centres named NE1, NE2 and NE3 have developed also exhibit bright yellow-green PL under near-UV excitation. In their PL spectra certain vibronic systems already well known from studies of natural diamonds can be recognised [12]. Further EPR investigations [13–17] expanded the NE family of defects, adding NE5–NE8 to NE1–NE3, all being Ni–N complexes derived from a basic structure, NE4, shown in Fig. 1. The

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

577

Table 1 Visible-wavelength vibronic systems corresponding to EPR centres NE1–NE3 EPR centre

NE1 NE2 NE3 Fig. 1. The NE4 centre emplaced in the diamond structure shown in projection on a ð1 1% 0Þ plane containing a chain of carbon atoms (open circles), 2 carbon vacancies (open squares) and the Ni ion (solid circle) midway between the carbon vacancy pffiffiffi sites. Double circles represent carbon atoms distant a0 =2 2 above and below the plane of section. The 6 carbon nearest neighbours of the Ni are numbered 1–6, and for simplicity are drawn undisplaced from their normal positions.

essential common feature in the NE family is a Ni ion at the centre of a carbon divacancy, where it occupies a slightly distorted octahedral site. Its 6 ligand carbons are numbered in Fig. 1. The 2 vacancies facilitate accommodation of the Ni ion without major distortion of the diamond matrix. This relaxed structure can be produced by a substitutional Ni shifting an adjacent carbon into an interstitial site where it will be mobile at the temperatures concerned. When the annealing temperature of a synthetic diamond containing both substitutional N impurity and NE4 centres is raised sufficiently to render the former mobile, aggregation of N into A centres will be accompanied by its condensation on NE4 centres, progressively substituting for the ligand carbons. Centres having n carbons replaced by N (which may include isomeric forms) each show a characteristic symmetry in EPR properties. Structures in accord with the EPR behaviour have been derived, assisted by analysis of hyperfine structure [15]. In the present work, we are concerned principally with NE1, NE2 and NE3, those centres optically active at visible wavelengths. (NE8, also optically active, emits in the near IR.) The NE1 structure has n ¼ 2; NE2 and NE3 are isomers having n ¼ 3: The optical analogues of centres NE1–NE3 have been identified by comparison of EPR and optical properties in specimens containing NE1, NE2 and

Optical system

S3 S2 523

Principal ZPL (eV)

(nm)

2.496 2.536 2.369

496.7 488.9 523.2

NE3 in different proportions, and by use of PLexcitation spectroscopy (PLE) and decay-time measurement to separate overlapping vibronic systems. Table 1 lists the principal correspondences adopted here for interpreting the PL spectra shown below. In the experiments reported in Refs. [13–17], centres NE1–NE3 were detected in over 100 Yakutian diamonds. In EPR studies of diamonds from the Argyle mine, W. Australia, [18] the NE2 centre was found in greyish-blue specimens, and optical work on similar Argyle material [19] found specimens producing all the PL emissions listed in Table 1. None of the EPR and optical observations on natural diamonds included in studies [13–19] indicated any relationship between content of Ni–N complexes and growth morphology. Some earlier optical observations, however, did so. See below.

3. Mixed-habit growth in natural diamonds It was first point out by Frank [20] that certain remarkable etch patterns produced on polished sections of natural diamonds could be explained as constituting a record of epochs of mixed-habit growth during which normal growth on flat octahedral facets had been accompanied by nonfaceted growth on curved surfaces termed ‘cuboid’ [21] of which only the mean orientation could be specified by simple indices, as {1 0 0}, and which locally could be inclined up to about 30 from cube orientation. X-ray topographic studies [21– 25] have highlighted the great variety of manifestations of mixed-habit growth in natural diamonds, illustrating the wide range of relative

ARTICLE IN PRESS 578

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

development of cuboid and {1 1 1} forms both within and between specimens, and also of lattice perfection in cuboid compared with {1 1 1}-growth material as evidenced by the intensity of X-rays diffracted at various wavelengths [26]. The dense populations of microscopic non-diamond bodies found in cuboid but not in {1 1 1} growth, and generally restricted to inner zones, vary greatly in particle size. Optically, the smaller particles may be detectable only by the Tyndall light-scattering they produce en masse, or by ultra-microscopic imaging individually [21], whereas occasionally individual crystallites in the population may be large enough for their birefringence properties to be analysed and their preferred orientation relative to the diamond matrix demonstrated [27]. In the IR absorption spectrum of natural diamonds narrow peaks at 3107 and 1405 cm1, now convincingly assigned respectively to C–H stretching and bending modes [28], are usually detectable. They are always very strong in the absorption spectra of cuboid growth. But they cannot serve as a unique fingerprint of such growth because they are also strong in the absorption spectrum of diamond coat [29]. However, diamond coat and cuboid growth are distinguishable by their different dislocation densities. The ‘fibrous’ structure of diamond coat is highly dislocation-rich [30,23], whereas mm3-scale volumes of cuboid growth can be dislocation-free. Precursors of the present investigation were certain studies of natural diamonds in which mixed-habit growth was recognised, and in some cases mapped, and that also recorded cuboidsector PL spectra containing one or more of the ZPLs listed in Table 1. Plotnikova et al. [31] exhibited a strain birefringence pattern and visible PL pattern characteristic of mixed-habit growth, and reported strong 496.7 nm (S3) and 523.3 nm ZPL emissions from cuboid sectors under Hg 365 nm excitation at 77 K. Under similar excitation conditions Welbourn et al. [32] demonstrated S3 ZPL emission from cuboid growth in the course of morphological and X-ray topographic studies of remarkable mixed-habit diamonds from the Jwaneng mine, Botswana. Other experiments [33], combining microtopographic light-scattering and PL techniques, found S3 centres similarly located.

4. Observations on SL-00/47 (Snap Lake, NW Territories, Canada) 4.1. Growth history and lattice perfection Snap Lake lies on the southeastern part of the Slave Craton. It is situated 210 km NE of Yellowknife and 90 km S of Lac de Gras, the latter an area rich in diamondiferous kimberlite discoveries. The Snap Lake intrusion is of unusual form: it is neither pipe nor vertical dyke, but meets the surface as a massive sheet dipping 16–20 from the horizontal. Discovered in the late 1980s, it is now established as the largest primary diamond deposit on the American Continent. Mineralogical analyses of suites of inclusions in Snap Lake diamonds [34] indicate diamond formation pressures in a range with upper limit not less than 11 GPa, a considerably higher maximum than that found for diamonds from kimberlites in other cratons studied. Whereas in the latter case the pressure (and depth) range runs from, say, B3.7 to B6.5 GPa (B140 to B190 km), for Snap Lake the maximum depth is at least 300 km. Specimen SL00/47 was examined in the form of a near-central polished slice, close to (0 0 1) orientation, whose roughly rectangular shape showed that it had been cut from an octahedron only slightly rounded. Interest in this crystal stemmed from viewing a CL topograph of the polished surface here indexed (0 0 1) taken in the laboratory of Dr. Judith Milledge, Department of Geological Sciences, University College London, to whom the authors are greatly indebted. Dominating the CL image was an irregularly bounded core area of intense but apparently featureless luminescence. Coming from underneath this bright area was light scattered by a cluster of small inclusions and cracks contained within the small central core volume that is seen silhouetted in Fig. 2. Also silhouetted is a large crack issuing from this inner core, widening to intersect both upper and lower surfaces of the plate, but not visibly extending to the plate periphery. (This crack termination can be explained by compressional stress in the peripheral zones due to higher N impurity content found there.) CL, X-ray and birefringence topographs showed that outside the bright CL core normal

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

579

Fig. 2. Transmitted light micrograph of SL-00/47 central section viewed through its cube-plane-orientation polished surfaces. Overall plate dimensions 3.25 mm  2.94 mm, thickness 0.5 mm. Scale mark 1 mm. Orientation convention: [0 0 1] points towards observer, [1 1 0] upwards, [1 1% 0] rightwards. Nearly straight segments of the plate periphery are traces of slightly rounded {1 1 1} facets of the uncut crystal. Seen silhouetted is an innermost zone of small cracks and inclusions, diameter B0.25 mm, plus a single major crack extending to the right therefrom. The black line superimposed on the photograph traces the boundary of the strongly CL-emitting core along its outcrop on the nearer polished surface, that with orientation indexed [0 0 1].

{1 1 1}-faceted growth was dominant, and had proceeded apparently free from nucleation of additional dislocations to augment the population of dislocations grown-in from the core. The birefringence topographs, Figs. 3a and b, show the intense strain birefringence locally generated within the inner core of cracks and inclusions, with resulting long-range strains extending throughout the whole plate. Strain gradients caused by lattice parameter variations between {1 1 1} growth layers in the outer zones are picked out with the polars orientation in Fig. 3a and minimised in Fig. 3b. These variations correlate well with variations in N impurity revealed by CL topography and FTIR absorption microscopy. In the zones of octahedral growth enclosing the luminous core the birefringence images of bundles of dislocations fanning outwards are seen especially well in Fig. 3b, but

Fig. 3. Strain birefringence. Axes of crossed polars: (a) [1 0 0] and [0 1 0]; (b) [1 1 0] and [1 1% 0].

the strain distribution in the core regions precludes determining in what proportions these dislocations were generated at the inclusions, or within the bright CL volume, or by imperfect epitaxy on its bounding surface. The inner core of small cracks and inclusions extends from B75 to B325 mm in actual depth below the (0 0 1)-polished surface. Most of the included matter is densely opaque and is of 2 types: curved sheets of various dimensions or bodies of rectangular outline with edges parallel to

ARTICLE IN PRESS 580

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

Fig. 4. Opaque inclusions and loop features within inner core 250 mm below (0 0 1)-polished surface. Transmitted light, field width 100 mm. Overall diameter of dark loop near field centre is 11 mm.

diamond matrix [1 1 0] and [1 1% 0], edge lengths B2 to >25 mm. Less obvious are dark loops, the largest observed is near the field centre in Fig. 4 and appears to lie in (0 0 1). The curved opaque sheets are probably mainly graphite-filled cracks; many appear to spread out from the straight-edged opaque material. Inclusion edge orientations parallel to diamond /1 1 0S suggest garnet or olivine. Olivine inclusions rendered opaque by complete coating with graphite, and with pyrrhotite/pentlandite, have been found in diamonds [35]. All the inner core volume is too remote from the polished specimen surfaces to permit probing nondestructively. Scrutiny of the larger loops under various optical conditions showed that they surrounded transparent inclusions. Some darkness of the loop image is accountable by matrix/ inclusion refractive index difference, but in the case of larger loops such as that prominent in Fig. 4 it does appear to be a genuine observation that the inclusion is ringed by opaque matter. 4.2. Cathodoluminescence and photoluminescence imaging A major discontinuity in growth history is marked by the irregular boundary of the strongly

luminescent core. The CL images of the enclosing zones not only provide stratigraphic detail of the normal {1 1 1}-faceted growth therein, but also reveal that except for the specimen’s outermost B0.6 mm-thick octahedral shell this growth had an abnormally low N content generally. The CL indicators of this condition are extremely low intensity of the spectrally continuous blue ‘Band A’ emission [5] coupled with the appearance of individually resolved, yellow-emitting ‘giant’ {1 0 0} platelets [36,37]. Measurements of image lengths of platelets on (1 0 0) and (0 1 0) cut by the (0 0 1)-polished surface of SL-00/47, made first by Dr. Milledge and confirmed in the present study, show platelet diameters ranging up to 150 mm, several times greater than the largest illustrated in Ref. [37]. Hence, the outer zones of SL-00/47 are of scientific interest in their own right. However, the present observations had the particular aim of looking for clues to the growth stratigraphy within the strongly luminescent core. Following previous Bristol practice [21,36], CL topographs were recorded by microphotography of the surface area of interest placed within the specimen chamber of an SEM and flooded with a broad, stationary electron beam. Kilovoltage, area irradiated and specimen current density all being independently controllable helped towards distinguishing between different CL-emitting defects. For PL topography the specimen surface was displaced from focus in the confocal microscope so that the illuminating laser beam spread to cover the area of interest, which was then studied with an independently mounted photomicrographic setup. Figs. 5a and b are, respectively, the visible luminescence images of the core recorded by CL excited by 30 keV electrons and PL excited by a 325 nm UV laser source. When comparing the CL and PL topographs, the following differences in their formation must be borne in mind. In the CL image, maximum light emission generated by 30 keV electrons comes from B5 mm depth. Hence the CL image provides a valid record of the structure within the luminescent core outcrop on the surface, and is sharply bounded. In the PL case, recall that the core is overgrown by nearly Nfree diamond, which only weakly absorbs 325 nm

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

581

Fig. 5. The strongly luminescent core of SL-00/47. (a) CL topograph, 30 keV electron energy; (b) PL topograph, 325 nm excitation; (c) key to core structure revealed by (a) and (b). In (c) outcropping areas of cuboid and faceted growth are distinguished as follows. Sectors of cuboid growth in the four 7[1 0 0] and 7[0 1 0] quadrants are shown by curved arcs sketching the curved growth layers identified in (a) and (b); octahedral faceted sectors are shown solid black. The interrupted line encloses the area within which light scattered from sub-surface cracks and inclusions masks diamond matrix luminescence. Particular points within the [0 1 0], [1 0 0] and [0 1% 0] cuboid sectors from which PL spectra were recorded are marked by crosses.

UV (measured absorption coefficient in this region roughly 3 cm1). Hence core elements at depth below the surface contribute to the PL image, which is consequently diffusely bounded. The CL topographs succeeded in revealing structure within the luminescent core, albeit with only weak contrast in brightness or colour. Discernible within the 7[1 0 0] and 7[0 1 0] quadrants were the tell-tale curvilinear growth horizons that unambiguously identified cuboid growth therein. Small areas of octahedral growth were revealed by slight excess CL intensity, notably

from a narrow column, direction [1 1 0] upward from the pattern centre, variable in width, and strongly reminiscent of the minor, variable, but generally persistent developments of octahedral growth sectors that are found in some mixedhabit, dominantly cuboid diamonds [22,32]. Other similarly revealed outcrops of {1 1 1} sectors detected in Fig. 5a are picked out in Fig. 5c. It is likely that the excess CL from octahedral growth sectors seen in Fig. 5a arises from the greater Nimpurity concentration and more advanced state of aggregation found in {1 1 1} compared with

ARTICLE IN PRESS 582

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

cuboid sectors. This difference would cause {1 1 1} sectors to produce a relatively stronger emission of the N3 vibronic system, ZPL 415 nm, which is excited both in CL and PL [1,2], but this explanation has not yet been experimentally verified by point-by-point CL spectroscopic probing in the case of SL-00/47. On the other hand the yellow-green luminescence systems observed from cuboid growth, including those in Table 1, are much more efficiently excited by UV than by electron irradiation. Consequently, one would expect a UV-excited PL topograph of SL-00/47 to be a better discriminator between {1 1 1} and cuboid growth than a CL topograph. This expected behaviour is well exhibited in monochrome in Fig. 5b, where it can be directly compared with Figs. 5a and c; and colour (Fig. 6) is additionally informative. In the PL images the narrow vertical band of luminescence deficiency between [1 0 0] and [0 1 0]-directed cuboid sectors is striking: it results from the sub-surface sheetwise extension of the (1 1 1)-faceted sector that forms the narrow, [1 1 0]-pointing outcrop strip on Fig. 5a, and it may also include at depth a sheet of growth on (1 1 1% ). The colour reproduction Fig. 6

Fig. 6. Colour micrograph of core PL, 325 nm excitation, recorded on Kodak Gold 200.

shows clearly that it is the yellow-green PL that is here deficient, not the blue of the N3 system. 4.3. Infrared absorption Two methods were used to obtain infrared absorption spectra, serving different purposes. The first provided an overall measure of the strikingly different absorption properties of the luminescent core compared with its enclosing shells of normal {1 1 1}-faceted growth. This was achieved by appropriately positioning on the (0 0 1)-polished surface a thin metal mask with a central 0.6 mm diameter aperture and recording transmission through the aperture using a Nicolet 510P FTIR spectrometer with condenser accessory. In the first mask position the aperture fell wholly within the core outcrop boundary. In the second position the aperture was displaced 1 mm in the [1% 1 0] direction, i.e. leftwards in Figs. 2 and 3. It then covered zones of lowest N impurity concentration, as judged by CL. The spectra obtained by this procedure will be referred to as ‘spot’ measurements. To obtain more spatial detail, the second method used a Bruker IFS 113 FTIR spectrometer plus optical microscope combination, enabling spectra to be recorded step-wise along selected trajectories on the (0 0 1) face. The specimen translation steps were 150 mm, the sampling beam diameter 100 mm, and 2 trajectories were taken. Trajectory 1 scanned in the [1% 1 0] direction, running below the major crack as viewed in Fig. 2; trajectory 2 was diagonal, roughly [1% 0 0], passing over the centre of the luminescent core. Spectra were recorded with 2 cm1 resolution by both spectrometers. To find the concentrations of N aggregated into A and B defects, NA and NB, respectively, the observed spectra were separated into their component A- and B-defect spectrum contributions. Then the currently accepted conversion rates from these absorptions to NA [38] and NB [39] were applied. For the core, average values given by the spot measurement were, in round figures, 15 atomic ppm NA and 600 atomic ppm NB. This NA/NB ratio is very low; NB is high but not exceptionally so. The spot measurement in the {1 1 1}-faceted area gave NAB25 atomic ppm,

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

NBB40 atomic ppm. The step-wise measurements showed that the outermost zones of {1 1 1}-faceted growth contain growth layers with up to 5–10 times the N content recorded by the spot average (a finding not unexpected from birefringence, CL and PL images) but with no great change in A/B ratio. The strong B-defect presence in the core is accompanied by a strong B0 , ‘platelet’ absorption line, for which the spot measurement recorded the following values: peak height 21 cm1, wavenumber of peak 1365 cm1 and (very roughly) integrated area B300 cm2. The ratio of this area to the B-defect absorption strength does not fall significantly below that expected for the most common diamonds, those called ‘regular’ by Woods [40]. Outside the core the platelet peak strength is variable, and generally weak except in the outermost zones where both A and B defect concentrations rise significantly. The identification of cuboid growth in the core was strongly supported by the strong presence of the sharp 3107 cm1 absorption line [28] referred to previously in Section 3. The resolution of 2 cm1 used in the present spectrometry is insufficient to give an accurate peak height value for this very sharp line but is adequate for describing its inter- and intra-crystal variations. The maximum peak height recorded along the trajectory crossing the core centre was 14 cm1, strikingly higher than the spot measurement outside the core, B0.5 cm1. Indeed, along trajectory segments outside the core crossing areas of least N content, the 3107 cm1 line was below detection limit above background, say 0.1 cm1 absorption coefficient. The peak height in the core, 14 cm1, is quite comparable with heights found in previous measurements (unpublished) on cuboid growth sectors of mixed-habit diamonds. 4.4. Photoluminescence spectroscopy Luminescence spectra excited by laser sources of 325, 488 and 514.5 nm radiations were recorded using Renishaw confocal micro-Raman ‘Ramascope’ spectrometers fitted with Oxford Instruments ‘Microstat’ specimen chambers allowing microscopic PL examination down to B6 K. Two procedures were employed. First was a step-

583

wise traverse across the specimen in the [1% 1 0] direction, trajectory 1 as used in IR absorption spectroscopy, passing over the lower cuboid sectors of the cuboid core as viewed in Figs. 2 and 5. Spectra were measured at 22 points separated by 150 mm. The second procedure, used only with the 325 nm UV and 488 nm sources, selected 4 positions on the specimen surface from which to take ‘spot’ measurements that could be easily related to the corresponding points on the CL and PL topographs, Figs. 5a,b and 6, photographed at room temperature. For positioning the laser probes accurately and reproducibly very helpful landmarks were a few easily recognisable small inclusions in the [0 1 0] cuboid sector near the core outcrop boundary and sufficiently close to the polished surface to be clearly seen under normal, wide-field epi-illumination when the surface lay in the confocal plane. Positions 1 and 2 (Fig. 5c) lie in the [0 1 0] and [1 0 0] cuboid sectors, respectively, 100 mm on either side of the narrow outcrop of (1 1 1)-faceted growth; position 3 is in the [0 1% 0] cuboid sector, 0.5 mm from 2. Position 4 is outside the field of Fig. 5c, 0.62 mm leftward from 1, in a region of (1% 1 1) growth and weak (but not the least) blue CL and PL. In measurements with the 488 nm source, the probe diameter on the surface was 3 mm and the depth resolution below the diamond surface roughly 10 mm. Using the 325 nm UV source the probe diameter on the surface was B10 mm and the depth resolution probably a few times larger than this figure. The principal findings concerning PL excited by the 325 nm source were as follows. The traverse along trajectory 1 demonstrated that the vibronic systems with ZPLs at 488.9 and 523.2 nm, which correlate with the NE2 and NE3 centres respectively, were emitted strongly from the luminescent core but not from the enclosing zones of {1 1 1} growth. They did, however, dip in strength when crossing the wedge of (1% 1% 1)-faceted growth lying between the [1% 0 0] and [0 1% 0] cuboid sectors that was recognised in the PL images Figs. 5b and 6. On the other hand, the N3 vibronic system appeared both within and outside the luminescent core, being relatively strong in the outermost {1 1 1} zones that contained several hundred at ppm NB. (Proportionality between N3, B-defect

ARTICLE IN PRESS 584

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

and B0 integrated absorptions was found by Woods [40] in ‘regular’ diamonds.) Within the luminescent core the N3 ZPL peak width was significantly broader than outside it: at 6 K, B0.6 cm1 compared with B0.2 cm1. Positions 1–3 were deemed best placed for revealing any important differences in the low-temperature PL spectra accompanying the evident differences in room temperature PL emitted by different parts of the luminescent core shown by Fig. 6. In fact, no differences of major significance concerning presence of nickel-related centres were discovered; and the spectrum from position 1, Fig. 7a, can be taken as representative of cuboid-sector diamond. In the wavelength range from the ZPL of the N3 system (415.3 nm) to about 475 nm the characteristic phonon side bands of this system dominate. The very broad band from B500 to B600 nm is the typical yellow-green PL excited by UV from diamonds containing NE centres both in HTannealed synthetics [12,16] and cuboid sectors of natural diamonds [32,33]. Diagnostically significant in the present context are the strongly seen sharp ZPLs of the NE2 and NE3 centres. The broad peak at 496.7 nm is the first phonon replica of the NE2 ZPL; it may also contain a weak ZPL of the NE1 centre (see Table 1.). Next in prominence in the longer-wavelength region is the ZPL at B604 nm. This is known to accompany the 488.9 nm line in PL from cuboid sectors in natural diamonds [2,31]. The structure of the 604 nm centre is not known. The spectrum from position 3 closely resembled that from 1; at 2 the N3 emission dominated over the longer-wavelength constituents. At 4 the only recognisable feature was the N3 system with sharp ZPL but with integrated intensity less than 1/20 of that at position 1. The PL spectra excited by the 488 nm source recorded from points 1, 2 and 3 were quite similar. Their dominating feature is a complex group of lines ranging from B650 to B750 nm (Fig. 7b). A generally similar group appears in the 488 nmexcited spectrum when Ni-containing synthetics are annealed at very high temperatures, B2300 K, e.g. curve 3 of Fig. 1 in Ref. [17]. Of the few weak lines seen in Fig. 7b at lower and higher wavelengths than the dominating group, viz. lines

at B604, B640, B787 and B794 nm, the first also appears in Fig. 7a and the last is the 793.6 nm ZPL of the NE8 centre, which contains four N [15]. The spectrum recorded from position 4 was featureless except for a very weak line at 741 nm, the GR1 line due to radiation damage. In the spectra recorded along trajectory 1, both those excited by 488 and 514 nm, some well-known luminescent systems produced by radiation damage and subsequent heating, e.g. systems H3, H4 and GR1, were observed from points close to the specimen plate edge and from a few points within the core, the latter presumably sited close to a-emitting inclusions.

5. Observations on GDO2/1 (Sierra Leone) 5.1. Growth history, lattice perfection and infrared absorption Specimen GDO2/1 differed strongly from specimen SL-00/47. As usually the case with diamonds found in alluvial deposits, the location and characteristics of the intrusive body from which this crystal originated are unknown. As also common with alluvial diamonds, it had suffered substantial dissolution, which had progressed to the stage where an outer zone of {1 1 1} growth, undoubtedly originally enveloping the mixed-habit core, was retained only in local remnant form. The initial non-destructive examination using X-ray section topographs [22] showed that this crystal belonged to the class of mixed habit diamonds in which the volumes of {1 1 1}-faceted growth are clearly distinguishable from cuboid growth by their stronger integrated X-reflecting power, attributable to greater number and diameter of {1 0 0} platelets within them compared with adjacent volumes of cuboid growth. This crystal possessed unusual and scientifically valuable features, viz. (1) exceptional freedom from growth banding, resulting in mm3-scale volumes of both cuboid and {1 1 1} growth apparently homogeneous in impurity and defect content, (2) freedom from dislocations, at least within the regions of most interest, and (3) the presence of {1 1 1}-faceted re-entrants, not involving twinning, located in the growth

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

585

Fig. 7. PL spectra recorded from point 1 within the luminescent core of SL-00/47, see Fig. 5c. The sharp first-order Raman-scattered lines are indicated by R. (a) Excitation by 325 nm, spectral range 325–650 nm, R at 340 nm, ZPLs corresponding to N3, NE2 and NE3 indicated. (b) Excitation by 488 nm, spectral range 480–900 nm, R at 522 nm.

sector boundaries between adjacent cuboid sectors. Feature 3 has been subjected to detailed birefringence and X-ray topographic study [24], which was performed on a central section (GDO2/ 1), area B27 mm2, cut from the whole crystal (GDO2) and fine-polished parallel to (1 1 0). Feature 1 made specimen GDO2/1 ideally suitable for probing differences between the material in

octahedral and cuboid growth sectors, the probes now including PL spectroscopy. Among a variety of experiments performed on GDO2/1, one involved bombardment of small areas, diameters B0.7 mm, with 17 MeV energy 19F ions. These penetrated B5 mm, producing at that depth a lenticle of radiation damage with lattice dilatation and consequent uplifting of the specimen surface

ARTICLE IN PRESS 586

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

above the lenticle to form an optically detectable mesa. The accompanying X-ray diffraction contrast effects were described in Ref. [26]. The damaged areas were avoided during spectroscopic probing. The homogeneity of the {1 1 1} sectors and the spatially gradual zonal variation of apparent defect content in the cuboid sectors simplified acquisition of IR absorption spectra representing the 3 types of diamond matrix requiring characterisation, viz. {1 1 1}-faceted growth, inner cuboid containing microscopic bodies and outer cuboid lacking them. These 3 locations are designated O, IC and OC, respectively. Absorption data acquired by FTIR microscopy some years ago were available. These were analysed similarly to the data for SL-00/47. High values of NB were found, the highest, 1600 at ppm, in location O. The sharp 3107 cm1 line was recorded with the same resolution, 2 cm1, as used for specimen SL-00/47. The peak heights are 0.9 cm1 in O, 36 cm1 in IC and 9 cm1 in OC. The peak in IC is very high, and, as in SL-00/47, many times the height in {1 1 1}-faceted growth. 5.2. Photoluminescence spectroscopy For the same reasons as applied in IR absorption microspectroscopy, only the three locations, O, IC and OC, needed to be probed by PL microspectroscopy in order to characterise the PL properties of specimen GDO2/1. The experimental arrangements for these ‘spot’ probings were identical with those used on specimen SL-00/47 and described in Section 4.4. Spectra excited by laser sources of 325 and 488 nm radiations were recorded, in this case at room temperature as well as at 7 K. Whereas low temperature measurements are essential for picking out the ZPLs of vibronic systems, the spectral changes occurring between low and ambient temperature are also of interest. Here, for brevity, report is restricted to the 7 K spectra. Observations made with the 325 nm source were as follows. Only the N3 system appeared in the O spectrum. In the IC spectrum N3 dominated but a number of identifiable longerwavelength emissions also appeared. The OC spectrum was a combination of the O and IC

spectra. In order not to saturate the detector by the very strong N3 ZPL when recording the IC spectrum (Fig. 8a), it was necessary to reduce laser power and thereby diminish peak intensities at longer wavelengths. Strongest among the latter is the B604 nm ZPL, imprecisely measured here as 603.5 nm (and listed as 603.8 nm, 2.053 eV [2]). The next highest peaks are those most significant in the present investigation: the ZPL at 488.9 nm due to the NE2 centre, and that at 523.3 nm due to the NE3 centre. Fig. 8b shows the IC spectrum excited by 488 nm laser. A strong sharp line appears at B503.5 nm. This line is found, but considerably weaker, at locations OC and O. It corresponds with the 3H vibronic system generated by radiation damage, ZPL 503.4 nm, 2.462 eV [2]. The small intensity hump seen under the Raman line in Fig. 8b fits the principal 3 H phonon side-band emission, a broad peak with maximum B65 meV below the ZPL energy as measured in near roomtemperature CL [36]. The profile of the ZPL and associated longer-wavelength features in Fig. 8b closely matches that recorded at B6 K from 300 keV-electron irradiated nitrogen-containing CVD diamond, see Fig. 1 of [41], and also observed in a variety of diamonds likewise treated. In untreated diamonds the 3H system was spectroscopically identified in CL from aradiation-damaged surface ‘rinds’ of natural specimens, see Fig. 13 of Ref. [36]. However, in specimen GDO2/1 there is little doubt that presence of the 3H system is an artefact created by stray irradiation during the 17 MeV 19F ion probing experiments mentioned in Section 5.1. The 3H emission was notably strongest from location IC, which lay nearest to visible patches of radiation damage. Proceeding to identifiable features at longer wavelengths, the 603.8 nm ZPL is again in evidence. The complicated spectral structures with l > 650 nm have significant features in common with those in Fig. 7b and in the spectrum in [17] referred to in connection with Fig. 7b. In Fig. 8b the ZPLs at wavelengths recorded here as 700.5 and 787.3 nm stand out strongly compared with Fig. 7b. The former line has been found in PL both from mixed-habit natural diamonds [31,33] and

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

587

Fig. 8. PL spectra recorded from the inner cuboid region (location IC) of specimen GDO2/1 at 7 K. R indicates the sharp first-order Raman-scattered line. (a) Excitation by 325 nm, spectral range 330–650 nm. R at 340 nm, very weak. ZPLs corresponding to N3, NE2, NE3 indicated. (b) Excitation by 488 nm, spectral range 500–900 nm, R at 522 nm.

from synthetics, but only those grown in a Nicontaining environment [15]. The ZPL recorded at 787.3 nm (usually quoted as at B788 nm) is known in mixed-habit natural diamonds [2,31], but not in synthetics. The OC spectrum was similar to the IC spectrum. The O spectrum was devoid of ‘cuboid’ features except for very weak peaks at B701 and B787 nm.

6. Concluding remarks The enquiry reported in this paper relies upon effective methods for detecting cuboid growth in diamonds. In absence of visible tell-tale features such as internal star-like distributions of lightscattering bodies or external morphologies like those shown remarkably by mixed-habit Jwaneng

ARTICLE IN PRESS 588

A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589

diamonds [32], X-ray topography is the most generally revealing method, at least for nondestructive initial diagnosis. However, in specimen GDO2/1 mixed-habit growth was patent in all inspection methods. Contrastingly, in SL-00/47 its detection in the luminescent core (a relatively much smaller volume) came about through close study of CL topographs, followed by PL topographs, both recorded by the techniques outlined in Section 4.2. Birefringence and transmission Xray topographs did not show up cuboid growth within the SL-00/47 core because of local masking by strains from cracks and inclusions. The conclusions reached in this paper stand upon twin foundations. The first is that the EPR centres NE1, NE2, etc. that develop when Ni-containing synthetic diamonds are heated post-growth to high temperatures are themselves Ni-containing structures. This is taken as well established by extensive prior work. Some NE centres also display optical activity, which in the present context implies visibly observable PL (and near-IR PL in the case of NE8). The second foundation is correctness in identifying these optical counterparts of NE centres in the spectra of the natural diamonds examined. Long-standing evidence from naturaldiamond studies backs such identifications. [The essential step represented in Table 1 is matching EPR centres with their ZPLs; allocation to a named optical system is less important. In Table 1 customary allocation of the ZPL of the NE2 centre to the S2 system is retained for simplicity.] The focus of the present work has been unequivocal demonstration that NE centres reside in cuboid growth sectors of natural diamonds. The 2 specimens investigated in detail represent such different manifestation of mixed habit growth as to encourage belief that presence of NE centres in cuboid growth is a general occurrence. Detection of NE8 centres in the luminescent core of SL-00/ 47, albeit weakly, but not at all in cuboid sectors of GDO2/1 is understandable. Among EPR-active Ni–N-vacancy centres, the NE8 centre with 4N represents the highest coordination of the Ni ion with N so far encountered. This fits in with the IR spectral characteristics of the core of SL-00/47 whose highly advanced nitrogen aggregation state, as shown by its very high NB/NA ratio, suggests

some residence at uncommonly high temperatures [11]. In GDO2/1 the profiles of spectra recorded from regions IC and OC differ little after subtracting the different contributions to them from the N3 and 3H systems that are not ‘cuboidcharacteristic’ features. The cuboid features, such as the NE2, NE3, B604, 700.5 and 787.3 nm ZPLs, are reduced to roughly half-strength in the OC region. This is a much smaller reduction than that shown from IC to OC in the population density of visibly light-scattering or detectable Xray diffraction-contrast-producing bodies. Regarding Ni concentrations in NE centres, studies involving both synthetic and natural diamonds, e.g. [15], usually find optical absorptions due to NE centres too weak to measure in the natural crystals, with Ni concentration 1–2 orders of magnitude less than in synthetics, perhaps o0.1 at ppm. Assaying Ni in cuboid growth faces the problem of distinguishing between Ni atomically dispersed in NE centres and Ni in finely dispersed non-diamond bodies.

Acknowledgements We thank Dr. Paul Spear, DTC Research Centre, Maidenhead, Berkshire SL6 6JW for IR absorption spectra of specimen GDO2/1 and again thank Dr. J. Milledge, Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, together with Dr. D. Zedgenizov, for CL topographs of specimen SL00/47. Two authors, A.P.Y. and N.P.P., gratefully acknowledge support for the work in part by Grants No. 98-05-65283 and 02-05-65075 from the Russian Foundation for Basic Research, and A.P.Y. thanks the Royal Society for an International Exchange Grant.

References [1] J.E. Field (Ed.), The properties of Natural and Synthetic Diamond, Academic Press Ltd., London, 1992. [2] A.M. Zaitsev, Optical Properties of Diamond: a Data Handbook, Springer, Berlin, 2001. [3] R.H. Wentorf, J. Phys. Chem. 75 (1971) 1833.

ARTICLE IN PRESS A.R. Lang et al. / Journal of Crystal Growth 263 (2004) 575–589 [4] R.C. Burns, G.J. Davies, in: J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992, pp. 395–422 (Chapter 10). [5] P.J. Dean, Phys. Rev. 139 (1965) A588. [6] A.T. Collins, P.M. Spear, J. Phys. C: Solid State Phys. 16 (1983) 963. [7] A.T. Collins, J. Phys.: Condens. Matter 1 (1989) 439. [8] G. Davies, A.J. Neves, M.H. Nazar!e, Europhys. Lett. 9 (1989) 47. [9] A.T. Collins, Diamond Relat. Mater. 9 (2000) 417. [10] C.D. Clark, A.T. Collins, G.S. Woods, in: J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992, pp. 35–79 (Chapter 2). [11] T. Evans, in: J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992, pp. 259–290 (Chapter 6). [12] V.A. Nadolinny, A.P. Yelisseyev, Diamond Relat. Mater. 3 (1994) 17. [13] V. Nadolinny, A. Yelisseyev, Diamond Relat. Mater. 3 (1994) 1196. [14] V.A. Nadolinny, A.P. Yelisseyev, O.P. Yuryeva, B.N. Feygelson, Appl. Magn. Reson. 12 (1997) 543. [15] V.A. Nadolinny, A.P. Yelisseyev, J.M. Baker, M.E. Newton, D.J. Twitchen, S.C. Lawson, O.P. Yuryeva, B.N. Feigelson, J. Phys.: Condens. Matter 11 (1999) 7357. [16] A. Yelisseyev, Yu. Babich, V. Nadolinny, D. Fisher, B. Feigelson, Diamond Relat. Mater. 11 (2002) 22. [17] A. Yelisseyev, V. Nadolinny, B. Feigelson, Yu. Babich, Int. J. Modern Phys. B 16 (2002) 900. [18] C.J. Noble, Th. Pawlik, J.-M. Spaeth, J. Phys.: Condens. Matter 10 (1998) 11781. [19] K. Iakoubovskii, G.J. Adriaenssens, Diamond Relat. Mater. 11 (2002) 125. [20] F.C. Frank, in: J. Burls (Ed.), Proceedings of the International Industrial Diamond Conference, Oxford, 1966, Vol. 1, Science, Industrial Diamond Information Bureau, London, 1967, pp. 119–135.

589

[21] A.R. Lang, Proc. R. Soc. London A 340 (1974) 233. [22] S. Suzuki, A.R. Lang, Diamond Research 1976. De Beers Industrial Diamond Division Ltd., Ascot, 1976, pp. 39–47. [23] A.R. Lang, J. Crystal Growth 24/25 (1974) 108. [24] S. Suzuki, A.R. Lang, J. Crystal Growth 34 (1976) 29. [25] A.R. Lang, in: J.E. Field (Ed.), The Properties of Diamond, Academic Press, London, 1979, pp. 425–469 (Chapter 14). [26] A.R. Lang, A.P.W. Makepeace, M. Moore, W.G. Machado, J. Appl. Cryst. 16 (1983) 113. [27] S. Suzuki, A.R. Lang, Philos. Mag. 32 (1975) 1083. [28] G.S. Woods, A.T. Collins, J. Phys. Chem. Solids 44 (1983) 471. [29] R.M. Chrenko, R.S. McDonald, K.A. Darrow, Nature, Lond. 312 (1967) 474. [30] Y. Kamiya, A.R. Lang, Philos. Mag. 11 (1965) 347. [31] S.P. Plotnikova, Yu.A. Klyuev, I.A. Parfianovich, Mineral. Zh. 2 (1980) 75. [32] C.M. Welbourn, M.-L.T. Rooney, D.J.F. Evans, J. Crystal Growth 94 (1989) 229. [33] W.J.P. Van Enckevort, E.P. Visser, Philos. Mag. B 62 (1990) 597. [34] N.P. Pokhilenko, N.V. Sobolev, J.A. MacDonald, A.E. Hall, E.S. Yefimova, D.A. Zedgenizov, A.M. Logvinova, L.F. Reimers, Dokl. Acad. Nauk 380 (2001) 374–379 [Doklady Earth Sciences 380 (2001) 800]. [35] J.W. Harris, Contr. Mineral. Petrol. 35 (1972) 22. [36] P.L. Hanley, I. Kiflawi, A.R. Lang, Philos. Trans. R. Soc. London A 284 (1977) 329. [37] A.R. Lang, J. Crystal Growth 42 (1977) 625. [38] S.R. Boyd, I. Kiflawi, G.S. Woods, Philos. Mag. B 69 (1994) 1149. [39] S.R. Boyd, I. Kiflawi, G.S. Woods, Philos. Mag. B 72 (1995) 351. [40] G.S. Woods, Proc. R. Soc. London A 407 (1986) 219. [41] J.W. Steeds, T.J. Davis, S.J. Charles, J.M. Hayes, J.E. Butler, Diamond Relat. Mater. 8 (1999) 1847.