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HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system
Diamond and Related Materials 11 (2002) 1863–1870
HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system Yu. Borzdova, Yu. Pal’yanova,*, I. Kupriyanova, V. Gusevb, A. Khokhryakova, A. Sokola, A. Efremova a
Institute of Mineralogy and Petrography, Siberian Branch of the Russian Academy of Sciences, Academician Koptyug Prospect, 3, 630090 Novosibirsk, Russia b Institute of Automation and Electrometry, SB RAS, Ac. Koptyug Prospect, 1, 630090 Novosibirsk, Russia Received 10 April 2002; received in revised form 17 June 2002; accepted 30 June 2002
Abstract The capability of iron nitride, Fe3 N for converting graphite to diamond was explored at Ps7 GPa and Ts1550–1850 8C in experiments with a duration of 20 h. It was established that depending on the synthesis temperature the iron nitride melt provides conditions for crystallisation of diamond andyor graphite, with the minimal temperature for spontaneous diamond nucleation being approximately 1700 8C. Based on the results obtained it was argued that the iron nitride acts as the solvent-catalyst for diamond formation. The crystallised diamonds were found to contain nitrogen in concentration up to approximately 3300 ppm, which depending on the synthesis temperature was present in either the A form or both A and C forms. Absorption peaks caused by hydrogen-related defects were observed in IR spectra of all diamonds examined. For the 3107 cmy1 line a tendency to increase in intensity with increasing the nitrogen content was found. The well-known blue band-A, N3, H3 and 2.156 eV systems as well as a band with zero-phonon energy at 1.787 eV were observed in cathodoluminescence. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: HPHT diamond; Fe3N solvent-catalyst; IR absorption; Cathodoluminescence
1. Introduction It is well known that nitrogen is the dominant impurity in diamond. Most of the physical properties of diamond essentially depend on its form and concentration. In the diamond lattice nitrogen may be present as single substitutional atoms (C centres) or in the form of aggregates of substitutional atoms (A and B centres). Diamonds containing nitrogen in the single substitutional form are classified as type Ib and those containing aggregated nitrogen forms are termed as type Ia. Most of synthetic diamonds produced by the conventional high pressure, high temperature (HPHT) techniques correspond to type Ib, whereas an overwhelming majority of natural diamonds are of Ia type. The formation of *Corresponding author. Tel.: q7-3832-34-25-01; fax: q7-383234-25-01. E-mail address: [email protected] (Y. Pal’yanov).
the aggregated forms of nitrogen is known to proceed through thermally-activated migration of nitrogen atoms and their aggregation. Concentration of substitutional nitrogen, which is another very important characteristic, is determined by nitrogen incorporation during diamond growth. This process due to its high complexity, is still poorly understood and represents a particular problem. As is known, nitrogen content in diamond varies from -1 ppm to a few thousand parts per million. The highest nitrogen concentrations of approximately 3000–5000 ppm were found in natural diamonds w1,2x. In synthetic diamonds grown from conventional metal solvent-catalysts nitrogen content is typically approximately 200– 300 ppm and can be increased by the addition of nitrogen-containing compounds to the growth system. However, the highest nitrogen concentrations reported so far for the metal-grown diamonds are only approxi-
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mately 800–900 ppm w3–5x. Kanda et al. w6x examining diamond crystals synthesised using non-metallic solventcatalysts, such as Na2SO4 and Na2CO3, have found that these diamonds may contain nitrogen in concentrations as high as 1200–1900 ppm. Although these values are still lower than the highest observed, this finding indicates that the incorporation of nitrogen in a growing diamond is strongly affected by the nature of the solventcatalyst. Further indication of the solvent-catalyst effect upon the nitrogen incorporation comes from the study of diamonds crystallised in the sulfur–graphite system w7x. Without any intentional nitrogen doping these diamonds may contain up to 700 ppm of nitrogen. In this connection and taking into consideration the very special role of nitrogen, it is of interest to explore if diamond could be grown from a nitrogen-containing compound. In the present work an iron nitride, Fe3N, having relatively low melting temperature, was chosen as a candidate of the solvent-catalyst for diamond synthesis. The experiments carried out at a fixed pressure of 7 GPa and temperatures ranging from 1550 to 1850 8C for 20 h confirmed that the iron nitride is capable of converting graphite to diamond. The crystallised diamonds were characterised by means of optical and scanning electron microscopy, Fourier transform IR microspectroscopy and cathodoluminescence. 2. Experimental Diamond crystallisation in the Fe3N–C system was studied at 7 GPa and temperatures in the range 1550– 1850 8C using a high-pressure apparatus of a ‘splitsphere’ type w8x. Details on the high-pressure cell design and pressure calibration were presented in our previous works w9,10x. The temperature was measured in every experiment using a PtRh30 yPtRh6 thermocouple. The thermocouple was calibrated at 7 GPa by the melting of Ni. Iron nitride used as the solvent-catalyst was synthesised from carbonyl iron with purity of 99.999% by nitriding in a stream of ammonia, NH3 in a running quartz reactor at 400–500 8C. An X-ray diffraction analysis made after the nitridation revealed only the Fe3N phase. Cube-octahedral synthetic diamonds approximately 0.5 mm in size were used as seed crystals. To determine the state of the crystallisation medium under the experimental conditions the seed crystals were pressed in Fe3N powder at three different levels in height. The pressed sample of iron nitride and seed crystals was loaded inside an ampoule of 7.2 mm in diameter and 7 mm height made from a graphite rod (99.99% purity), which was the carbon source. The ampoule was then embedded into a high-pressure cell. All the runs were performed with a fixed duration of 20 h. The recovered samples were examined by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). HCl and HClO3 acids were
Table 1 Experimental results on diamond and graphite crystallisation in the Fe3N–C system at Ps7 GPa for 20 h Run no.
Temp. (8C)
Diamond nucleation
a (%)
Diamond growth on seed
Newly formed graphitea
IN1 IN2 IN3 IN4 IN5 IN6 IN7
1850 1800 1750 1700 1650 1600 1550
q q q q y y y
100 90 70 -1 0 0 0
q q q q q q y
y y y q q q q
a
Graphite formed upon quenching has not been included.
used to dissolve iron nitride and graphite, respectively. The amount of spontaneously nucleated diamond (MDm) was measured by microbalance and its ratio to the initial graphite weight (MGr) showed the degree of graphite conversion to diamond, a(%)s(MDm yMGr )=100. Infrared absorption spectra were recorded using a Bruker IFS66 FTIR spectrophotometer fitted with a Bruker model A590 microscope. A circular aperture providing a 100 mm diameter sampling area was applied. To convert the recorded spectra into the absorption coefficient units each spectrum was fitted to the standard infrared spectrum of type IIa diamond so that to obtain the best fit of the intrinsic two-phonon absorption bands (2700–1700 cmy1). Cathodoluminescence measurements were performed using a custom-made set-up based on a Tesla BS250 transmission electron microscope. The luminescence from a sample was focused onto the entrance slit of an MDR-23 diffraction monochromator fitted with a 1200-grooves mmy1 grating. An FEU-100 photomultiplier was used as the detector. The CL spectra were recorded with a sample held at approximately 90 K. The electron microscope was operated at an accelerating voltage of 40 kV with typical beam current density of 10 mAymm2. None of the CL spectra presented here has been corrected either for the transfer function of the monochromator or for the spectral response of the detector. 3. Results and discussion 3.1. Diamond crystallisation Experiments on diamond crystallisation in the Fe3N– C system were carried out at 7 GPa in the temperature range 1550–1850 8C with a duration of 20 h. The results of the experiments are presented in Table 1. The seed diamond crystals after the experiments were always found in the top part of the ampoules. This definitely indicates that the iron nitride was in a molten state under the experimental conditions. After each experiment the recovered samples were analysed by XRD and
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Fig. 1. X-Ray diffraction pattern of the recovered sample from run IN2 (7 GPa, 1800 8C). Question-marks highlight unidentified peaks.
in all the cases Fe3N was revealed as the dominant phase. Typical X-ray diffraction pattern is shown in Fig. 1. The main peaks correspond to iron nitride and in addition, depending on the temperature as shown in Table 1, peaks of diamond andyor graphite were present. The XRD patterns also exhibited some minor peaks, which we were not able to identify. These are marked in Fig. 1 by the question marks. At temperatures in the range 1750–1850 8C substantial crystallisation of diamond was established. As the temperature increased the degree of graphite conversion to diamond changed from approximately 70 up to almost 100%. Diamond crystals up to 500 mm in size and their aggregates were found both at the graphite ampoule and directly within the Fe3N catalyst. The crystallisation of diamond in these experiments proceeded most probably through two different processes: synthesis through a film of the iron nitride melt and recrystallisation inside the ampoule caused by the temperature gradient. The crystallised diamonds had octahedral habit and looked either colourless or yellowish to brown tinted. The crystals were rather specific. First of all, the twinning was unusually prominent. All diamonds contained a lot of microtwins and frequently formed polysynthetic and cyclic twins and aggregates (Fig. 2). Another feature of the crystallised diamonds is a specific surface structure of the faces. Octahedral faces were not flat but always exhibited vicinal hillocks. When the hillocks were individual the face was rounded and convex, and if they were numerous the surface had a cellular structure (Fig. 2b, Fig. 3). Also characteristics of these diamond crystals were splitting and block structure. In the experiment at 1700 8C no diamond crystals were established near the boundary between the graphite ampoule and the Fe3N catalyst despite the ampoule at the contact with the catalyst was considerably dissolved.
Fig. 2. SEM micrographs of crystallised diamond: (a) polysynthetic; and (b) cyclic aggregates from runs IN1 (1850 8C) and IN2 (1800 8C), respectively.
Fig. 3. SEM micrograph of a diamond aggregate showing cellular structure of the {111} faces (run IN3, 1750 8C).
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Fig. 4. SEM micrograph of graphite dendrites formed in run IN4 (1700 8C).
Fig. 6. SEM micrograph of a seed diamond crystal after the experiment at 7 GPa and 1700 8C for 20 h (run IN4).
Inside the Fe3N sample after dissolution a lot of large graphite crystals, numerous small dendrites of graphite (Fig. 4) a few diamond crystals (Fig. 5) were found (Fig. 5). The crystallised diamonds also exhibited vicinal surface structure and twinning, but these features were less pronounced than in the experiments at higher temperatures. The graphite dendrites formed probably upon quenching from carbon dissolved in the iron nitride melt. Diamond layers grown on the {111} faces and octahedral pyramids on the {100} faces, indicating the regeneration of crystals by the {100}™{111} sequence, were established on the cube-octahedral seed crystals (Fig. 6). The maximum thickness of the grown diamond layers was up to 25 mm and 80 mm for the {111} and {100} faces, respectively.
Fig. 5. SEM micrographs of diamond crystals synthesised in run IN4 (1700 8C).
Fig. 7. SEM micrograph of a seed diamond crystal after the experiment at 7 GPa and 1650 8C for 20 h (run IN5): (a) overall view; and (b) growth pattern on the (100) face.
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Fig. 8. Infrared absorption spectra of diamonds synthesised at 1850 8C (run IN1). Spectrum (b) has been displaced vertically for clarity.
At 1650 8C no newly formed diamonds were established. Graphite crystals were found at the boundary between the Fe3N melt and the graphite ampoule. Slight diamond growth on the seeds with formation of rounded vicinal surfaces on the {111} faces and microfacet structure on the {100} faces (Fig. 7) was observed. At 1600 8C diamond growth on the seeds was even lesser. The thickness of the grown diamond layers did not exceed 1 mm. In the experiment at 1550 8C, only small crystals of newly formed graphite were established.
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Fig. 9. Infrared absorption spectra of diamonds synthesised at 17508C (run IN3). Spectrum (b) has been displaced vertically for clarity.
nitrogen (C centres). The presence of the C centres in these samples is apparently a consequence of the synthesis temperature effect on the nitrogen aggregation
3.2. Spectroscopic characterisation Spectroscopic characterisation was performed for diamonds synthesised in runs IN1 (1850 8C), IN3 (1750 8C) and IN4 (1700 8C). From each run, two samples of sizes and quality appropriate for FTIR absorption measurements were selected. These were both individual diamond crystals and particles resulted from fracturing of diamond aggregates. Infrared absorption spectra recorded for these diamonds are shown in Figs. 8–10. As it follows from the spectra all the samples exhibit in the defect-induced one-phonon region (1400–900 cmy1) strong absorption caused by nitrogen impurities. For the samples synthesised at 1850 8C and 1750 8C (Figs. 8 and 9) this absorption is predominantly due to the A aggregates of nitrogen (pairs of nearest-neighbouring nitrogen atoms). The IR spectra taken from the samples synthesised at 1700 8C (Fig. 10) show components of both nitrogen pairs and single substitutional
Fig. 10. Infrared absorption spectra of diamonds synthesised at 1700 8C (run IN4). Spectrum (b) has been displaced vertically for clarity.
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process. Also, as it is seen from the recorded spectra diamonds crystallised in the same runs, especially at higher temperatures, exhibit substantial variations in the strength of the nitrogen-related absorption and consequently, nitrogen content. Taking into account a very high intensity of diamond crystallisation at these temperatures as well as rather long duration of the synthesis experiments, the reasons for the observed variability of the nitrogen uptake could be quite various. The crystals selected for the infrared measurements could crystallise on different points in time, under different physical and chemical conditions and by different growth processes. Concentrations of nitrogen in the form of A and C centres were derived from the infrared spectra using standard procedures w11x. It was found that in the samples from runs IN1 and IN3 the nitrogen is contained in concentrations from 1350 wFig. 9, spectrum (a)x to 2650 wFig. 9, spectrum (b)x ppm. For the samples from run IN4 the calculations gave values of 1500y1800 ppm wFig. 10, spectrum (a)x and 1700y1300 ppm wFig. 10, spectrum (b)x of nitrogen in the form of A (figures before slash) and C (figures after slash) centres. Thus, the total nitrogen content in these samples was approximately 3300 and 3000 ppm. It is necessary to note that due to small sizes, irregular shape and rough surfaces of the samples the baseline of the recorded spectra was not exactly straight. This giving rise to some uncertainty in the fitting confidence was the major source of error in determining the absorption coefficients and consequently, nitrogen concentrations. This error was sampledependent and, as estimated, varied within 10–20%. In addition to the nitrogen-related bands, the samples also exhibited sharp absorption lines related to hydrogen impurity. An absorption peak at 3107 cmy1, which is attributed to a C–H bond stretching vibration w12,13x, was present in the IR spectra of all diamonds examined. The intensity of the peak varied from sample to sample ranging from 10 wFig. 10, spectrum (b)x up to 30 cmy1 wFig. 9, spectrum (b)x. As compared to the data available for natural and synthetic diamonds these values of the absorption coefficient appear to be rather high. A peak at 1405 cmy1, which accompanies the 3107 cmy1 line and is assigned to a C–H bond bending vibration w12,13x, was readily detected in the spectra, where atmospheric absorption interference was not too high. In samples, showing strong 3107 cmy1 line, weak peaks at 3237 and 2786 cmy1 were found. Both these lines are commonly observed as minor features in IR spectra of natural hydrogen-containing diamonds w13,14x. The 2786 cmy1 peak is believed to be the first overtone of the 1405 cmy1 line w14x. The origin of the peak at 3237 cmy1 is still under the question. The occurrence of hydrogen-related absorption peaks of high intensity in the crystallised diamonds is very curious. It should be noted that despite the fact that hydrogen has been recognised as a common impurity in natural type Ia diamonds, data on its occurrence in the
Fig. 11. The concentration of nitrogen vs. the absorption strength of the 3107 cmy1 line in the examined diamonds. Open triangles correspond to samples from run IN4 for which the concentration of the A-form nitrogen rather than the total nitrogen content is plotted. Solid line shows the best linear fit to the data points.
IR-active form in synthetic HPHT diamonds are very scarce. In fact, we can refer only to work of Kiflawi et al. w15x, where it has been found that the 3107 cmy1 absorption line can be produced in some HPHT grown diamonds by annealing them at high temperatures. The main question emerging from the results of the present work concerns the possible source of hydrogen in the diamond crystallisation environment. In principle, taking into account the abundance of hydrogen in nature, its high diffusivity and chemical activity, one could reasonably assume that the growth environment would contain some atmospheric hydrogen. Besides, it is necessary to note that the iron nitride used for the diamond synthesis was produced by nitriding iron in stream of ammonia, NH3 and consequently contained hydrogen. It is most probable that the Fe3N reagent was in fact the major source of hydrogen in the synthesis experiments performed in this work. It has been shown previously that the intensity of the hydrogen-related infrared absorption could correlate with the nitrogen content in both natural w16x and synthetic w15x diamonds. To check whether such a correlation exists for the diamonds crystallised in the Fe3N–C system the intensity of the 3107 cmy1 line was plotted vs. the nitrogen concentration (see Fig. 11). For the crystals synthesised at 1700 8C, which contain nitrogen in both the A and C forms, the concentration of the aggregated nitrogen rather than the total nitrogen content was shown in Fig. 11. This appears to be reasonable
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this band represents a particularly interesting question. It is apparent that the 1.787 eV centre is due to an impurity-related defect. Nitrogen and hydrogen alone can hardly be the relevant species and some other impurities should also be taken into consideration. In this respect, it is worth noting that the specific feature of the studied diamonds is very high concentrations of nitrogen. In principal, local lattice dilatation caused by nitrogen defects might facilitate the incorporation of other impurities, for instance, iron. Such an effect of enhanced incorporation of phosphorus with nitrogen content increase has been observed for CVD diamond w17x. Obviously, some additional examinations are necessary to clarify the nature of a defect responsible for the 1.787 eV vibronic band observed in this study. 4. Summary and conclusions
Fig. 12. Cathodoluminescence spectra, recorded at 90 K, of diamonds synthesised at 1850 8C (run IN1). Spectrum (b) has been displaced vertically for clarity.
since in synthetic HPHT diamonds the 3107 cmy1 line was found to appear on annealing them at temperatures where the nitrogen aggregation takes place w15x. As it is seen from Fig. 11 there is an apparent tendency for the 3107 cmy1 peak intensity to increase with increasing the nitrogen content. This observation gives further support to the idea that the conditions favouring the incorporation of nitrogen in a growing diamond might also favour the incorporation of hydrogen. Cathodoluminescence spectra measured for the samples from runs IN1 and IN3 are shown in Figs. 12 and 13. The samples from run IN4 emitted very weakly under electron beam excitation so that we were not able to acquire their CL spectra. For the most part the recorded CL spectra are dominated by bands, which are commonly observed in diamond. This concerns the broad featureless band peaking at approximately 2.8 eV, so-called blue band-A, and vibronic bands of the N3, H3 and 2.156 eV centres. Another feature observed in cathodoluminescence is a vibronic system whose zerophonon line (ZPL) peaks at 1.787 eV. A much weaker peak separated by (14"1) meV from the 1.787 eV ZPL is seen on its higher photon energy foot. The peak correlates in intensity with the 1.787 eV line suggesting a multiplet structure of the ZPL. The dominant phonon energies for this vibronic band lie at (39"2) and (80"5) meV. From an analysis of the existing data on optical bands in diamond we may infer that the observed 1.787 eV system has not been reported previously. The origin of
We have shown that under HPHT conditions spontaneous diamond nucleation and diamond growth on seeds can be realised in the Fe3N–C system. In all experiments, which were carried out at 7 GPa and temperatures in the range 1550–1850 8C for 20 h, the crystallisation environment was established to be in a molten state and upon quenching the melt crystallised predominantly as the Fe3N phase. This allows us to infer that the iron nitride when melted under the experimental conditions did not undergo any significant phase decomposition and its stochiometric ratio of iron and nitrogen corresponded to the initial Fe3N. Therefore, we may conclude
Fig. 13. Cathodoluminescence spectra, recorded at 90 K, of diamonds synthesised at 1750 8C (run IN3). Spectrum (b) has been displaced vertically for clarity.
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that the iron nitride can be regarded as a new solventcatalyst for diamond formation. Acting as the solventcatalyst the iron nitride exhibits some peculiar features, which are not common to most of the catalysts, both metal and non-metallic, explored so far. The crystallisation of diamond in the Fe3N–C system was found to depend strongly on the temperature. At 1850 8C, virtually all initial graphite was converted into diamond, at 1700 8C the degree of the conversion decreased to as low as approximately 1%, and at lower temperatures no spontaneously nucleated diamonds were established in the run products. At the same time, starting from 1700 8C and down to 1550 8C, which was the minimum temperature in this study, rather intense crystallisation of graphite in the diamond stability field occurred. This fact indicates that at these temperatures the solubility of carbon in the iron nitride melt is still rather high but the properties of the melt is likely to change significantly. Crystallisation of metastable graphite at the P–T parameters of thermodynamical stability of diamond was previously observed for the Cu–C system w18x and a number of non-metallic catalysts, such as alkali halides w19x, fluid-containing carbonates w20x and C–O–H fluids w21,22x. The reasons for this phenomenon, however, are still under the question. Taking into account a considerably long duration of the synthesis experiments it can be inferred that the minimum temperature for spontaneous diamond nucleation in the Fe3N–C system is approximately 1700 8C, with this temperature being of some critical character allowing joint crystallisation of diamond and metastable graphite. The minimum temperature for diamond growth on seeds was found to be approximately 1600 8C. FTIR absorption measurements made on a number of crystallised diamonds revealed that they contain very high amount of nitrogen, which depending on the synthesis temperature, was present either in paired form or in both paired and single substitutional forms. The maximum nitrogen concentration measured was approximately 3300 ppm, which is the highest value reported so far for synthetic diamonds and close to the maximum concentrations found in natural diamonds. Hydrogenrelated absorption peaks of relatively high intensity were observed in the IR spectra of all diamonds examined. The most probable source of hydrogen in the diamond crystallisation environment was supposed to be the Fe3N solvent-catalyst whose synthesis involved the use of an NH3 gas. For the 3107 cmy1 line an apparent tendency to increase in intensity with increasing the nitrogen concentration was found. This provides further indications of the possibility of correlated incorporation of nitrogen and hydrogen during diamond growth. The well-known blue band-A, N3, H3 and 2.156 eV system as well as a vibronic band with ZPL structure peaking at 1.787 eV were observed in cathodoluminescence. It was noted that the 1.787 eV band might arise from a
Fe-related defect, this suggestion, however, has yet to be verified. Acknowledgments The authors are grateful to Dr M.Yu. Mikhaylov (Institute of Mineralogy and Petrography, Novosibirsk, Russia) for his assistance in the course of the work. The support from the Interdisciplinary Basic Research Programme of the Siberian Branch of the Russian Academy of Sciences (Project no. 72) and Science Support Foundation is acknowledged. References w1x G.S. Woods, Phil. Mag. B 50 (1984) 673. w2x K. De Corte, P. Cartigny, V.S. Shatsky, P. De Paepe, N.V. Sobolev, M. Javoy, Proceedings of the Seventh International Kimberlite Conference, April 13–17, Red Roof Design cc, Goodwood, Cape Town, South Africa, 1999, p. 174. w3x A.T. Collins, S.C. Lawson, Phil. Mag. Lett. 60 (1989) 117. w4x N.V. Surovtsev, I.N. Kupriyanov, V.K. Malinovsky, V.A. Gusev, Yu.N. Pal’yanov, J. Phys. Condens. Matter 11 (1999) 4767. w5x Yu.M. Borzdov, I.N. Kupriyanov, A.V. Efremov, Yu.N. Pal’yanov, Proceedings of the Fourth International Conference on Crystals: Growth, Properties, Real Structure, Application, October 18–22, vol. 1, VNIISIMS, Aleksandrov, Russia, 1999, p. 342 (in Russian). w6x H. Kanda, M. Akaishi, S. Yamaoka, Diamond Rel. Mater. 8 (1999) 1441. w7x Yu. Pal’yanov, Yu. Borzdov, I. Kupriyanov, V. Gusev, A. Khokhryakov, A. Sokol, Diamond Rel. Mater. 10 (2001) 2145. w8x Yu.N. Pal’yanov, A.F. Khokhryakov, Yu.M. Borzdov, et al., Russ. Geol. Geophys. 38 (1997) 920. w9x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, A.F. Shatsky, N.V. Sobolev, Diamond Rel. Mater. 8 (1999) 1118. w10x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, Lithos 60 (2002) 145. w11x A.M. Zaitsev, in: M. Prelas, G. Popovici, L. Bigelow (Eds.), Handbook of Industrial Diamonds and Films, Dekker, New York, 1998, pp. 227–376. w12x W.A. Runciman, T. Carter, Solid State Commun. 9 (1971) 315. w13x G.S. Woods, A.T. Collins, J. Phys. Chem. Solids 44 (1983) 471. w14x G. Davies, A.T. Collins, P. Spear, Solid State Commun. 49 (1984) 433. w15x I. Kiflawi, D. Fisher, H. Kanda, G. Sittas, Diamond Rel. Mater. 5 (1996) 1516. w16x K. Iakoubovskii, G.J. Adriaenssens, Diamond Rel. Mater. 11 (2002) 125. w17x G.Z. Cao, W.J.P. van Enckevort, L.J. Giling, R.C.M. de Kruif, Appl. Phys. Lett. 66 (1995) 688. w18x S.K. Singhal, H. Kanda, J. Cryst. Growth 154 (1995) 297. w19x Y. Wang, H. Kanda, Diamond Rel. Mater. 7 (1998) 57. w20x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, N.V. Sobolev, Nature 400 (1999) 417. w21x M. Akaishi, M.D.S. Kumar, H. Kanda, S. Yamaoka, Proceedings of the Eighth NIRIM International Symposium on Advanced Materials, March 4–8, NIRIM, Tsukuba, Japan, 2001, p. 43. w22x A.G. Sokol, Y.N. Pal’yanov, G.A. Pal’yanova, A.F. Khokhryakov, Y.M. Borzdov, Diamond Rel. Mater. 10 (2001) 2131.
HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system Yu. Borzdova, Yu. Pal’yanova,*, I. Kupriyanova, V. Gusevb, A. Khokhryakova, A. Sokola, A. Efremova a
Institute of Mineralogy and Petrography, Siberian Branch of the Russian Academy of Sciences, Academician Koptyug Prospect, 3, 630090 Novosibirsk, Russia b Institute of Automation and Electrometry, SB RAS, Ac. Koptyug Prospect, 1, 630090 Novosibirsk, Russia Received 10 April 2002; received in revised form 17 June 2002; accepted 30 June 2002
Abstract The capability of iron nitride, Fe3 N for converting graphite to diamond was explored at Ps7 GPa and Ts1550–1850 8C in experiments with a duration of 20 h. It was established that depending on the synthesis temperature the iron nitride melt provides conditions for crystallisation of diamond andyor graphite, with the minimal temperature for spontaneous diamond nucleation being approximately 1700 8C. Based on the results obtained it was argued that the iron nitride acts as the solvent-catalyst for diamond formation. The crystallised diamonds were found to contain nitrogen in concentration up to approximately 3300 ppm, which depending on the synthesis temperature was present in either the A form or both A and C forms. Absorption peaks caused by hydrogen-related defects were observed in IR spectra of all diamonds examined. For the 3107 cmy1 line a tendency to increase in intensity with increasing the nitrogen content was found. The well-known blue band-A, N3, H3 and 2.156 eV systems as well as a band with zero-phonon energy at 1.787 eV were observed in cathodoluminescence. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: HPHT diamond; Fe3N solvent-catalyst; IR absorption; Cathodoluminescence
1. Introduction It is well known that nitrogen is the dominant impurity in diamond. Most of the physical properties of diamond essentially depend on its form and concentration. In the diamond lattice nitrogen may be present as single substitutional atoms (C centres) or in the form of aggregates of substitutional atoms (A and B centres). Diamonds containing nitrogen in the single substitutional form are classified as type Ib and those containing aggregated nitrogen forms are termed as type Ia. Most of synthetic diamonds produced by the conventional high pressure, high temperature (HPHT) techniques correspond to type Ib, whereas an overwhelming majority of natural diamonds are of Ia type. The formation of *Corresponding author. Tel.: q7-3832-34-25-01; fax: q7-383234-25-01. E-mail address: [email protected] (Y. Pal’yanov).
the aggregated forms of nitrogen is known to proceed through thermally-activated migration of nitrogen atoms and their aggregation. Concentration of substitutional nitrogen, which is another very important characteristic, is determined by nitrogen incorporation during diamond growth. This process due to its high complexity, is still poorly understood and represents a particular problem. As is known, nitrogen content in diamond varies from -1 ppm to a few thousand parts per million. The highest nitrogen concentrations of approximately 3000–5000 ppm were found in natural diamonds w1,2x. In synthetic diamonds grown from conventional metal solvent-catalysts nitrogen content is typically approximately 200– 300 ppm and can be increased by the addition of nitrogen-containing compounds to the growth system. However, the highest nitrogen concentrations reported so far for the metal-grown diamonds are only approxi-
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mately 800–900 ppm w3–5x. Kanda et al. w6x examining diamond crystals synthesised using non-metallic solventcatalysts, such as Na2SO4 and Na2CO3, have found that these diamonds may contain nitrogen in concentrations as high as 1200–1900 ppm. Although these values are still lower than the highest observed, this finding indicates that the incorporation of nitrogen in a growing diamond is strongly affected by the nature of the solventcatalyst. Further indication of the solvent-catalyst effect upon the nitrogen incorporation comes from the study of diamonds crystallised in the sulfur–graphite system w7x. Without any intentional nitrogen doping these diamonds may contain up to 700 ppm of nitrogen. In this connection and taking into consideration the very special role of nitrogen, it is of interest to explore if diamond could be grown from a nitrogen-containing compound. In the present work an iron nitride, Fe3N, having relatively low melting temperature, was chosen as a candidate of the solvent-catalyst for diamond synthesis. The experiments carried out at a fixed pressure of 7 GPa and temperatures ranging from 1550 to 1850 8C for 20 h confirmed that the iron nitride is capable of converting graphite to diamond. The crystallised diamonds were characterised by means of optical and scanning electron microscopy, Fourier transform IR microspectroscopy and cathodoluminescence. 2. Experimental Diamond crystallisation in the Fe3N–C system was studied at 7 GPa and temperatures in the range 1550– 1850 8C using a high-pressure apparatus of a ‘splitsphere’ type w8x. Details on the high-pressure cell design and pressure calibration were presented in our previous works w9,10x. The temperature was measured in every experiment using a PtRh30 yPtRh6 thermocouple. The thermocouple was calibrated at 7 GPa by the melting of Ni. Iron nitride used as the solvent-catalyst was synthesised from carbonyl iron with purity of 99.999% by nitriding in a stream of ammonia, NH3 in a running quartz reactor at 400–500 8C. An X-ray diffraction analysis made after the nitridation revealed only the Fe3N phase. Cube-octahedral synthetic diamonds approximately 0.5 mm in size were used as seed crystals. To determine the state of the crystallisation medium under the experimental conditions the seed crystals were pressed in Fe3N powder at three different levels in height. The pressed sample of iron nitride and seed crystals was loaded inside an ampoule of 7.2 mm in diameter and 7 mm height made from a graphite rod (99.99% purity), which was the carbon source. The ampoule was then embedded into a high-pressure cell. All the runs were performed with a fixed duration of 20 h. The recovered samples were examined by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). HCl and HClO3 acids were
Table 1 Experimental results on diamond and graphite crystallisation in the Fe3N–C system at Ps7 GPa for 20 h Run no.
Temp. (8C)
Diamond nucleation
a (%)
Diamond growth on seed
Newly formed graphitea
IN1 IN2 IN3 IN4 IN5 IN6 IN7
1850 1800 1750 1700 1650 1600 1550
q q q q y y y
100 90 70 -1 0 0 0
q q q q q q y
y y y q q q q
a
Graphite formed upon quenching has not been included.
used to dissolve iron nitride and graphite, respectively. The amount of spontaneously nucleated diamond (MDm) was measured by microbalance and its ratio to the initial graphite weight (MGr) showed the degree of graphite conversion to diamond, a(%)s(MDm yMGr )=100. Infrared absorption spectra were recorded using a Bruker IFS66 FTIR spectrophotometer fitted with a Bruker model A590 microscope. A circular aperture providing a 100 mm diameter sampling area was applied. To convert the recorded spectra into the absorption coefficient units each spectrum was fitted to the standard infrared spectrum of type IIa diamond so that to obtain the best fit of the intrinsic two-phonon absorption bands (2700–1700 cmy1). Cathodoluminescence measurements were performed using a custom-made set-up based on a Tesla BS250 transmission electron microscope. The luminescence from a sample was focused onto the entrance slit of an MDR-23 diffraction monochromator fitted with a 1200-grooves mmy1 grating. An FEU-100 photomultiplier was used as the detector. The CL spectra were recorded with a sample held at approximately 90 K. The electron microscope was operated at an accelerating voltage of 40 kV with typical beam current density of 10 mAymm2. None of the CL spectra presented here has been corrected either for the transfer function of the monochromator or for the spectral response of the detector. 3. Results and discussion 3.1. Diamond crystallisation Experiments on diamond crystallisation in the Fe3N– C system were carried out at 7 GPa in the temperature range 1550–1850 8C with a duration of 20 h. The results of the experiments are presented in Table 1. The seed diamond crystals after the experiments were always found in the top part of the ampoules. This definitely indicates that the iron nitride was in a molten state under the experimental conditions. After each experiment the recovered samples were analysed by XRD and
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Fig. 1. X-Ray diffraction pattern of the recovered sample from run IN2 (7 GPa, 1800 8C). Question-marks highlight unidentified peaks.
in all the cases Fe3N was revealed as the dominant phase. Typical X-ray diffraction pattern is shown in Fig. 1. The main peaks correspond to iron nitride and in addition, depending on the temperature as shown in Table 1, peaks of diamond andyor graphite were present. The XRD patterns also exhibited some minor peaks, which we were not able to identify. These are marked in Fig. 1 by the question marks. At temperatures in the range 1750–1850 8C substantial crystallisation of diamond was established. As the temperature increased the degree of graphite conversion to diamond changed from approximately 70 up to almost 100%. Diamond crystals up to 500 mm in size and their aggregates were found both at the graphite ampoule and directly within the Fe3N catalyst. The crystallisation of diamond in these experiments proceeded most probably through two different processes: synthesis through a film of the iron nitride melt and recrystallisation inside the ampoule caused by the temperature gradient. The crystallised diamonds had octahedral habit and looked either colourless or yellowish to brown tinted. The crystals were rather specific. First of all, the twinning was unusually prominent. All diamonds contained a lot of microtwins and frequently formed polysynthetic and cyclic twins and aggregates (Fig. 2). Another feature of the crystallised diamonds is a specific surface structure of the faces. Octahedral faces were not flat but always exhibited vicinal hillocks. When the hillocks were individual the face was rounded and convex, and if they were numerous the surface had a cellular structure (Fig. 2b, Fig. 3). Also characteristics of these diamond crystals were splitting and block structure. In the experiment at 1700 8C no diamond crystals were established near the boundary between the graphite ampoule and the Fe3N catalyst despite the ampoule at the contact with the catalyst was considerably dissolved.
Fig. 2. SEM micrographs of crystallised diamond: (a) polysynthetic; and (b) cyclic aggregates from runs IN1 (1850 8C) and IN2 (1800 8C), respectively.
Fig. 3. SEM micrograph of a diamond aggregate showing cellular structure of the {111} faces (run IN3, 1750 8C).
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Fig. 4. SEM micrograph of graphite dendrites formed in run IN4 (1700 8C).
Fig. 6. SEM micrograph of a seed diamond crystal after the experiment at 7 GPa and 1700 8C for 20 h (run IN4).
Inside the Fe3N sample after dissolution a lot of large graphite crystals, numerous small dendrites of graphite (Fig. 4) a few diamond crystals (Fig. 5) were found (Fig. 5). The crystallised diamonds also exhibited vicinal surface structure and twinning, but these features were less pronounced than in the experiments at higher temperatures. The graphite dendrites formed probably upon quenching from carbon dissolved in the iron nitride melt. Diamond layers grown on the {111} faces and octahedral pyramids on the {100} faces, indicating the regeneration of crystals by the {100}™{111} sequence, were established on the cube-octahedral seed crystals (Fig. 6). The maximum thickness of the grown diamond layers was up to 25 mm and 80 mm for the {111} and {100} faces, respectively.
Fig. 5. SEM micrographs of diamond crystals synthesised in run IN4 (1700 8C).
Fig. 7. SEM micrograph of a seed diamond crystal after the experiment at 7 GPa and 1650 8C for 20 h (run IN5): (a) overall view; and (b) growth pattern on the (100) face.
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Fig. 8. Infrared absorption spectra of diamonds synthesised at 1850 8C (run IN1). Spectrum (b) has been displaced vertically for clarity.
At 1650 8C no newly formed diamonds were established. Graphite crystals were found at the boundary between the Fe3N melt and the graphite ampoule. Slight diamond growth on the seeds with formation of rounded vicinal surfaces on the {111} faces and microfacet structure on the {100} faces (Fig. 7) was observed. At 1600 8C diamond growth on the seeds was even lesser. The thickness of the grown diamond layers did not exceed 1 mm. In the experiment at 1550 8C, only small crystals of newly formed graphite were established.
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Fig. 9. Infrared absorption spectra of diamonds synthesised at 17508C (run IN3). Spectrum (b) has been displaced vertically for clarity.
nitrogen (C centres). The presence of the C centres in these samples is apparently a consequence of the synthesis temperature effect on the nitrogen aggregation
3.2. Spectroscopic characterisation Spectroscopic characterisation was performed for diamonds synthesised in runs IN1 (1850 8C), IN3 (1750 8C) and IN4 (1700 8C). From each run, two samples of sizes and quality appropriate for FTIR absorption measurements were selected. These were both individual diamond crystals and particles resulted from fracturing of diamond aggregates. Infrared absorption spectra recorded for these diamonds are shown in Figs. 8–10. As it follows from the spectra all the samples exhibit in the defect-induced one-phonon region (1400–900 cmy1) strong absorption caused by nitrogen impurities. For the samples synthesised at 1850 8C and 1750 8C (Figs. 8 and 9) this absorption is predominantly due to the A aggregates of nitrogen (pairs of nearest-neighbouring nitrogen atoms). The IR spectra taken from the samples synthesised at 1700 8C (Fig. 10) show components of both nitrogen pairs and single substitutional
Fig. 10. Infrared absorption spectra of diamonds synthesised at 1700 8C (run IN4). Spectrum (b) has been displaced vertically for clarity.
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process. Also, as it is seen from the recorded spectra diamonds crystallised in the same runs, especially at higher temperatures, exhibit substantial variations in the strength of the nitrogen-related absorption and consequently, nitrogen content. Taking into account a very high intensity of diamond crystallisation at these temperatures as well as rather long duration of the synthesis experiments, the reasons for the observed variability of the nitrogen uptake could be quite various. The crystals selected for the infrared measurements could crystallise on different points in time, under different physical and chemical conditions and by different growth processes. Concentrations of nitrogen in the form of A and C centres were derived from the infrared spectra using standard procedures w11x. It was found that in the samples from runs IN1 and IN3 the nitrogen is contained in concentrations from 1350 wFig. 9, spectrum (a)x to 2650 wFig. 9, spectrum (b)x ppm. For the samples from run IN4 the calculations gave values of 1500y1800 ppm wFig. 10, spectrum (a)x and 1700y1300 ppm wFig. 10, spectrum (b)x of nitrogen in the form of A (figures before slash) and C (figures after slash) centres. Thus, the total nitrogen content in these samples was approximately 3300 and 3000 ppm. It is necessary to note that due to small sizes, irregular shape and rough surfaces of the samples the baseline of the recorded spectra was not exactly straight. This giving rise to some uncertainty in the fitting confidence was the major source of error in determining the absorption coefficients and consequently, nitrogen concentrations. This error was sampledependent and, as estimated, varied within 10–20%. In addition to the nitrogen-related bands, the samples also exhibited sharp absorption lines related to hydrogen impurity. An absorption peak at 3107 cmy1, which is attributed to a C–H bond stretching vibration w12,13x, was present in the IR spectra of all diamonds examined. The intensity of the peak varied from sample to sample ranging from 10 wFig. 10, spectrum (b)x up to 30 cmy1 wFig. 9, spectrum (b)x. As compared to the data available for natural and synthetic diamonds these values of the absorption coefficient appear to be rather high. A peak at 1405 cmy1, which accompanies the 3107 cmy1 line and is assigned to a C–H bond bending vibration w12,13x, was readily detected in the spectra, where atmospheric absorption interference was not too high. In samples, showing strong 3107 cmy1 line, weak peaks at 3237 and 2786 cmy1 were found. Both these lines are commonly observed as minor features in IR spectra of natural hydrogen-containing diamonds w13,14x. The 2786 cmy1 peak is believed to be the first overtone of the 1405 cmy1 line w14x. The origin of the peak at 3237 cmy1 is still under the question. The occurrence of hydrogen-related absorption peaks of high intensity in the crystallised diamonds is very curious. It should be noted that despite the fact that hydrogen has been recognised as a common impurity in natural type Ia diamonds, data on its occurrence in the
Fig. 11. The concentration of nitrogen vs. the absorption strength of the 3107 cmy1 line in the examined diamonds. Open triangles correspond to samples from run IN4 for which the concentration of the A-form nitrogen rather than the total nitrogen content is plotted. Solid line shows the best linear fit to the data points.
IR-active form in synthetic HPHT diamonds are very scarce. In fact, we can refer only to work of Kiflawi et al. w15x, where it has been found that the 3107 cmy1 absorption line can be produced in some HPHT grown diamonds by annealing them at high temperatures. The main question emerging from the results of the present work concerns the possible source of hydrogen in the diamond crystallisation environment. In principle, taking into account the abundance of hydrogen in nature, its high diffusivity and chemical activity, one could reasonably assume that the growth environment would contain some atmospheric hydrogen. Besides, it is necessary to note that the iron nitride used for the diamond synthesis was produced by nitriding iron in stream of ammonia, NH3 and consequently contained hydrogen. It is most probable that the Fe3N reagent was in fact the major source of hydrogen in the synthesis experiments performed in this work. It has been shown previously that the intensity of the hydrogen-related infrared absorption could correlate with the nitrogen content in both natural w16x and synthetic w15x diamonds. To check whether such a correlation exists for the diamonds crystallised in the Fe3N–C system the intensity of the 3107 cmy1 line was plotted vs. the nitrogen concentration (see Fig. 11). For the crystals synthesised at 1700 8C, which contain nitrogen in both the A and C forms, the concentration of the aggregated nitrogen rather than the total nitrogen content was shown in Fig. 11. This appears to be reasonable
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this band represents a particularly interesting question. It is apparent that the 1.787 eV centre is due to an impurity-related defect. Nitrogen and hydrogen alone can hardly be the relevant species and some other impurities should also be taken into consideration. In this respect, it is worth noting that the specific feature of the studied diamonds is very high concentrations of nitrogen. In principal, local lattice dilatation caused by nitrogen defects might facilitate the incorporation of other impurities, for instance, iron. Such an effect of enhanced incorporation of phosphorus with nitrogen content increase has been observed for CVD diamond w17x. Obviously, some additional examinations are necessary to clarify the nature of a defect responsible for the 1.787 eV vibronic band observed in this study. 4. Summary and conclusions
Fig. 12. Cathodoluminescence spectra, recorded at 90 K, of diamonds synthesised at 1850 8C (run IN1). Spectrum (b) has been displaced vertically for clarity.
since in synthetic HPHT diamonds the 3107 cmy1 line was found to appear on annealing them at temperatures where the nitrogen aggregation takes place w15x. As it is seen from Fig. 11 there is an apparent tendency for the 3107 cmy1 peak intensity to increase with increasing the nitrogen content. This observation gives further support to the idea that the conditions favouring the incorporation of nitrogen in a growing diamond might also favour the incorporation of hydrogen. Cathodoluminescence spectra measured for the samples from runs IN1 and IN3 are shown in Figs. 12 and 13. The samples from run IN4 emitted very weakly under electron beam excitation so that we were not able to acquire their CL spectra. For the most part the recorded CL spectra are dominated by bands, which are commonly observed in diamond. This concerns the broad featureless band peaking at approximately 2.8 eV, so-called blue band-A, and vibronic bands of the N3, H3 and 2.156 eV centres. Another feature observed in cathodoluminescence is a vibronic system whose zerophonon line (ZPL) peaks at 1.787 eV. A much weaker peak separated by (14"1) meV from the 1.787 eV ZPL is seen on its higher photon energy foot. The peak correlates in intensity with the 1.787 eV line suggesting a multiplet structure of the ZPL. The dominant phonon energies for this vibronic band lie at (39"2) and (80"5) meV. From an analysis of the existing data on optical bands in diamond we may infer that the observed 1.787 eV system has not been reported previously. The origin of
We have shown that under HPHT conditions spontaneous diamond nucleation and diamond growth on seeds can be realised in the Fe3N–C system. In all experiments, which were carried out at 7 GPa and temperatures in the range 1550–1850 8C for 20 h, the crystallisation environment was established to be in a molten state and upon quenching the melt crystallised predominantly as the Fe3N phase. This allows us to infer that the iron nitride when melted under the experimental conditions did not undergo any significant phase decomposition and its stochiometric ratio of iron and nitrogen corresponded to the initial Fe3N. Therefore, we may conclude
Fig. 13. Cathodoluminescence spectra, recorded at 90 K, of diamonds synthesised at 1750 8C (run IN3). Spectrum (b) has been displaced vertically for clarity.
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that the iron nitride can be regarded as a new solventcatalyst for diamond formation. Acting as the solventcatalyst the iron nitride exhibits some peculiar features, which are not common to most of the catalysts, both metal and non-metallic, explored so far. The crystallisation of diamond in the Fe3N–C system was found to depend strongly on the temperature. At 1850 8C, virtually all initial graphite was converted into diamond, at 1700 8C the degree of the conversion decreased to as low as approximately 1%, and at lower temperatures no spontaneously nucleated diamonds were established in the run products. At the same time, starting from 1700 8C and down to 1550 8C, which was the minimum temperature in this study, rather intense crystallisation of graphite in the diamond stability field occurred. This fact indicates that at these temperatures the solubility of carbon in the iron nitride melt is still rather high but the properties of the melt is likely to change significantly. Crystallisation of metastable graphite at the P–T parameters of thermodynamical stability of diamond was previously observed for the Cu–C system w18x and a number of non-metallic catalysts, such as alkali halides w19x, fluid-containing carbonates w20x and C–O–H fluids w21,22x. The reasons for this phenomenon, however, are still under the question. Taking into account a considerably long duration of the synthesis experiments it can be inferred that the minimum temperature for spontaneous diamond nucleation in the Fe3N–C system is approximately 1700 8C, with this temperature being of some critical character allowing joint crystallisation of diamond and metastable graphite. The minimum temperature for diamond growth on seeds was found to be approximately 1600 8C. FTIR absorption measurements made on a number of crystallised diamonds revealed that they contain very high amount of nitrogen, which depending on the synthesis temperature, was present either in paired form or in both paired and single substitutional forms. The maximum nitrogen concentration measured was approximately 3300 ppm, which is the highest value reported so far for synthetic diamonds and close to the maximum concentrations found in natural diamonds. Hydrogenrelated absorption peaks of relatively high intensity were observed in the IR spectra of all diamonds examined. The most probable source of hydrogen in the diamond crystallisation environment was supposed to be the Fe3N solvent-catalyst whose synthesis involved the use of an NH3 gas. For the 3107 cmy1 line an apparent tendency to increase in intensity with increasing the nitrogen concentration was found. This provides further indications of the possibility of correlated incorporation of nitrogen and hydrogen during diamond growth. The well-known blue band-A, N3, H3 and 2.156 eV system as well as a vibronic band with ZPL structure peaking at 1.787 eV were observed in cathodoluminescence. It was noted that the 1.787 eV band might arise from a
Fe-related defect, this suggestion, however, has yet to be verified. Acknowledgments The authors are grateful to Dr M.Yu. Mikhaylov (Institute of Mineralogy and Petrography, Novosibirsk, Russia) for his assistance in the course of the work. The support from the Interdisciplinary Basic Research Programme of the Siberian Branch of the Russian Academy of Sciences (Project no. 72) and Science Support Foundation is acknowledged. References w1x G.S. Woods, Phil. Mag. B 50 (1984) 673. w2x K. De Corte, P. Cartigny, V.S. Shatsky, P. De Paepe, N.V. Sobolev, M. Javoy, Proceedings of the Seventh International Kimberlite Conference, April 13–17, Red Roof Design cc, Goodwood, Cape Town, South Africa, 1999, p. 174. w3x A.T. Collins, S.C. Lawson, Phil. Mag. Lett. 60 (1989) 117. w4x N.V. Surovtsev, I.N. Kupriyanov, V.K. Malinovsky, V.A. Gusev, Yu.N. Pal’yanov, J. Phys. Condens. Matter 11 (1999) 4767. w5x Yu.M. Borzdov, I.N. Kupriyanov, A.V. Efremov, Yu.N. Pal’yanov, Proceedings of the Fourth International Conference on Crystals: Growth, Properties, Real Structure, Application, October 18–22, vol. 1, VNIISIMS, Aleksandrov, Russia, 1999, p. 342 (in Russian). w6x H. Kanda, M. Akaishi, S. Yamaoka, Diamond Rel. Mater. 8 (1999) 1441. w7x Yu. Pal’yanov, Yu. Borzdov, I. Kupriyanov, V. Gusev, A. Khokhryakov, A. Sokol, Diamond Rel. Mater. 10 (2001) 2145. w8x Yu.N. Pal’yanov, A.F. Khokhryakov, Yu.M. Borzdov, et al., Russ. Geol. Geophys. 38 (1997) 920. w9x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, A.F. Shatsky, N.V. Sobolev, Diamond Rel. Mater. 8 (1999) 1118. w10x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, Lithos 60 (2002) 145. w11x A.M. Zaitsev, in: M. Prelas, G. Popovici, L. Bigelow (Eds.), Handbook of Industrial Diamonds and Films, Dekker, New York, 1998, pp. 227–376. w12x W.A. Runciman, T. Carter, Solid State Commun. 9 (1971) 315. w13x G.S. Woods, A.T. Collins, J. Phys. Chem. Solids 44 (1983) 471. w14x G. Davies, A.T. Collins, P. Spear, Solid State Commun. 49 (1984) 433. w15x I. Kiflawi, D. Fisher, H. Kanda, G. Sittas, Diamond Rel. Mater. 5 (1996) 1516. w16x K. Iakoubovskii, G.J. Adriaenssens, Diamond Rel. Mater. 11 (2002) 125. w17x G.Z. Cao, W.J.P. van Enckevort, L.J. Giling, R.C.M. de Kruif, Appl. Phys. Lett. 66 (1995) 688. w18x S.K. Singhal, H. Kanda, J. Cryst. Growth 154 (1995) 297. w19x Y. Wang, H. Kanda, Diamond Rel. Mater. 7 (1998) 57. w20x Yu.N. Pal’yanov, A.G. Sokol, Yu.M. Borzdov, A.F. Khokhryakov, N.V. Sobolev, Nature 400 (1999) 417. w21x M. Akaishi, M.D.S. Kumar, H. Kanda, S. Yamaoka, Proceedings of the Eighth NIRIM International Symposium on Advanced Materials, March 4–8, NIRIM, Tsukuba, Japan, 2001, p. 43. w22x A.G. Sokol, Y.N. Pal’yanov, G.A. Pal’yanova, A.F. Khokhryakov, Y.M. Borzdov, Diamond Rel. Mater. 10 (2001) 2131.