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Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology, CCL-SEM and CL spectroscopy studies
Diamond and Related Materials 8 (1999) 267–272
Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology, CCL-SEM and CL spectroscopy studies Yu.A. Litvin a,*, L.T. Chudinovskikh a, G.V. Saparin b, S.K. Obyden b, M.V. Chukichev b, V.S. Vavilov b a Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow, District 142432, Russia b Department of Physics, Moscow State University, Moscow 119899, Russia Received 27 July 1998; accepted 13 October 1998
Abstract Colorless octahedral diamonds up to 150 mm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na Mg(CO ) –graphite and NaKMg(CO ) –graphite systems at pressures of 8–10 GPa and temperatures of 2 32 32 1700–1800 °C. Seeded growth of carbonate–carbon (CC ) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic ‘‘metal–carbon’’ (MC ) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM ) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbonate-solution growth; Cathodoluminescence; Pressure; Scanning electron microscopy; Spectroscopy; Synthetic diamond
1. Introduction Crystallization of diamond in high alkaline–carbonate systems at high pressures and temperatures is of considerable promise for experimental modeling of diamond genesis in the Earth’s mantle [1]. Inclusions of high alkaline mineral phases, both silicate [2] and carbonatelike [3] were found in natural diamonds of the mantle origin. Alkaline carbonate–graphite systems Li CO –C, 2 3 K CO –C, Na CO –C [4] and K Mg(CO ) –C [1,5] 2 3 2 3 2 32 have been successfully used in the series of high pressure experiments on ‘‘carbonate–carbon’’ (CC ) diamond synthesis. Diamond is a promising material in the electronic industry, and this has stimulated a search for new methods of its synthesis. The objective of this work is the experimental study of CC diamond crystallization in Na Mg(CO ) – 2 32 graphite and NaKMg(CO ) –graphite systems at pres32 * Corresponding author. Fax: +7 95 913 2112; e-mail: [email protected]
sures of 8–10 GPa and temperatures of 1700–1800 °C. Scanning electron microscopy (SEM ), colour cathodoluminescence scanning electron microscopy (CCL-SEM ) and cathodoluminescence (CL) spectroscopy methods were used for characterization of the CC diamonds grown under conditions of spontaneous and seed-stimulated nucleation.
2. Experimental techniques High pressure experiments at 8–10 GPa have been carried out using an ‘‘anvil-with-hole’’ apparatus [6 ]. The high pressure cell configuration ( Fig. 1) is analogous to that previously described [1]. Pressure in the cell at room temperature was determined using Bi and Sn transducers (accuracy ±0.05 GPa). The temperature was measured by a Pt Rh /Pt Rh thermocouple 70 30 94 6 (accuracy ±5 °C ). The starting materials were homogeneous mixtures of pure graphite and carbonates Na Mg(CO ) and NaKMg(CO ) synthesized from 2 32 32
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Fig. 1. High pressure cell configuration. (1) Lithographic limestone container; (2) graphite tube heater (d=4–8 mm, h=8 mm); (3) graphite or MgO ceramics end plug; and (4) high pressure reaction zone.
homogeneous stoichiometric mixtures of Na CO , 2 3 K CO and MgCO at 2 GPa and 700 °C for 4–6 h. The 2 3 3 crystal morphology of synthetic CC diamonds was studied by SEM with the use of a CamScan electron microscope. Color CL images of the diamonds (gold evaporated) were obtained at room temperature using a highly sensitive CCL-SEM method [7]. Color CL was excited by a 40 keV electron beam in the impulse regime (average current 1 mA). The thickness of the luminescing layer was 3 mm. The image on the monitor screen was formed by real colors in accordance with the local CL spectrum for each pixel in the interval 400–800 nm. This technique is a unique way for demonstration of CL spectral distribution over the surfaces of diamond crystals (512×512 pixels×24 bit per 6 s). The CL spectra were measured at the liquid nitrogen temperature with a setup consisting of a pulsed electron gun (accelerating voltage 7–50 kV, impulse frequency 200 Hz, duration of impulse 0.4 ms, electrical current density in the impulse 10−3–20 A cm−2, diameter of the probe beam 0.1–0.3 mm),a vacuum cryostat (vacuum to 5×10−7 mmHg) for the samples under study, and a high resolution diffraction spectrometer (5 A mm−1).
3. Experimental results 3.1. Scanning electron microscopy morphology of diamonds grown in alkaline carbonate–carbon systems Experiments on crystallization of CC diamond in the Na Mg(CO ) –C and NaKMg(CO ) –C systems were 2 32 32 carried out with the powders of starting alkaline carbonates and graphite mixed in 1:1 weight ratio. Under conditions of high-pressure experiments, melting of the carbonates is congruent. Carbonate melts dissolve graphite forming supersaturated CC solutions. It was previously found [1] that the supersaturation of carbon solution in the alkaline carbonate melt in respect to diamond can be labile and metastable. As a result, labile solution field (LSF ) and metastable solution field (MSF ) are formed. Spontaneous nucleation and crystallization of diamond occur at the LSF conditions, and the seeded growth of diamond is compatible with the MSF conditions. It is possible to designate the position
Fig. 2. PT conditions of CC diamond crystallization in the Na Mg(CO ) –graphite system. (1) Graphite–diamond equilibrium 2 32 curve [20]; (2) rough position of the eutectic PT melting curve for the Na Mg(CO ) –carbon system; (3) field of MSF of carbon solution in 2 32 the Na Mg(CO ) melt with respect to diamond; (4) field of LSF; and 2 32 (5) boundary between the fields MSF and LSF. #, Run conditions for spontaneous diamond nucleation and crystallization; $, seeded diamond growth.
of the boundary between the LSF and MSF fields in the terms of pressure and temperature, and combine that with the PT equilibrium curve graphite–diamond and the PT melting curve of the carbonate solvent used for the high-pressure process of diamond synthesis ( Fig. 2). On experimental evidence, the position of the LSF–MSF boundary for the Na Mg(CO ) –C system is 2 32 steep and corresponds to 8.8 GPa at 1800 °C and 8.5 GPa at 1600 °C. It should be remembered that the boundary characterizes the conditions of diamond spontaneous nucleation and is kinetic by its very nature. Crystallization of CC diamonds with both spontaneous and seeded nucleation was realized at pressures of 8–10 GPa and temperatures of 1700–1800 °C within the PT conditions of the LSF field and the LSF–MSF boundary. Colorless diamond single crystals of preferentially octahedral habit up to 150 mm in size (Fig. 3a and b) and twins (Fig. 3c) were grown in the course of spontaneous crystallization at relatively lower pressures. At higher pressures (and supersaturations), crystalline aggregates ( Fig. 3d ) were formed. Cubo-octahedral MC diamonds (0.5–0.7 mm in size)
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(b)
(c)
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Fig. 3. SEM images of spontaneous CC diamonds crystallized from carbon solutions in alkaline CC melts at 8–10 GPa and 1700–1800 °C: (a) octahedral single crystals [NaKMg(CO ) –C system, 10.5 GPa, 1800 °C ]; (b) a face of octahedral single crystal [Na Mg(CO ) –C system, 9.5 GPa, 32 2 32 1700 °C ]; (c) twin crystal [Na Mg(CO ) –C system, 9 GPa, 1600 °C ]; and (d ) aggregate of CC diamonds [Na Mg(CO ) –C system, 10 GPa, 2 32 2 32 1800 °C ].
synthesized in the Ni–Mn–C system were used as the seed crystals for CC diamond growth from carbon solutions in carbonate melts. Seeds of natural Yakutian diamonds were applied as well. The newly formed diamond layers were deposited on both octahedral {111} and cubic {100} faces of the seed crystals (Fig. 4a). The growing layers on the {111} faces have octahedral orientation. As a result the plane faces are developed. Some peculiarities of the growth are shown in Fig. 4b–d. But the newly grown layers over cubic faces are formed with intimately contacted octahedral microcrystals (Fig. 4c and d ). Eventually the coarsely rugged faces are formed. Similar mode of deposition of CC diamond layers was observed when using natural diamonds as the seeds. Thus, the newly grown diamond material precipitated by layers on the surfaces in parallel to faces
{111}, and much the same way in the case of natural diamonds [8]. 3.2. Color cathodoluminescence scanning electron microscopy images The CCL–SEM study of spontaneous diamonds crystallized in the alkaline–carbonate melts showed either a lack of luminescence on the octahedral faces or red emission of extra low intensity. The red emission became more intensive towards octahedron tops, sometimes possessing small cubic shapes. The layers of CC diamond precipitated on the seed crystals of both MC synthetic and natural Yakutian diamond show no emission on their surface and yellow– green–red emission on cleavages.
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(a)
(b)
(c)
(d)
Fig. 4. MC diamond cubo-octahedral seed crystal overgrown with CC diamond layers in the NaKMg(CO ) –C system at 10 GPa, 1700 °C, 33 min: 32 (a) whole view of the resulting crystal; (b) growth forms on octahedral faces; (c) growth forms as smooth layers on octahedral face (at the top) and as octahedral micropyramids on cubic face (at the bottom), the faces are adjucent; and (d ) octahedral micropyramids on cubic face (top part of the crystal ) and smooth layers and growth trigons on octahedral face (central and bottom parts).
The CCL peculiarities were inherent in the CC diamonds: they are not proper for both natural and MC diamonds [9]. 3.3. Cathodoluminescence spectroscopy data CL spectra of CC diamonds have been obtained for both spontaneous and seed grown single crystals. All the CL patterns present three characteristic bands (Fig. 5). For the shortest wavelength interval, a wide, low intensity, smooth blue band with peak at 470 nm shifting from sample to sample within the 440–480 nm range is exhibited. Then, a green band with dominating structure of the peaks at 503.6±0.6, 512.6±0.6 and 521.6±0.6 nm is observed. The peak at 503.6 nm is a line of non-phonon transformation, and the two other are its phonon replicas with the phonon energy
41±2 MeV. In the longest-wavelength interval 575–640 nm, the nonphonon line at 576.3±0.5 nm and its phonon replicas with the phonon energy 40±3 MeV are seen. The green band is the most intensive for the CC diamonds. But the intensity of the bands is variable from sample to sample. Sometimes the peak 576 nm has a maximal intensity. Overall, the blue band showed the lowest intensity.
4. Discussion It is known [8] that natural diamonds grow by layers in parallel with the octahedral faces, whereas synthetic MC diamonds grow by layers in parallel with both octahedral and cubic faces. This accounts for the physi-
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Fig. 5. CL spectrum of CC diamonds.
cal inhomogeneity of the MC diamonds in connection with the capture of metallic solvent inclusions into the space between the growth pyramids and an irregular distribution of isomorphic impurities (e.g. nitrogen, nickel ) and vacancies within the different growth sectors. The preferential growing mechanism for the CC diamond is the deposition of newly formed material by layers in parallel with the octahedral faces much as in the case of natural growing. Crystallization of CC and MC diamonds is physicochemically similar. In general, the difference between solubilities of thermodynamically unstable graphite and stable diamond phases in the carbonatic or metallic melts for high-pressure conditions of diamond crystallization is responsible for the appearance of carbon solutions supersaturated with respect to diamond and formation of the motive force for diamond crystallization [10]. When the source of carbon is diamond material, the carbon supersaturated solutions necessary for diamond-to-diamond re-crystallization do not occur in the case of isothermal conditions in the sample, and the thermal gradient field should be induced to provide the motive force and supersaturation of carbon solution in the melt which are reasonable for diamond nucleation and crystallization. In the real experiments, when graphite is used as a carbon source, both motive forces of carbon transfer in the melt-solution are likely to work out: the difference between graphite and diamond solubilities in the alkaline CC and metal–carbon melts and temperature gradients in the reaction zone of the high
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pressure cell. The temperature gradients produce the additional supersaturation for the carbon solutions in the melts used. Similarly to the MC diamond crystallization, spontaneous CC diamond crystallization occurs in the PT field of LSF [1]. The seed stimulated nucleation of CC-diamond is bound with the field of MSF. The LSF and MSF fields as the important parts of the pressure– temperature–concentration (PTN ) diagram of supersaturation for the metal–carbon system at high pressures were designed previously [11]. CC diamond growth on the seed crystals was detected in the LSF as well. In this case, the seed growth was accompanied by spontaneous diamond crystallization. The specific CCL-SEM images of the synthetic CC diamonds were revealed [9]. The demonstration of the lack of luminescence by both spontaneous and seedgrown diamond material is of the greatest importance. The seeds of MC diamonds are characterized by the concentration of paramagnetic nitrogen centres within (0.5–3)×1019 at cm−3 [12] and belong to type Ib. The natural diamonds used as the seed crystals show a blue luminiscence color and belong to type Ia. Special tests showed that natural diamonds of type IIa (concentration of nitrogen impurity <1018 at cm−3) are characterized by the lack of CCL-SEM luminescence and bears a general resemblance to the CC diamonds. The CCLSEM technique used has a sensibility limit of nitrogen impurity detection at close to 1018 at cm−3. There are three major bands in the CL-spectra of the synthetic CC diamonds: (1) a broad nonstructural blue band within 445–480 nm (‘‘band A’’) having a maximal peak at 470 nm; intensity of the band is several times lower than in the case of the other bands observed in the 500–540 nm interval; (2) a green band with the dominating system of the 503 nm line which is a zero-phonon line, and two following peaks at 512.6 and 521 nm that appear as its phonon replica (H3-system); (3) a band system at 575–640 nm consisting of a zerophonon line at 576.3 nm and its phonon replicas with a phonon energy 40±3 MeV. The band system is reproducible in the CL spectra of the CC diamond layers grown on the octahedral {111} faces of both synthetic MC and natural diamond seed crystals. The green band line system at 503–515 nm is most intense for the greater part of spontaneous CC diamonds. For some particular samples, the peak 576.3 is most intense. The blue band was lowest in intensity for all the CC diamond samples. The distinctive feature of the synthetic CC and MC diamonds is the absence in CC diamonds of the 485 and 884 nm which are characteristic for the MC diamond synthesized with the use of the Ni solvent [13].
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The band A of the CC diamonds is characterized by a variable position of the emission maximum at 442.6–415.5 nm, much like that of diamonds of type IIa [14,15]. The 435 nm peak is interpreted on the base of a dislocation centre model [16 ]. The alternative is a model based on radiative electron displacement from the deep acceptor levels into the valency zone [17]. The line 503.4 nm is connected with a system with a 3H-centre and characteristic for the type IIb diamonds being stable >400 °C. ‘‘Vacancy’’ and ‘‘oxygenvacancy’’ models are proposed for the centre [18]. The 575 nm peak within the longest-wavelength region is generally supposed to be observed in diamonds containing nitrogen impurity in the case of both natural and synthetic MC diamonds. The concept of an interstitial nitrogen atom bound with a vacancy in the direction [001] was suggested as the centre model [19]. But, no correlation was found between the nitrogen content in diamonds and the appearance of the 575 nm centre, and a proposal was made that the centre may be formed without participation of nitrogen (e.g. with involvement of H and O )[12]. For the CC diamonds, 2 2 the intensity of the 575 nm peak varies from sample to sample up to the point of disappearance. It is believed that further studies of CC diamonds will give a closer approximation to a correct model of the 575 nm centre. Significant differences are observed between the synthetic CC and MC diamonds. These observations suggest that carbonate melts seems to play a role of screen for atmospheric nitrogen diffusion to the growth interface boundary protecting to a great extent the growing CC diamonds from nitrogen incorporation into the diamond real lattice.
5. Conclusions (1) Diamond was experimentally crystallized in carbon solutions in alkaline–carbonate melts in the systems Na Mg(CO ) –graphite and NaKMg(CO ) – 2 32 32 graphite at 8–10 GPa and 1700–1800 °C. Colorless CC diamond single crystals of octahedral habit up to 100–150 mm in size were grown. (2) The effects of CC diamond deposition on the octahedral {111} and cubic {100} faces of the MC diamond seed crystals were observed. The CC diamond layers precipitated on both octahedral {111} and cubic {100} faces of seed single crystals had octahedral orientation, much as in the case for the growing of natural diamonds. (3) Color CL investigation of CC diamonds showed a lack of emission at their surface as in natural
diamonds with lower concentrations of impure nitrogen (type II ). This is a distinctive feature of the synthetic CC diamonds in comparison with both MC diamonds and type I natural diamonds. (4) CL spectroscopy reveals the simultaneous luminescent systems H3, 484 and 575 nm, which, in case of the the H3 band structure resembles that of type IIa natural diamonds.
Acknowledgements This study was funded by the Russian Federal Program ‘‘Integration’’ (Project No. N250) and the Russian Foundation for Basic Research (Grant No. 96-05-64786). The authors would like to thank N.N. Korotayeva, E.V. Guseva and A. Lobus for their help.
References [1] Yu.A. Litvin, L.T. Chudinovskikh, V.A. Zharikov, Trans. (Dokl.) Russian Acad. Sci. A 355 (6) (1997) 908. [2] P.G. Novgorodov, G.P. Bulanova, L.A. Pavlova, V.N. Mikhailov, V.V. Ugarov, A.P. Shebanin, K.P. Argunov, Dokl. Akad. Nauk SSSR 310 (1990) 439 (in Russian) [3] O. Navon, I.D. Hutcheon, C.R. Rossman, G.J. Wasserburg, Nature 335 (1988) 784. [4] M. Akaishi, Diamond Relat. Mater. 2 (1993) 183. [5] T. Taniguchi, D. Dobson, A.P. Jones, R. Rabe, H.J. Milledge, J. Mater. Res. 11 (10) (1996) 1. [6 ] L.G. Khvostantsev, L.F. Vereshchagin, A.P. Novikov, High. Pres.-High Temp. 9 (1977) 637. [7] G.V. Saparin, S.K. Obyden, Electron Technol. 27 (1994) 111. [8] J.E. Shigley, E. Fritsch, C.M. Stockton, J.I. Koivula, C.W. Fryer, R.E. Kane, D.R. Hargett, C.W. Welch, Gems Gemol. 23 (1987) 187. [9] Yu.A. Litvin, L.T. Chudinovskikh, G.V. Saparin, S.K. Obyden, M.V. Chukichev, V.S. Vavilov, Scanning 20 (1998) 380. [10] Y.A. Litvin, Izv. Akad. Nauk SSSR, Neorg. Mater. 9 (1968) 637 (in Russian) [11] Yu.A. Litvin, Zapisky Vsesoyuzn. Mineral. Obshchestva 98 (1) (1969) 116 (in Russian) [12] A.S. Vishnevsky, V.G. Malogolovets, A.F. Nikityuk, L.S. Posun’ko, in: Sintetich. Almazy, vol. 3, Naukova Dumka, Kiev, 1975, p. 7. [13] M.H. Nazare, A.J. Nevas, Phys. Rev. B 3 (1991) 14196. [14] W.A. Patlow, J. Ruan, R.E. Witkowski, W.J. Choyke, D.S. Knight, J. Appl. Phys. 67 (1990) 7019. [15] N. Yamamoto, J.C.H. Spence, D. Fathy, Phil. Mag. 49 (6) (1984) 609. [16 ] J.F. Prins, Diamond Relat. Mater. 3 (1994) 922. [17] J. Ruan, K. Kobayashi, W.J. Choyke, Appl. Phys. Lett. 60 (1992) 3138. [18] J. Walker, Rep. Prog. Phys. 42 (1979) 1605. [19] A.M. Zaitsev, A. Uliyashin, H. Ali Noor, in: Superhard Materials, vol. 1, Naukova Dumka, Kiev, 1991, p. 18. [20] C.S. Kennedy, G.C. Kennedy, J. Geophys. Res. 8 (14) (1976) 2467.
Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology, CCL-SEM and CL spectroscopy studies Yu.A. Litvin a,*, L.T. Chudinovskikh a, G.V. Saparin b, S.K. Obyden b, M.V. Chukichev b, V.S. Vavilov b a Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow, District 142432, Russia b Department of Physics, Moscow State University, Moscow 119899, Russia Received 27 July 1998; accepted 13 October 1998
Abstract Colorless octahedral diamonds up to 150 mm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na Mg(CO ) –graphite and NaKMg(CO ) –graphite systems at pressures of 8–10 GPa and temperatures of 2 32 32 1700–1800 °C. Seeded growth of carbonate–carbon (CC ) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic ‘‘metal–carbon’’ (MC ) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM ) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbonate-solution growth; Cathodoluminescence; Pressure; Scanning electron microscopy; Spectroscopy; Synthetic diamond
1. Introduction Crystallization of diamond in high alkaline–carbonate systems at high pressures and temperatures is of considerable promise for experimental modeling of diamond genesis in the Earth’s mantle [1]. Inclusions of high alkaline mineral phases, both silicate [2] and carbonatelike [3] were found in natural diamonds of the mantle origin. Alkaline carbonate–graphite systems Li CO –C, 2 3 K CO –C, Na CO –C [4] and K Mg(CO ) –C [1,5] 2 3 2 3 2 32 have been successfully used in the series of high pressure experiments on ‘‘carbonate–carbon’’ (CC ) diamond synthesis. Diamond is a promising material in the electronic industry, and this has stimulated a search for new methods of its synthesis. The objective of this work is the experimental study of CC diamond crystallization in Na Mg(CO ) – 2 32 graphite and NaKMg(CO ) –graphite systems at pres32 * Corresponding author. Fax: +7 95 913 2112; e-mail: [email protected]
sures of 8–10 GPa and temperatures of 1700–1800 °C. Scanning electron microscopy (SEM ), colour cathodoluminescence scanning electron microscopy (CCL-SEM ) and cathodoluminescence (CL) spectroscopy methods were used for characterization of the CC diamonds grown under conditions of spontaneous and seed-stimulated nucleation.
2. Experimental techniques High pressure experiments at 8–10 GPa have been carried out using an ‘‘anvil-with-hole’’ apparatus [6 ]. The high pressure cell configuration ( Fig. 1) is analogous to that previously described [1]. Pressure in the cell at room temperature was determined using Bi and Sn transducers (accuracy ±0.05 GPa). The temperature was measured by a Pt Rh /Pt Rh thermocouple 70 30 94 6 (accuracy ±5 °C ). The starting materials were homogeneous mixtures of pure graphite and carbonates Na Mg(CO ) and NaKMg(CO ) synthesized from 2 32 32
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 31 8 - 5
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Fig. 1. High pressure cell configuration. (1) Lithographic limestone container; (2) graphite tube heater (d=4–8 mm, h=8 mm); (3) graphite or MgO ceramics end plug; and (4) high pressure reaction zone.
homogeneous stoichiometric mixtures of Na CO , 2 3 K CO and MgCO at 2 GPa and 700 °C for 4–6 h. The 2 3 3 crystal morphology of synthetic CC diamonds was studied by SEM with the use of a CamScan electron microscope. Color CL images of the diamonds (gold evaporated) were obtained at room temperature using a highly sensitive CCL-SEM method [7]. Color CL was excited by a 40 keV electron beam in the impulse regime (average current 1 mA). The thickness of the luminescing layer was 3 mm. The image on the monitor screen was formed by real colors in accordance with the local CL spectrum for each pixel in the interval 400–800 nm. This technique is a unique way for demonstration of CL spectral distribution over the surfaces of diamond crystals (512×512 pixels×24 bit per 6 s). The CL spectra were measured at the liquid nitrogen temperature with a setup consisting of a pulsed electron gun (accelerating voltage 7–50 kV, impulse frequency 200 Hz, duration of impulse 0.4 ms, electrical current density in the impulse 10−3–20 A cm−2, diameter of the probe beam 0.1–0.3 mm),a vacuum cryostat (vacuum to 5×10−7 mmHg) for the samples under study, and a high resolution diffraction spectrometer (5 A mm−1).
3. Experimental results 3.1. Scanning electron microscopy morphology of diamonds grown in alkaline carbonate–carbon systems Experiments on crystallization of CC diamond in the Na Mg(CO ) –C and NaKMg(CO ) –C systems were 2 32 32 carried out with the powders of starting alkaline carbonates and graphite mixed in 1:1 weight ratio. Under conditions of high-pressure experiments, melting of the carbonates is congruent. Carbonate melts dissolve graphite forming supersaturated CC solutions. It was previously found [1] that the supersaturation of carbon solution in the alkaline carbonate melt in respect to diamond can be labile and metastable. As a result, labile solution field (LSF ) and metastable solution field (MSF ) are formed. Spontaneous nucleation and crystallization of diamond occur at the LSF conditions, and the seeded growth of diamond is compatible with the MSF conditions. It is possible to designate the position
Fig. 2. PT conditions of CC diamond crystallization in the Na Mg(CO ) –graphite system. (1) Graphite–diamond equilibrium 2 32 curve [20]; (2) rough position of the eutectic PT melting curve for the Na Mg(CO ) –carbon system; (3) field of MSF of carbon solution in 2 32 the Na Mg(CO ) melt with respect to diamond; (4) field of LSF; and 2 32 (5) boundary between the fields MSF and LSF. #, Run conditions for spontaneous diamond nucleation and crystallization; $, seeded diamond growth.
of the boundary between the LSF and MSF fields in the terms of pressure and temperature, and combine that with the PT equilibrium curve graphite–diamond and the PT melting curve of the carbonate solvent used for the high-pressure process of diamond synthesis ( Fig. 2). On experimental evidence, the position of the LSF–MSF boundary for the Na Mg(CO ) –C system is 2 32 steep and corresponds to 8.8 GPa at 1800 °C and 8.5 GPa at 1600 °C. It should be remembered that the boundary characterizes the conditions of diamond spontaneous nucleation and is kinetic by its very nature. Crystallization of CC diamonds with both spontaneous and seeded nucleation was realized at pressures of 8–10 GPa and temperatures of 1700–1800 °C within the PT conditions of the LSF field and the LSF–MSF boundary. Colorless diamond single crystals of preferentially octahedral habit up to 150 mm in size (Fig. 3a and b) and twins (Fig. 3c) were grown in the course of spontaneous crystallization at relatively lower pressures. At higher pressures (and supersaturations), crystalline aggregates ( Fig. 3d ) were formed. Cubo-octahedral MC diamonds (0.5–0.7 mm in size)
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(b)
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Fig. 3. SEM images of spontaneous CC diamonds crystallized from carbon solutions in alkaline CC melts at 8–10 GPa and 1700–1800 °C: (a) octahedral single crystals [NaKMg(CO ) –C system, 10.5 GPa, 1800 °C ]; (b) a face of octahedral single crystal [Na Mg(CO ) –C system, 9.5 GPa, 32 2 32 1700 °C ]; (c) twin crystal [Na Mg(CO ) –C system, 9 GPa, 1600 °C ]; and (d ) aggregate of CC diamonds [Na Mg(CO ) –C system, 10 GPa, 2 32 2 32 1800 °C ].
synthesized in the Ni–Mn–C system were used as the seed crystals for CC diamond growth from carbon solutions in carbonate melts. Seeds of natural Yakutian diamonds were applied as well. The newly formed diamond layers were deposited on both octahedral {111} and cubic {100} faces of the seed crystals (Fig. 4a). The growing layers on the {111} faces have octahedral orientation. As a result the plane faces are developed. Some peculiarities of the growth are shown in Fig. 4b–d. But the newly grown layers over cubic faces are formed with intimately contacted octahedral microcrystals (Fig. 4c and d ). Eventually the coarsely rugged faces are formed. Similar mode of deposition of CC diamond layers was observed when using natural diamonds as the seeds. Thus, the newly grown diamond material precipitated by layers on the surfaces in parallel to faces
{111}, and much the same way in the case of natural diamonds [8]. 3.2. Color cathodoluminescence scanning electron microscopy images The CCL–SEM study of spontaneous diamonds crystallized in the alkaline–carbonate melts showed either a lack of luminescence on the octahedral faces or red emission of extra low intensity. The red emission became more intensive towards octahedron tops, sometimes possessing small cubic shapes. The layers of CC diamond precipitated on the seed crystals of both MC synthetic and natural Yakutian diamond show no emission on their surface and yellow– green–red emission on cleavages.
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(a)
(b)
(c)
(d)
Fig. 4. MC diamond cubo-octahedral seed crystal overgrown with CC diamond layers in the NaKMg(CO ) –C system at 10 GPa, 1700 °C, 33 min: 32 (a) whole view of the resulting crystal; (b) growth forms on octahedral faces; (c) growth forms as smooth layers on octahedral face (at the top) and as octahedral micropyramids on cubic face (at the bottom), the faces are adjucent; and (d ) octahedral micropyramids on cubic face (top part of the crystal ) and smooth layers and growth trigons on octahedral face (central and bottom parts).
The CCL peculiarities were inherent in the CC diamonds: they are not proper for both natural and MC diamonds [9]. 3.3. Cathodoluminescence spectroscopy data CL spectra of CC diamonds have been obtained for both spontaneous and seed grown single crystals. All the CL patterns present three characteristic bands (Fig. 5). For the shortest wavelength interval, a wide, low intensity, smooth blue band with peak at 470 nm shifting from sample to sample within the 440–480 nm range is exhibited. Then, a green band with dominating structure of the peaks at 503.6±0.6, 512.6±0.6 and 521.6±0.6 nm is observed. The peak at 503.6 nm is a line of non-phonon transformation, and the two other are its phonon replicas with the phonon energy
41±2 MeV. In the longest-wavelength interval 575–640 nm, the nonphonon line at 576.3±0.5 nm and its phonon replicas with the phonon energy 40±3 MeV are seen. The green band is the most intensive for the CC diamonds. But the intensity of the bands is variable from sample to sample. Sometimes the peak 576 nm has a maximal intensity. Overall, the blue band showed the lowest intensity.
4. Discussion It is known [8] that natural diamonds grow by layers in parallel with the octahedral faces, whereas synthetic MC diamonds grow by layers in parallel with both octahedral and cubic faces. This accounts for the physi-
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Fig. 5. CL spectrum of CC diamonds.
cal inhomogeneity of the MC diamonds in connection with the capture of metallic solvent inclusions into the space between the growth pyramids and an irregular distribution of isomorphic impurities (e.g. nitrogen, nickel ) and vacancies within the different growth sectors. The preferential growing mechanism for the CC diamond is the deposition of newly formed material by layers in parallel with the octahedral faces much as in the case of natural growing. Crystallization of CC and MC diamonds is physicochemically similar. In general, the difference between solubilities of thermodynamically unstable graphite and stable diamond phases in the carbonatic or metallic melts for high-pressure conditions of diamond crystallization is responsible for the appearance of carbon solutions supersaturated with respect to diamond and formation of the motive force for diamond crystallization [10]. When the source of carbon is diamond material, the carbon supersaturated solutions necessary for diamond-to-diamond re-crystallization do not occur in the case of isothermal conditions in the sample, and the thermal gradient field should be induced to provide the motive force and supersaturation of carbon solution in the melt which are reasonable for diamond nucleation and crystallization. In the real experiments, when graphite is used as a carbon source, both motive forces of carbon transfer in the melt-solution are likely to work out: the difference between graphite and diamond solubilities in the alkaline CC and metal–carbon melts and temperature gradients in the reaction zone of the high
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pressure cell. The temperature gradients produce the additional supersaturation for the carbon solutions in the melts used. Similarly to the MC diamond crystallization, spontaneous CC diamond crystallization occurs in the PT field of LSF [1]. The seed stimulated nucleation of CC-diamond is bound with the field of MSF. The LSF and MSF fields as the important parts of the pressure– temperature–concentration (PTN ) diagram of supersaturation for the metal–carbon system at high pressures were designed previously [11]. CC diamond growth on the seed crystals was detected in the LSF as well. In this case, the seed growth was accompanied by spontaneous diamond crystallization. The specific CCL-SEM images of the synthetic CC diamonds were revealed [9]. The demonstration of the lack of luminescence by both spontaneous and seedgrown diamond material is of the greatest importance. The seeds of MC diamonds are characterized by the concentration of paramagnetic nitrogen centres within (0.5–3)×1019 at cm−3 [12] and belong to type Ib. The natural diamonds used as the seed crystals show a blue luminiscence color and belong to type Ia. Special tests showed that natural diamonds of type IIa (concentration of nitrogen impurity <1018 at cm−3) are characterized by the lack of CCL-SEM luminescence and bears a general resemblance to the CC diamonds. The CCLSEM technique used has a sensibility limit of nitrogen impurity detection at close to 1018 at cm−3. There are three major bands in the CL-spectra of the synthetic CC diamonds: (1) a broad nonstructural blue band within 445–480 nm (‘‘band A’’) having a maximal peak at 470 nm; intensity of the band is several times lower than in the case of the other bands observed in the 500–540 nm interval; (2) a green band with the dominating system of the 503 nm line which is a zero-phonon line, and two following peaks at 512.6 and 521 nm that appear as its phonon replica (H3-system); (3) a band system at 575–640 nm consisting of a zerophonon line at 576.3 nm and its phonon replicas with a phonon energy 40±3 MeV. The band system is reproducible in the CL spectra of the CC diamond layers grown on the octahedral {111} faces of both synthetic MC and natural diamond seed crystals. The green band line system at 503–515 nm is most intense for the greater part of spontaneous CC diamonds. For some particular samples, the peak 576.3 is most intense. The blue band was lowest in intensity for all the CC diamond samples. The distinctive feature of the synthetic CC and MC diamonds is the absence in CC diamonds of the 485 and 884 nm which are characteristic for the MC diamond synthesized with the use of the Ni solvent [13].
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The band A of the CC diamonds is characterized by a variable position of the emission maximum at 442.6–415.5 nm, much like that of diamonds of type IIa [14,15]. The 435 nm peak is interpreted on the base of a dislocation centre model [16 ]. The alternative is a model based on radiative electron displacement from the deep acceptor levels into the valency zone [17]. The line 503.4 nm is connected with a system with a 3H-centre and characteristic for the type IIb diamonds being stable >400 °C. ‘‘Vacancy’’ and ‘‘oxygenvacancy’’ models are proposed for the centre [18]. The 575 nm peak within the longest-wavelength region is generally supposed to be observed in diamonds containing nitrogen impurity in the case of both natural and synthetic MC diamonds. The concept of an interstitial nitrogen atom bound with a vacancy in the direction [001] was suggested as the centre model [19]. But, no correlation was found between the nitrogen content in diamonds and the appearance of the 575 nm centre, and a proposal was made that the centre may be formed without participation of nitrogen (e.g. with involvement of H and O )[12]. For the CC diamonds, 2 2 the intensity of the 575 nm peak varies from sample to sample up to the point of disappearance. It is believed that further studies of CC diamonds will give a closer approximation to a correct model of the 575 nm centre. Significant differences are observed between the synthetic CC and MC diamonds. These observations suggest that carbonate melts seems to play a role of screen for atmospheric nitrogen diffusion to the growth interface boundary protecting to a great extent the growing CC diamonds from nitrogen incorporation into the diamond real lattice.
5. Conclusions (1) Diamond was experimentally crystallized in carbon solutions in alkaline–carbonate melts in the systems Na Mg(CO ) –graphite and NaKMg(CO ) – 2 32 32 graphite at 8–10 GPa and 1700–1800 °C. Colorless CC diamond single crystals of octahedral habit up to 100–150 mm in size were grown. (2) The effects of CC diamond deposition on the octahedral {111} and cubic {100} faces of the MC diamond seed crystals were observed. The CC diamond layers precipitated on both octahedral {111} and cubic {100} faces of seed single crystals had octahedral orientation, much as in the case for the growing of natural diamonds. (3) Color CL investigation of CC diamonds showed a lack of emission at their surface as in natural
diamonds with lower concentrations of impure nitrogen (type II ). This is a distinctive feature of the synthetic CC diamonds in comparison with both MC diamonds and type I natural diamonds. (4) CL spectroscopy reveals the simultaneous luminescent systems H3, 484 and 575 nm, which, in case of the the H3 band structure resembles that of type IIa natural diamonds.
Acknowledgements This study was funded by the Russian Federal Program ‘‘Integration’’ (Project No. N250) and the Russian Foundation for Basic Research (Grant No. 96-05-64786). The authors would like to thank N.N. Korotayeva, E.V. Guseva and A. Lobus for their help.
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