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Cobalt impurities in synthetic diamond
Diamond and Related Materials 8 (1999) 1895–1899 www.elsevier.com/locate/diamond
Cobalt impurities in synthetic diamond X. Jia a, *,1, S. Hayakawa b, W. Li a, Y. Gohshi b, M. Wakatsuki a a Institute of Materials Science, University of Tsukuba, Ibaraki 305-0006, Japan b Department of Applied Chemistry, Faculty of Engineering, University of Tokyo, Tokyo 113-8586, Japan Received 30 October 1998; accepted 18 May 1999
Abstract High-quality single crystals of diamond were grown by the temperature-gradient method using cobalt-containing metals as the solvent-catalyst. Cobalt and nickel impurities in the crystals were measured by X-ray fluorescence using synchrotron radiation. Their distributions were imaged by a mapping technique. Their concentrations were also measured at a number of points of the crystals. X-ray absorption near edge structure ( XANES ) spectroscopic measurements were carried out to investigate the bonding nature of the impurities. It was confirmed that the cobalt impurity distributes preferentially in the {111} sectors like nickel. We found that the concentration of cobalt is in proportional correlation with that of nickel in crystals grown from alloys containing both cobalt and nickel, and that cobalt is more difficult to incorporate than nickel. The XANES measurements revealed that the cobalt impurity occupies tetrahedral sites like nickel. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Impurity and growth sector; X-ray fluorescence
1. Introduction Metallic solvent-catalysts used for diamond growth under high pressure and temperature conditions are mainly composed of nickel, cobalt, iron, manganese and chromium [1–4]. Small quantities of aluminium, titanium or zirconium are added as nitrogen getters to the solvent-catalysts in order to reduce nitrogen concentrations in the grown diamond [3,4]. When these metallic elements are used, they may be incorporated into the growing diamond as atomic impurities as well as inclusions. The atomic-scale impurities usually modify the electronic structure of diamond, producing absorption and luminescence bands. It has been established that nickel and cobalt impurities are present as optically active centres [5–7]. We have reported that the X-ray fluorescence ( XRF ) technique using synchrotron radiation is sensitive enough to detect these impurities and that they are restricted only to {111} sectors, which is consistent with absorption and luminescence images [8–10]. In this report, the * Corresponding author. Tel.: +81-298-59-2608; fax: +81-298-59-2601. E-mail address: [email protected] ( X. Jia) 1 Present address: National Research Institute for Metals, 4th Research Group, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.
results of an additional X-ray fluorescence study are given: (1) a quantitative analysis of the concentrations and (2) the chemical state of the cobalt impurity examined by X-ray adsorption near edge structure ( XANES).
2. Experimental High-quality single crystals of diamond were grown by the temperature-gradient method at 5.5 GPa and 1200–1300°C for 20–40 h [11]. Pure cobalt (99.9%), Fe Ni Co and Ni Mn Co alloys were used as the 55 29 16 70 25 5 solvent-catalysts. Some data on the crystals investigated in this study are listed in Table 1. Crystal sizes are 2 to 3 mm, but it is noted that sample L was grown at a rate more than three times higher than the others. The crystals were mechanically polished to thin plates with a thickness of 150 to 250 mm. The X-ray fluorescence measurements were carried out by using synchrotron radiation monochromatized at 9 keV at the Photon Factory of KEK (National Laboratory for High Energy Physics). Details of the measurement technique have been presented elsewhere [8]. The X-ray beam, focused with an ellipsoidal mirror, was irradiated through a 100 mm×100 mm aperture. The microscopic fluorescence spectra were taken at different locations with steps of 100 mm to obtain two-dimen-
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X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
Table 1 Samples for X-ray fluorescence analyses Sample
Metal-solvent
Temperature (°C )
Growth rate (mg h−1)
Growth sector
L J S O R
Fe Ni Co 55 29 16 Fe Ni Co 55 29 16 Fe Ni Co 55 29 16 Ni Mn Co 70 25 5 Co (99.9%)
1200 1200 1200 1200 1300
2.3 0.8 0.8 0.8 1.0
{100}, {100}, {100}, {100}, {100},
{111}, {111}, {111}, {111}, {111},
{110} {110} {311} {110} {311}
sional mapping data. Precise values of concentrations of nickel and cobalt were measured with a larger time constant, 4 to 10 min, at selected points. The detection limit of this technique is 0.05 ppm. XANES spectra were taken by energy scanning of the incident X-rays.
3. Results and discussion 3.1. Relationship between concentrations of cobalt and nickel XRF mapping images of sample L, as shown in Figs. 1a–d, clearly show that the cobalt impurities are located in {111} growth sectors like the nickel. On the other hand, the intensity of the iron signal is low and its sector dependence cannot be seen clearly in Fig. 1b, indicating that the amount of iron impurity is too low to be detected by this method. Negligible amounts of iron are present in diamond in spite of its being a major component of the alloy. On the other hand, the presence of cobalt and nickel is distinct. The difference allows us to conclude definitely that the cobalt and nickel detected in the diamond are not metallic inclusions, because metallic inclusions should have the same composition as the solvent-catalyst. Results of quantitative measurements of cobalt and nickel impurities at points shown in Fig. 1e are presented in Table 2. The result confirms that cobalt and nickel are located in {111} sectors. Their concentrations are above the detection limit even in the other sectors. However, it is too early to conclude that they are present in all sectors, because the measurements may detect nickel and cobalt in adjacent {111} sectors. The cobalt concentrations are plotted against nickel concentrations in Fig. 2. This indicates that their concentrations are in a proportional relation in {111} sectors, although both nickel and cobalt concentrations are variable between 7.76 and 31.0 ppm and 1.57 and 4.77 ppm, respectively, suggesting that nickel and cobalt are incorporated in the same manner. However, regarding sectors other than {111}, the concentrations of cobalt and nickel are too low to conclude the relationship. The scattering of the concentrations of nickel and cobalt is due to their inhomogeneous distribution. It has been shown that a change of growth rate causes
Fig. 1. X-ray images and points analysis for sample L: (a) incident X-ray; (b) iron; (c) cobalt; (d ) nickel; and (e) points of quantitative analysis. The scanned width was 4 mm.
non-uniform distribution of nickel and cobalt even in the same {111} sector [7]. Fig. 3 shows the correlation for other specimens, J and S, grown from the same composition of alloy. The same composition of alloy is seen to give the same ratio of cobalt concentration to that of nickel. XRF measurement was attempted for crystals grown from an alloy containing a smaller amount of cobalt, i.e., Ni Mn Co . As shown in Figs. 4a–d, cobalt was 70 25 5 detected clearly in {111} sectors. However, a strong intensity of the manganese signal appeared only in the seed side and its sector dependence was not same as
X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
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Table 2 Concentrations of nickel and cobalt impurities in sample L Point
Growth sector
[Ni ] (ppm)
[Co] (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Origin point {111} {111} {111} {111} {111} {111} {111}+{100} {111} {110} {110} {100} {111} {100} {100} {100} {100} {100} {111} {100}+{111} {110}+{111} {100} {100} {111}
0.09 23.3 24.5 27.7 16.7 21.2 20.6 14.5 25.5 1.19 1.02 1.01 7.76 0.72 0.76 0.85 0.44 0.75 31.0 3.87 4.97 1.11 1.11 19.6
0.15 3.23 3.20 3.56 2.51 3.92 2.97 3.12 3.47 0.79 0.90 0.83 1.57 0.65 0.24 0.88 0.58 0.76 4.77 1.14 1.43 0.82 0.70 3.32
Fig. 4. X-ray images and points analysis for sample O: (a) nickel; (b) cobalt; (c) manganese; (d) incident X-rays; and (e) points of quantitative analysis. The scanned width was 4 mm.
Fig. 2. Relationship between the concentrations of cobalt and nickel impurities in various sectors for sample L.
Fig. 3. Relationship between concentrations of cobalt and nickel impurities in {111} sectors for samples L, J and S.
that shown for nickel in Fig. 4a. This result indicates that the distribution behaviour of the manganese impurity is different from that of nickel or cobalt. The difference between the images of cobalt/nickel and that of manganese again gives evidence that the cobalt or nickel detected here is not from metallic inclusions. Quantitative measurements at positions indicated in Fig. 4e are listed in Table 3. Concentrations of cobalt are much lower for sample O than for samples L, J and S, whereas nickel concentrations are almost the same in the four samples. This clearly shows that the cobalt content in the metallic solvent-catalyst is reflected in the cobalt concentration in the grown diamond. The values in Table 3 are plotted in Fig. 5. It is clearly seen that proportional correlation between cobalt and nickel concentrations is kept in {111} sectors, even for a crystal grown from a different composition of alloy. Regarding the crystal grown from pure cobalt, the same sector dependence of the cobalt impurity was
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X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
Table 3 Concentrations of nickel and cobalt impurities in sample O Point
Growth sector
[Ni ] (ppm)
[Co] (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
{111} {111} {110} {111} {111} {110} {110} {111}+{110} {110} {111} {111}+{100} {111} a {100} {100} {100} {100} {100} {100} {100} {100} {100} {100} {100}b
31.8 19.2 1.39 8.9 12.9 1.08 1.09 1.14 0.81 18.6 15.2 30.0 496 2.23 1.99 2.71 0.21 0.18 0.25 0.26 1.11 1.78 0.27 1.25
0.63 0.32 <0.05 <0.05 0.33 <0.05 0.13 <0.05 <0.05 0.39 0.40 0.50 22.0 <0.05 0.18 <0.05 0.18 0.19 <0.05 <0.05 0.17 0.19 0.23 0.13
a On an inclusion. b Near the boundary.
confirmed as shown in Fig. 6c. Table 4 gives cobalt concentrations at various points in Fig. 6e. The concentrations in {111} sectors are much higher than those of crystals grown from the alloys described above. The highest value (detected at the position with strongest image signal on the mapping measurement) of cobalt concentrations (point 7) is 18.3 ppm in the {111} sector. 3.2. Correlation of Co/Ni ratio in the {111} sector with compositional ratio of the alloys A simple calculation was carried out to investigate the correlation of the Co/Ni concentration ratio in {111} sectors with the compositional ratio of the alloys.
Fig. 6. X-ray images and point analysis for sample R: (a) incident X-ray; (b) iron; (c) Co Ka; (d) Co Kb; and (e) points of quantitative analysis. The scanned width was 4 mm.
Let a represent the Co/Ni concentration ratio in {111} sectors, b represent the Co/Ni compositional ratio in the alloys and c represent the ratio a/b. Averaged values of a were used for the calculation, which were obtained by grade values shown in Figs 3 and 5, respectively. Table 5 shows the result of the calculation. We found that the values of the correlation factor c are smaller than 1 and nearly consistent for the four samples, Table 4 Concentrations of cobalt impurity in sample R
Fig. 5. Relationship between concentrations of cobalt and nickel impurities in various sectors for sample O.
Point
Growth sector
[Co] (ppm)
1 2 3 4 5 6 7 8 9
{311} {311} {311}+{111} {100} {100} {111} {111} {111} {100}
1.4 <0.1 5.7 <0.1 0.8 12.6 18.3 5.3 13.5
X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899 Table 5 Correlation between a values in the {111} sector of diamond and b values in the alloys Sample
a
b
c
L J S O
0.12 0.14 0.14 0.021
0.55 0.55 0.55 0.071
0.22 0.25 0.25 0.30
1899
[8]; the pre-edge peak has been confirmed as being due to nickel atoms occupying a tetrahedral site of diamond. Because the XANES spectrum shown in Fig. 7 has peaks in the pre-edge region, and gives the same pattern as nickel, we conclude that cobalt is dissolved in diamond and occupies the same tetrahedral sites as does nickel.
4. Summary although use of averaged a values for the calculations meant that they were not so precise. This result implies that: (1) the composition ratio of cobalt to nickel in an alloy would determine the ratio of cobalt to nickel concentrations in {111} sectors of diamond; and (2) that cobalt is more difficult to incorporate than nickel. 3.3. XANES measurement for cobalt The XANES measurement was carried out on sample R, to investigate the chemical state of dissolved cobalt. The XANES spectrum for cobalt measured at an inclusion-free position is shown in Fig. 7. As reference, a XANES spectrum for dissolved nickel is shown in Fig. 8
Direct evidence has been given showing that cobalt is incorporated into diamond from the metal-solvent. It was confirmed that the cobalt impurity distributes preferentially in the {111} growth sector, like nickel. The concentrations of cobalt are in proportional correlation with those of nickel in crystals grown from alloys containing both cobalt and nickel. However, cobalt is more difficult to incorporate than nickel. The highest concentration measured at a variety of locations was 20 ppm. The cobalt impurity occupies tetrahedral sites like nickel.
Acknowledgements The authors are grateful to Ogura Jewel Industry Co., Ltd for kind help in cutting and polishing the samples. The experiments using synchrotron radiation were performed as the subjects, 90-202 and 92G247. This study was supported by Grants-in-Aid for Priority Area Research (03204003, 04204002) and by a Grantin-Aid for Encouragement of Young Scientists (04780060) from the Ministry of Education, Science and Culture of Japan.
References Fig. 7. Cobalt XANES spectrum obtained from a small region in sample R where the cobalt concentration is 18.6 ppm.
Fig. 8. Nickel XANES spectrum (after Hayakawa et al. [8]).
[1] H.P. Bovenkerk, F.P. Bundy, H.T. Hall, H.M. Strong, R.H. Wentrof, Nature 184 (1959) 1097. [2] R.H. Wentorf Jr., Ber. 70 (1966) 975. [3] R.H. Wentorf Jr., J. Phys. Chem. 75 (1971) 1833. [4] H.M. Strong, R.M. Cherenko, J. Phys. Chem. 75 (1971) 1838. [5] A.T. Collins, H. Kanada, R.C. Burns, Phys. Mag. B 61 (1990) 797. [6 ] S. Lawson, H. Kanada, K. Watanabe, I. Kiflawi, Y. Sato, J. Appl. Phys. 79 (1996) 4348. [7] H. Kanda, K. Watanabe, Diamond Relat. Mater. 6 (1997) 708. [8] M. Wakatsuki, S. Hayakawa, S. Aoki, Y. Gohshi, A. Iida, in: R. Messier et al. (Eds.), New Diamond Science and Technology, MRS, Pittsburgh, PA, 1991, p. 143. [9] X. Jia, H. Kagi, S. Hayakawa, M. Wakatsuki, Y. Gohshi, Proc. 4th ICNDST, MYU, Tokyo, 1994, p. 525. [10] X. Jia, H. Kagi, W. Li, M. Wakatsuki, S. Hayakawa, Y. Gohshi, Proc. 15th Int. Conf. On High Pressure Science and Technology, Ptc. Ltd., Warsaw, 1996, p. 565. [11] X. Jia, M. Wakatsuki,ISAM ’96, Proc. 3rd NIRIM International Symposium on Advanced Materials, Japan, NIRIM, Tsukuba, 1996, p. 267.
Cobalt impurities in synthetic diamond X. Jia a, *,1, S. Hayakawa b, W. Li a, Y. Gohshi b, M. Wakatsuki a a Institute of Materials Science, University of Tsukuba, Ibaraki 305-0006, Japan b Department of Applied Chemistry, Faculty of Engineering, University of Tokyo, Tokyo 113-8586, Japan Received 30 October 1998; accepted 18 May 1999
Abstract High-quality single crystals of diamond were grown by the temperature-gradient method using cobalt-containing metals as the solvent-catalyst. Cobalt and nickel impurities in the crystals were measured by X-ray fluorescence using synchrotron radiation. Their distributions were imaged by a mapping technique. Their concentrations were also measured at a number of points of the crystals. X-ray absorption near edge structure ( XANES ) spectroscopic measurements were carried out to investigate the bonding nature of the impurities. It was confirmed that the cobalt impurity distributes preferentially in the {111} sectors like nickel. We found that the concentration of cobalt is in proportional correlation with that of nickel in crystals grown from alloys containing both cobalt and nickel, and that cobalt is more difficult to incorporate than nickel. The XANES measurements revealed that the cobalt impurity occupies tetrahedral sites like nickel. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Impurity and growth sector; X-ray fluorescence
1. Introduction Metallic solvent-catalysts used for diamond growth under high pressure and temperature conditions are mainly composed of nickel, cobalt, iron, manganese and chromium [1–4]. Small quantities of aluminium, titanium or zirconium are added as nitrogen getters to the solvent-catalysts in order to reduce nitrogen concentrations in the grown diamond [3,4]. When these metallic elements are used, they may be incorporated into the growing diamond as atomic impurities as well as inclusions. The atomic-scale impurities usually modify the electronic structure of diamond, producing absorption and luminescence bands. It has been established that nickel and cobalt impurities are present as optically active centres [5–7]. We have reported that the X-ray fluorescence ( XRF ) technique using synchrotron radiation is sensitive enough to detect these impurities and that they are restricted only to {111} sectors, which is consistent with absorption and luminescence images [8–10]. In this report, the * Corresponding author. Tel.: +81-298-59-2608; fax: +81-298-59-2601. E-mail address: [email protected] ( X. Jia) 1 Present address: National Research Institute for Metals, 4th Research Group, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.
results of an additional X-ray fluorescence study are given: (1) a quantitative analysis of the concentrations and (2) the chemical state of the cobalt impurity examined by X-ray adsorption near edge structure ( XANES).
2. Experimental High-quality single crystals of diamond were grown by the temperature-gradient method at 5.5 GPa and 1200–1300°C for 20–40 h [11]. Pure cobalt (99.9%), Fe Ni Co and Ni Mn Co alloys were used as the 55 29 16 70 25 5 solvent-catalysts. Some data on the crystals investigated in this study are listed in Table 1. Crystal sizes are 2 to 3 mm, but it is noted that sample L was grown at a rate more than three times higher than the others. The crystals were mechanically polished to thin plates with a thickness of 150 to 250 mm. The X-ray fluorescence measurements were carried out by using synchrotron radiation monochromatized at 9 keV at the Photon Factory of KEK (National Laboratory for High Energy Physics). Details of the measurement technique have been presented elsewhere [8]. The X-ray beam, focused with an ellipsoidal mirror, was irradiated through a 100 mm×100 mm aperture. The microscopic fluorescence spectra were taken at different locations with steps of 100 mm to obtain two-dimen-
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 1 6 4- 8
1896
X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
Table 1 Samples for X-ray fluorescence analyses Sample
Metal-solvent
Temperature (°C )
Growth rate (mg h−1)
Growth sector
L J S O R
Fe Ni Co 55 29 16 Fe Ni Co 55 29 16 Fe Ni Co 55 29 16 Ni Mn Co 70 25 5 Co (99.9%)
1200 1200 1200 1200 1300
2.3 0.8 0.8 0.8 1.0
{100}, {100}, {100}, {100}, {100},
{111}, {111}, {111}, {111}, {111},
{110} {110} {311} {110} {311}
sional mapping data. Precise values of concentrations of nickel and cobalt were measured with a larger time constant, 4 to 10 min, at selected points. The detection limit of this technique is 0.05 ppm. XANES spectra were taken by energy scanning of the incident X-rays.
3. Results and discussion 3.1. Relationship between concentrations of cobalt and nickel XRF mapping images of sample L, as shown in Figs. 1a–d, clearly show that the cobalt impurities are located in {111} growth sectors like the nickel. On the other hand, the intensity of the iron signal is low and its sector dependence cannot be seen clearly in Fig. 1b, indicating that the amount of iron impurity is too low to be detected by this method. Negligible amounts of iron are present in diamond in spite of its being a major component of the alloy. On the other hand, the presence of cobalt and nickel is distinct. The difference allows us to conclude definitely that the cobalt and nickel detected in the diamond are not metallic inclusions, because metallic inclusions should have the same composition as the solvent-catalyst. Results of quantitative measurements of cobalt and nickel impurities at points shown in Fig. 1e are presented in Table 2. The result confirms that cobalt and nickel are located in {111} sectors. Their concentrations are above the detection limit even in the other sectors. However, it is too early to conclude that they are present in all sectors, because the measurements may detect nickel and cobalt in adjacent {111} sectors. The cobalt concentrations are plotted against nickel concentrations in Fig. 2. This indicates that their concentrations are in a proportional relation in {111} sectors, although both nickel and cobalt concentrations are variable between 7.76 and 31.0 ppm and 1.57 and 4.77 ppm, respectively, suggesting that nickel and cobalt are incorporated in the same manner. However, regarding sectors other than {111}, the concentrations of cobalt and nickel are too low to conclude the relationship. The scattering of the concentrations of nickel and cobalt is due to their inhomogeneous distribution. It has been shown that a change of growth rate causes
Fig. 1. X-ray images and points analysis for sample L: (a) incident X-ray; (b) iron; (c) cobalt; (d ) nickel; and (e) points of quantitative analysis. The scanned width was 4 mm.
non-uniform distribution of nickel and cobalt even in the same {111} sector [7]. Fig. 3 shows the correlation for other specimens, J and S, grown from the same composition of alloy. The same composition of alloy is seen to give the same ratio of cobalt concentration to that of nickel. XRF measurement was attempted for crystals grown from an alloy containing a smaller amount of cobalt, i.e., Ni Mn Co . As shown in Figs. 4a–d, cobalt was 70 25 5 detected clearly in {111} sectors. However, a strong intensity of the manganese signal appeared only in the seed side and its sector dependence was not same as
X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
1897
Table 2 Concentrations of nickel and cobalt impurities in sample L Point
Growth sector
[Ni ] (ppm)
[Co] (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Origin point {111} {111} {111} {111} {111} {111} {111}+{100} {111} {110} {110} {100} {111} {100} {100} {100} {100} {100} {111} {100}+{111} {110}+{111} {100} {100} {111}
0.09 23.3 24.5 27.7 16.7 21.2 20.6 14.5 25.5 1.19 1.02 1.01 7.76 0.72 0.76 0.85 0.44 0.75 31.0 3.87 4.97 1.11 1.11 19.6
0.15 3.23 3.20 3.56 2.51 3.92 2.97 3.12 3.47 0.79 0.90 0.83 1.57 0.65 0.24 0.88 0.58 0.76 4.77 1.14 1.43 0.82 0.70 3.32
Fig. 4. X-ray images and points analysis for sample O: (a) nickel; (b) cobalt; (c) manganese; (d) incident X-rays; and (e) points of quantitative analysis. The scanned width was 4 mm.
Fig. 2. Relationship between the concentrations of cobalt and nickel impurities in various sectors for sample L.
Fig. 3. Relationship between concentrations of cobalt and nickel impurities in {111} sectors for samples L, J and S.
that shown for nickel in Fig. 4a. This result indicates that the distribution behaviour of the manganese impurity is different from that of nickel or cobalt. The difference between the images of cobalt/nickel and that of manganese again gives evidence that the cobalt or nickel detected here is not from metallic inclusions. Quantitative measurements at positions indicated in Fig. 4e are listed in Table 3. Concentrations of cobalt are much lower for sample O than for samples L, J and S, whereas nickel concentrations are almost the same in the four samples. This clearly shows that the cobalt content in the metallic solvent-catalyst is reflected in the cobalt concentration in the grown diamond. The values in Table 3 are plotted in Fig. 5. It is clearly seen that proportional correlation between cobalt and nickel concentrations is kept in {111} sectors, even for a crystal grown from a different composition of alloy. Regarding the crystal grown from pure cobalt, the same sector dependence of the cobalt impurity was
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X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899
Table 3 Concentrations of nickel and cobalt impurities in sample O Point
Growth sector
[Ni ] (ppm)
[Co] (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
{111} {111} {110} {111} {111} {110} {110} {111}+{110} {110} {111} {111}+{100} {111} a {100} {100} {100} {100} {100} {100} {100} {100} {100} {100} {100}b
31.8 19.2 1.39 8.9 12.9 1.08 1.09 1.14 0.81 18.6 15.2 30.0 496 2.23 1.99 2.71 0.21 0.18 0.25 0.26 1.11 1.78 0.27 1.25
0.63 0.32 <0.05 <0.05 0.33 <0.05 0.13 <0.05 <0.05 0.39 0.40 0.50 22.0 <0.05 0.18 <0.05 0.18 0.19 <0.05 <0.05 0.17 0.19 0.23 0.13
a On an inclusion. b Near the boundary.
confirmed as shown in Fig. 6c. Table 4 gives cobalt concentrations at various points in Fig. 6e. The concentrations in {111} sectors are much higher than those of crystals grown from the alloys described above. The highest value (detected at the position with strongest image signal on the mapping measurement) of cobalt concentrations (point 7) is 18.3 ppm in the {111} sector. 3.2. Correlation of Co/Ni ratio in the {111} sector with compositional ratio of the alloys A simple calculation was carried out to investigate the correlation of the Co/Ni concentration ratio in {111} sectors with the compositional ratio of the alloys.
Fig. 6. X-ray images and point analysis for sample R: (a) incident X-ray; (b) iron; (c) Co Ka; (d) Co Kb; and (e) points of quantitative analysis. The scanned width was 4 mm.
Let a represent the Co/Ni concentration ratio in {111} sectors, b represent the Co/Ni compositional ratio in the alloys and c represent the ratio a/b. Averaged values of a were used for the calculation, which were obtained by grade values shown in Figs 3 and 5, respectively. Table 5 shows the result of the calculation. We found that the values of the correlation factor c are smaller than 1 and nearly consistent for the four samples, Table 4 Concentrations of cobalt impurity in sample R
Fig. 5. Relationship between concentrations of cobalt and nickel impurities in various sectors for sample O.
Point
Growth sector
[Co] (ppm)
1 2 3 4 5 6 7 8 9
{311} {311} {311}+{111} {100} {100} {111} {111} {111} {100}
1.4 <0.1 5.7 <0.1 0.8 12.6 18.3 5.3 13.5
X. Jia et al. / Diamond and Related Materials 8 (1999) 1895–1899 Table 5 Correlation between a values in the {111} sector of diamond and b values in the alloys Sample
a
b
c
L J S O
0.12 0.14 0.14 0.021
0.55 0.55 0.55 0.071
0.22 0.25 0.25 0.30
1899
[8]; the pre-edge peak has been confirmed as being due to nickel atoms occupying a tetrahedral site of diamond. Because the XANES spectrum shown in Fig. 7 has peaks in the pre-edge region, and gives the same pattern as nickel, we conclude that cobalt is dissolved in diamond and occupies the same tetrahedral sites as does nickel.
4. Summary although use of averaged a values for the calculations meant that they were not so precise. This result implies that: (1) the composition ratio of cobalt to nickel in an alloy would determine the ratio of cobalt to nickel concentrations in {111} sectors of diamond; and (2) that cobalt is more difficult to incorporate than nickel. 3.3. XANES measurement for cobalt The XANES measurement was carried out on sample R, to investigate the chemical state of dissolved cobalt. The XANES spectrum for cobalt measured at an inclusion-free position is shown in Fig. 7. As reference, a XANES spectrum for dissolved nickel is shown in Fig. 8
Direct evidence has been given showing that cobalt is incorporated into diamond from the metal-solvent. It was confirmed that the cobalt impurity distributes preferentially in the {111} growth sector, like nickel. The concentrations of cobalt are in proportional correlation with those of nickel in crystals grown from alloys containing both cobalt and nickel. However, cobalt is more difficult to incorporate than nickel. The highest concentration measured at a variety of locations was 20 ppm. The cobalt impurity occupies tetrahedral sites like nickel.
Acknowledgements The authors are grateful to Ogura Jewel Industry Co., Ltd for kind help in cutting and polishing the samples. The experiments using synchrotron radiation were performed as the subjects, 90-202 and 92G247. This study was supported by Grants-in-Aid for Priority Area Research (03204003, 04204002) and by a Grantin-Aid for Encouragement of Young Scientists (04780060) from the Ministry of Education, Science and Culture of Japan.
References Fig. 7. Cobalt XANES spectrum obtained from a small region in sample R where the cobalt concentration is 18.6 ppm.
Fig. 8. Nickel XANES spectrum (after Hayakawa et al. [8]).
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