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Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal-carbon system
Diamond & Related Materials 14 (2005) 1932 – 1935 www.elsevier.com/locate/diamond
Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal-carbon system Z.Z. Liang a, X. Jia a,b, H.A. Ma a, C.Y. Zang a, P.W. Zhu a, Q.F. Guan a, H. Kanda c,* a
c
National Lab of Superhard Materials, Jilin University, Changchun 130012, China b Henan Polytechnic University, Jiaozuo 454000, China National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Available online 8 August 2005
Abstract Diamond crystals have been synthesized from FeNi alloy — carbon system under high pressure and temperature of 5.0 to 5.8 GPa and 1500 to 1750 K. Concentrations of the nitrogen impurities of the crystals have been determined from FTIR spectra, and it is found that the concentrations of single substitutional nitrogen impurities exceeds 1000 ppm, when NaN3 is added as dopant to the metal-carbon system. The crystals are 0.2 – 0.5 mm in diameter with cubo – octahedral morphology, although octahedron and cube were also found in the diamond products. They exhibit yellow green to deep green color. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond; Nitrogen impurity; HPHT
1. Introduction Nitrogen is the most dominant impurity found in diamond, and optical properties of diamond directly depend on its concentration and structures [1– 4]. Study on the nitrogen impurities has long history. The nitrogen is present in several different forms in diamond [1]. It is well known that diamond is classified into type IaA, IaB, Ib, IIa and IIb, based on the form of nitrogen and boron. Type IaA and IaB diamond contain nitrogen atoms in forms of pairs and four atoms accompanying vacancy, respectively, and type Ib contains nitrogen in a single substitutional form. On the other hand, type II diamond contains little nitrogen atoms undetectable with FTIR spectroscopy. The concentrations of nitrogen impurities vary from stone to stone. Nitrogen rich diamonds in which the nitrogen concentrations exceed 1000 ppm have commonly been found in natural diamond. The nitrogen impurities are present in the aggregated form, i.e. typ Ia. Such a nitrogen rich diamond has been synthesized [5]. On the other hand, type Ib diamond usually contains up * Corresponding author. Tel.: +81 29 860 4305; fax: +81 29 851 4005. E-mail address: [email protected] (H. Kanda). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.06.041
to 200 ppm, which are synthesized in the conventional way under high pressure and high temperature conditions. The highest concentrations of nitrogen reported so far are 800 ppm [6]. It is an interesting topic to know how the high concentrations of nitrogen are incorporated into diamond. The diamond is expected to show some unusual properties. In the present work, we attempted to synthesize nitrogen rich diamond crystals. NaN3 was used as dopant of nitrogen in NiFe alloy — carbon system to grow the diamond under high pressure and temperature conditions. Diamond crystals containing single substitutional nitrogen exceeding 1000 ppm were successfully grown.
2. Experimental We used scale like graphite powders as carbon source, Fe90Ni10 alloy with 75 Am in diameter as solvent-catalyst and NaN3 (99.99%) as nitrogen dopant. After mixing the powders for 4 h, they were shaped with the form of disk to fit in a cylindrical space surrounded by a ceramic material [7]. Composition of the graphite and the alloy was fixed to be 1 : 1, and the NaN3 additive varied from 0 to 0.5 wt.%.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
The sample assembly was compressed in a cubic anvil high pressure and high temperature apparatus (SPD-6 1200) with a sample chamber of 23 mm on an edge, and then heated for 15 min. Pressure and temperature were 5.0 to 5.8 GPa and 1500 to 1750 K. Higher pressure and temperature were required to synthesize diamond, when NaN3 was added to the metal-carbon system. Nucleation density of diamond crystals tends to decrease with an increase of the NaN3 additive at a fixed P-T condition. The samples recovered from the high pressure vessel were treated in acids to isolate grown diamond from unconverted graphite and the metal solvent catalyst. The concentrations of nitrogen in diamond were measured by Fourier transform infrared (FTIR) spectroscopy. For the infrared absorption measurements, a Bomem M110 Fourier transform infrared (FTIR) spectrometer fitted with a Spectra Tech IR-PLANTM microscope was used. The IR beam size is limited to 150 Am square by apertures so as to pass only the diamond particle.
3. Results and discussion 3.1. Size, morphology and color of diamond The synthetic diamond crystals exhibit a range of size, morphology and color. Fig. 1 shows typical examples of the crystals. They are 0.2 to 0.5 mm in diameter, and {111} and/ or {100} surfaces are dominant. Cubo –octahedral shape is common, but ideal octahedrons and cubes are also found. The size tends to decrease with the addition of NaN3. It is noted that the addition of NaN3 influences color of the crystals. The crystals exhibit yellowish green or deep green color, while crystals made without adding NaN3 show pale yellow color. Cube crystals made with addition of NaN3 tend to show darker color than octahedral one. Some cube crystals are so opaque that FTIR absorption spectra were not acquired, and it is expected that the nitrogen concentrations in the cube crystals are higher than those in the octahedral ones.
1933
Fig. 2. Diamond crystal under transmitted light. Fine inclusions are observed.
Observation using an optical microscope reveals that the grown crystals contain a number of fine inclusions (Fig. 2), which disturb to take high quality IR spectra as described in the following section. 3.2. FTIR spectra Fig. 3 shows typical FTIR spectra of a diamond synthesized with addition of NaN3 and a type Ib diamond commercially available. It is found that the concentrations of nitrogen in diamond synthesized with addition of NaN3 are much higher than usual, based on the absorption intensity in a one-phonon region between 1000 and 1400 cm 1 relative to that in a two phonon region [1]. Before quantitative determination of the concentrations, some characteristics of FTIR spectra recorded in this study are described. The nitrogen impurities are judged to be in a single substitutional form, i.e. type Ib, from the shape of spectra in the one-phonon region. Components of nitrogen aggregates such as IaA and IaB are not detectable in the spectra. However, the shape in the one-phonon region is different from ideal ones reported. Two types of spectra shape were observed as shown in Fig. 4. One type has a peak at 1130 cm 1 which is greater than that of ideal Ib diamond, and small peaks at 1050 and 1260 cm 1. The other type has 0.9
One phonon
Two phonon
0.8
Absorbance
0.7 0.6 0.5 0.4 0.3
(a)
0.2
(b)
0.1 0 800
1300
1800
2300
2800
Wavenumber(cm-1) Fig. 1. Two examples of diamond crystals synthesized with addition of NaN3 as a nitrogen dopant in FeNi alloy – carbon system.
Fig. 3. FTIR spectra of (a) synthetic diamond made with addition of NaN3 and (b) synthetic diamond commercially available.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
1.4
0.9
1.2
0.8
1 0.8 0.6
(a) 0.4
2033 cm-1
0.6 0.5
1130 cm-1 1290 cm-1
0.4
2120 cm-1
1400 cm-1
0.3
Base line
0.1
(b) 1300
1800
Wavenumber(cm-1)
2300
0 800
2800
1300
1800
2300
2800
Wavenumber(cm-1)
Fig. 4. FTIR spectra of diamond grown from FeNi alloy without addition of NaN3 containing different type of inclusions.
peaks at 900, 970 and 1045 cm 1. These may be peaks related to inclusions, although it was not possible to make a correlation between the spectral shape and morphology of inclusions observed with the optical microscope. The overlapping of the inclusion related absorption makes determination of the concentrations of nitrogen difficult. Fig. 5 shows three spectra of diamond made with addition of 0%, 0.3% and 0.5% NaN3, respectively. These spectra also show that the absorption intensity in the onephonon region increase with addition of NaN3. There is a tendency that an increase of the NaN3 additive increases the concentrations of nitrogen. The concentrations of nitrogen have been determined as described in the following section. 3.3. Concentrations of nitrogen It is well known that the concentrations of Ib nitrogen is proportional to intensity of absorbance in the one-phonon region between 1000 and 1350 cm 1 [1]. A value of absorption coefficient of a peak at 1130 cm 1 can be converted to the nitrogen concentration multiplied by a factor of 25 [8]. Absorption coefficient of the two phonon region is independent of impurity, in which the absorption coefficient of a small dip at 2000 cm 1 is 1.23/mm [1], and the absorption intensity at 2000 cm 1 can be used to
Fig. 6. FTIR spectrum of diamond containing single substitutional nitrogen atoms. Thick arrows indicate intensity used to evaluate the concentrations of nitrogen. See text.
normalize the intensity of the one-phonon region related to nitrogen impurity. It is, therefore, very simple to determine the concentrations of nitrogen from comparison of intensity of absorbance at 1130 and 2000 cm 1. However, IR spectra taken in this study were deformed so much that it was not easy to determine intensity of the absorbance. We may assume that absorbance is zero at 1400 cm 1, but it is difficult to determine intensity of absorption at 2000 cm 1, because base line is much curved between 800 and 3000 cm 1 in spectra of the diamond made in this study, as seen in Figs. 3 – 5. We, therefore, employed another internal standard instead of the dip at 2000 cm 1. We measured an amount of depth of a dip at 2120 cm 1 from two peaks at 2033 and 2160 cm 1 as shown in Fig. 6, assuming that a base line is straight between the two peaks. This assumption may cause little error, because the distance between the two peaks is not big. When the dip is compared with intensity at 2000 cm 1, the depth of the dip corresponds to 5.5/cm of absorption coefficient, which can be used as an internal standard to determine absorption coefficient at another wave number in the spectrum. Shape of the spectra in the one-phonon region has also problem to determine the concentration of nitrogen. Peak 3000
Concentration(ppm)
1.2 1
Absorbance
2160 cm-1
0.2
0.2 0 800
2000 cm-1
0.7
Absorbance
Absorbance
1934
0.8
(a)
0.6 0.4
2500 0.5 wt%
2000 1500 1000
(b)
500
(c)
0
0.3 wt% 0 wt%
0.2 0 800
1300
1800
2300
Wavenumber(cm-1)
2800
Fig. 5. FTIR spectra of diamond synthesized from FeNi alloy with addition of (a) 0%, (b) 0.3% and (c) 0.5% NaN3.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Number of measurement Fig. 7. Concentrations of nitrogen impurities in diamonds made with 0, 0.3 and 0.5 wt.% of NaN3 additives. The concentrations of nitrogen from 37 crystals are shown.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
intensity at 1130 cm 1 is not available, because absorption due to inclusion is overlapped around 1130 cm 1. We, therefore, measured absorption intensity around 1300 cm 1, where no influence of inclusions is observed. Absorption coefficient at 1290 cm 1 is estimated to be 0.31 times of that at 1130 cm 1 from the ideal Ib spectrum as shown in Fig. 6. Summarizing the above consideration, the nitrogen concentrations are determined using the formula as follows: lð1130 cm1 Þ ¼ ½ Að1290 cm1 Þ Að1370 cm1 Þ=0:31 l 2120 cm1 ¼ 40 A 2030 cm1 þ 87 A 2160 cm1 =127 A 2120 cm1 Nitrogen concentration(ppm) = l (1130 cm 1) / l (2120 cm 1) 5.5 25, where l and A are absorption intensity and recorded values of absorbance, respectively. Spectra taken from 37 crystals were analyzed with the above way, and the concentrations of nitrogen obtained are shown in Fig. 7. Fig. 7 indicates that crystals synthesized with addition of NaN3 contain nitrogen impurities exceeding 1000 ppm, and that the nitrogen concentrations tend to increase with an increase of NaN3 additives. The concentrations measured are scattered because of noisy and deformed spectra which are caused by low transparency, small size and the presence of considerable inclusions, but it is definitely concluded that the concentrations are much higher than those reported so far.
1935
4. Summary Diamond crystals, 0.2 to 0.5 mm across, were synthesized from FeNi alloy with addition of NaN3 as a nitrogen dopant under high pressure and temperature conditions of 5.0 to 5.8 GPa and 1500 to 1750 K. It is found that the crystals contain 1000 to 2000 ppm of single substitutional nitrogen atoms, which are the highest concentrations reported so far.
References [1] G. Davies, in: P.L. Walker Jr., P.A. Thrower (Eds.), Chemistry and Physics of Carbon, vol. 13, Marcel Dekker, New York, 1977, p. 1. [2] J.E. Field, The Properties of Diamond, Academic Press, London, 1979. [3] Field, The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992. [4] A.M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer-Verlag, Berlin, 2001. [5] H. Kanda, M. Akaishi, S. Yamaoka, Diamond Relat. Mater. 8 (1999) 1441. [6] A.T. Collins, S.C. Lawson, Philos. Mag. Lett. 60 (1989) 117. [7] Z.Z. Liang, X. Jia, C.Y. Zang, P.W. Zhu, H.A. Ma, G.Z. Ren, Diamond Relat. Mater. 14 (2005) 243. [8] I. Kiflawi, A.E. Mayer, P.M. Spear, J.A. van Wyk, G.S. Woods, Philos. Mag., B 9 (1994) 1141.
Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal-carbon system Z.Z. Liang a, X. Jia a,b, H.A. Ma a, C.Y. Zang a, P.W. Zhu a, Q.F. Guan a, H. Kanda c,* a
c
National Lab of Superhard Materials, Jilin University, Changchun 130012, China b Henan Polytechnic University, Jiaozuo 454000, China National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Available online 8 August 2005
Abstract Diamond crystals have been synthesized from FeNi alloy — carbon system under high pressure and temperature of 5.0 to 5.8 GPa and 1500 to 1750 K. Concentrations of the nitrogen impurities of the crystals have been determined from FTIR spectra, and it is found that the concentrations of single substitutional nitrogen impurities exceeds 1000 ppm, when NaN3 is added as dopant to the metal-carbon system. The crystals are 0.2 – 0.5 mm in diameter with cubo – octahedral morphology, although octahedron and cube were also found in the diamond products. They exhibit yellow green to deep green color. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond; Nitrogen impurity; HPHT
1. Introduction Nitrogen is the most dominant impurity found in diamond, and optical properties of diamond directly depend on its concentration and structures [1– 4]. Study on the nitrogen impurities has long history. The nitrogen is present in several different forms in diamond [1]. It is well known that diamond is classified into type IaA, IaB, Ib, IIa and IIb, based on the form of nitrogen and boron. Type IaA and IaB diamond contain nitrogen atoms in forms of pairs and four atoms accompanying vacancy, respectively, and type Ib contains nitrogen in a single substitutional form. On the other hand, type II diamond contains little nitrogen atoms undetectable with FTIR spectroscopy. The concentrations of nitrogen impurities vary from stone to stone. Nitrogen rich diamonds in which the nitrogen concentrations exceed 1000 ppm have commonly been found in natural diamond. The nitrogen impurities are present in the aggregated form, i.e. typ Ia. Such a nitrogen rich diamond has been synthesized [5]. On the other hand, type Ib diamond usually contains up * Corresponding author. Tel.: +81 29 860 4305; fax: +81 29 851 4005. E-mail address: [email protected] (H. Kanda). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.06.041
to 200 ppm, which are synthesized in the conventional way under high pressure and high temperature conditions. The highest concentrations of nitrogen reported so far are 800 ppm [6]. It is an interesting topic to know how the high concentrations of nitrogen are incorporated into diamond. The diamond is expected to show some unusual properties. In the present work, we attempted to synthesize nitrogen rich diamond crystals. NaN3 was used as dopant of nitrogen in NiFe alloy — carbon system to grow the diamond under high pressure and temperature conditions. Diamond crystals containing single substitutional nitrogen exceeding 1000 ppm were successfully grown.
2. Experimental We used scale like graphite powders as carbon source, Fe90Ni10 alloy with 75 Am in diameter as solvent-catalyst and NaN3 (99.99%) as nitrogen dopant. After mixing the powders for 4 h, they were shaped with the form of disk to fit in a cylindrical space surrounded by a ceramic material [7]. Composition of the graphite and the alloy was fixed to be 1 : 1, and the NaN3 additive varied from 0 to 0.5 wt.%.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
The sample assembly was compressed in a cubic anvil high pressure and high temperature apparatus (SPD-6 1200) with a sample chamber of 23 mm on an edge, and then heated for 15 min. Pressure and temperature were 5.0 to 5.8 GPa and 1500 to 1750 K. Higher pressure and temperature were required to synthesize diamond, when NaN3 was added to the metal-carbon system. Nucleation density of diamond crystals tends to decrease with an increase of the NaN3 additive at a fixed P-T condition. The samples recovered from the high pressure vessel were treated in acids to isolate grown diamond from unconverted graphite and the metal solvent catalyst. The concentrations of nitrogen in diamond were measured by Fourier transform infrared (FTIR) spectroscopy. For the infrared absorption measurements, a Bomem M110 Fourier transform infrared (FTIR) spectrometer fitted with a Spectra Tech IR-PLANTM microscope was used. The IR beam size is limited to 150 Am square by apertures so as to pass only the diamond particle.
3. Results and discussion 3.1. Size, morphology and color of diamond The synthetic diamond crystals exhibit a range of size, morphology and color. Fig. 1 shows typical examples of the crystals. They are 0.2 to 0.5 mm in diameter, and {111} and/ or {100} surfaces are dominant. Cubo –octahedral shape is common, but ideal octahedrons and cubes are also found. The size tends to decrease with the addition of NaN3. It is noted that the addition of NaN3 influences color of the crystals. The crystals exhibit yellowish green or deep green color, while crystals made without adding NaN3 show pale yellow color. Cube crystals made with addition of NaN3 tend to show darker color than octahedral one. Some cube crystals are so opaque that FTIR absorption spectra were not acquired, and it is expected that the nitrogen concentrations in the cube crystals are higher than those in the octahedral ones.
1933
Fig. 2. Diamond crystal under transmitted light. Fine inclusions are observed.
Observation using an optical microscope reveals that the grown crystals contain a number of fine inclusions (Fig. 2), which disturb to take high quality IR spectra as described in the following section. 3.2. FTIR spectra Fig. 3 shows typical FTIR spectra of a diamond synthesized with addition of NaN3 and a type Ib diamond commercially available. It is found that the concentrations of nitrogen in diamond synthesized with addition of NaN3 are much higher than usual, based on the absorption intensity in a one-phonon region between 1000 and 1400 cm 1 relative to that in a two phonon region [1]. Before quantitative determination of the concentrations, some characteristics of FTIR spectra recorded in this study are described. The nitrogen impurities are judged to be in a single substitutional form, i.e. type Ib, from the shape of spectra in the one-phonon region. Components of nitrogen aggregates such as IaA and IaB are not detectable in the spectra. However, the shape in the one-phonon region is different from ideal ones reported. Two types of spectra shape were observed as shown in Fig. 4. One type has a peak at 1130 cm 1 which is greater than that of ideal Ib diamond, and small peaks at 1050 and 1260 cm 1. The other type has 0.9
One phonon
Two phonon
0.8
Absorbance
0.7 0.6 0.5 0.4 0.3
(a)
0.2
(b)
0.1 0 800
1300
1800
2300
2800
Wavenumber(cm-1) Fig. 1. Two examples of diamond crystals synthesized with addition of NaN3 as a nitrogen dopant in FeNi alloy – carbon system.
Fig. 3. FTIR spectra of (a) synthetic diamond made with addition of NaN3 and (b) synthetic diamond commercially available.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
1.4
0.9
1.2
0.8
1 0.8 0.6
(a) 0.4
2033 cm-1
0.6 0.5
1130 cm-1 1290 cm-1
0.4
2120 cm-1
1400 cm-1
0.3
Base line
0.1
(b) 1300
1800
Wavenumber(cm-1)
2300
0 800
2800
1300
1800
2300
2800
Wavenumber(cm-1)
Fig. 4. FTIR spectra of diamond grown from FeNi alloy without addition of NaN3 containing different type of inclusions.
peaks at 900, 970 and 1045 cm 1. These may be peaks related to inclusions, although it was not possible to make a correlation between the spectral shape and morphology of inclusions observed with the optical microscope. The overlapping of the inclusion related absorption makes determination of the concentrations of nitrogen difficult. Fig. 5 shows three spectra of diamond made with addition of 0%, 0.3% and 0.5% NaN3, respectively. These spectra also show that the absorption intensity in the onephonon region increase with addition of NaN3. There is a tendency that an increase of the NaN3 additive increases the concentrations of nitrogen. The concentrations of nitrogen have been determined as described in the following section. 3.3. Concentrations of nitrogen It is well known that the concentrations of Ib nitrogen is proportional to intensity of absorbance in the one-phonon region between 1000 and 1350 cm 1 [1]. A value of absorption coefficient of a peak at 1130 cm 1 can be converted to the nitrogen concentration multiplied by a factor of 25 [8]. Absorption coefficient of the two phonon region is independent of impurity, in which the absorption coefficient of a small dip at 2000 cm 1 is 1.23/mm [1], and the absorption intensity at 2000 cm 1 can be used to
Fig. 6. FTIR spectrum of diamond containing single substitutional nitrogen atoms. Thick arrows indicate intensity used to evaluate the concentrations of nitrogen. See text.
normalize the intensity of the one-phonon region related to nitrogen impurity. It is, therefore, very simple to determine the concentrations of nitrogen from comparison of intensity of absorbance at 1130 and 2000 cm 1. However, IR spectra taken in this study were deformed so much that it was not easy to determine intensity of the absorbance. We may assume that absorbance is zero at 1400 cm 1, but it is difficult to determine intensity of absorption at 2000 cm 1, because base line is much curved between 800 and 3000 cm 1 in spectra of the diamond made in this study, as seen in Figs. 3 – 5. We, therefore, employed another internal standard instead of the dip at 2000 cm 1. We measured an amount of depth of a dip at 2120 cm 1 from two peaks at 2033 and 2160 cm 1 as shown in Fig. 6, assuming that a base line is straight between the two peaks. This assumption may cause little error, because the distance between the two peaks is not big. When the dip is compared with intensity at 2000 cm 1, the depth of the dip corresponds to 5.5/cm of absorption coefficient, which can be used as an internal standard to determine absorption coefficient at another wave number in the spectrum. Shape of the spectra in the one-phonon region has also problem to determine the concentration of nitrogen. Peak 3000
Concentration(ppm)
1.2 1
Absorbance
2160 cm-1
0.2
0.2 0 800
2000 cm-1
0.7
Absorbance
Absorbance
1934
0.8
(a)
0.6 0.4
2500 0.5 wt%
2000 1500 1000
(b)
500
(c)
0
0.3 wt% 0 wt%
0.2 0 800
1300
1800
2300
Wavenumber(cm-1)
2800
Fig. 5. FTIR spectra of diamond synthesized from FeNi alloy with addition of (a) 0%, (b) 0.3% and (c) 0.5% NaN3.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Number of measurement Fig. 7. Concentrations of nitrogen impurities in diamonds made with 0, 0.3 and 0.5 wt.% of NaN3 additives. The concentrations of nitrogen from 37 crystals are shown.
Z.Z. Liang et al. / Diamond & Related Materials 14 (2005) 1932 – 1935
intensity at 1130 cm 1 is not available, because absorption due to inclusion is overlapped around 1130 cm 1. We, therefore, measured absorption intensity around 1300 cm 1, where no influence of inclusions is observed. Absorption coefficient at 1290 cm 1 is estimated to be 0.31 times of that at 1130 cm 1 from the ideal Ib spectrum as shown in Fig. 6. Summarizing the above consideration, the nitrogen concentrations are determined using the formula as follows: lð1130 cm1 Þ ¼ ½ Að1290 cm1 Þ Að1370 cm1 Þ=0:31 l 2120 cm1 ¼ 40 A 2030 cm1 þ 87 A 2160 cm1 =127 A 2120 cm1 Nitrogen concentration(ppm) = l (1130 cm 1) / l (2120 cm 1) 5.5 25, where l and A are absorption intensity and recorded values of absorbance, respectively. Spectra taken from 37 crystals were analyzed with the above way, and the concentrations of nitrogen obtained are shown in Fig. 7. Fig. 7 indicates that crystals synthesized with addition of NaN3 contain nitrogen impurities exceeding 1000 ppm, and that the nitrogen concentrations tend to increase with an increase of NaN3 additives. The concentrations measured are scattered because of noisy and deformed spectra which are caused by low transparency, small size and the presence of considerable inclusions, but it is definitely concluded that the concentrations are much higher than those reported so far.
1935
4. Summary Diamond crystals, 0.2 to 0.5 mm across, were synthesized from FeNi alloy with addition of NaN3 as a nitrogen dopant under high pressure and temperature conditions of 5.0 to 5.8 GPa and 1500 to 1750 K. It is found that the crystals contain 1000 to 2000 ppm of single substitutional nitrogen atoms, which are the highest concentrations reported so far.
References [1] G. Davies, in: P.L. Walker Jr., P.A. Thrower (Eds.), Chemistry and Physics of Carbon, vol. 13, Marcel Dekker, New York, 1977, p. 1. [2] J.E. Field, The Properties of Diamond, Academic Press, London, 1979. [3] Field, The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992. [4] A.M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer-Verlag, Berlin, 2001. [5] H. Kanda, M. Akaishi, S. Yamaoka, Diamond Relat. Mater. 8 (1999) 1441. [6] A.T. Collins, S.C. Lawson, Philos. Mag. Lett. 60 (1989) 117. [7] Z.Z. Liang, X. Jia, C.Y. Zang, P.W. Zhu, H.A. Ma, G.Z. Ren, Diamond Relat. Mater. 14 (2005) 243. [8] I. Kiflawi, A.E. Mayer, P.M. Spear, J.A. van Wyk, G.S. Woods, Philos. Mag., B 9 (1994) 1141.