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Initial description of a new carbon-nitride phase synthesized at high pressures and temperatures
MATERIAlS
SCIEMCEIt EMCIMEERIMC ELSEVIER
A
Materials Science and Engineering A209 (1996) 23 25
Initial description of a new carbon-nitride phase synthesized at high pressures and temperatures Jeffrey H. Nguyena, Raymond Jeanloz b "Department oj Physics, UnilHSitl' of California, Berkeley, CA 94720, USA of CeoioKY and Ceophl'sic,\', Universitl' of California, Berkeley. C A 94720, USA
b Department
Abstract We describe a new crystalline phase synthesized from a mixture of carbon and nitrogen heated to 2000-2500 K at 30( ± 5) GPa, This phase, having an X-ray diffraction pattern compatible with cubic symmetry, is the first example of a dense carbon nitride formed in the bulk. Keywords: High pressures; High temperatures; Carbon-nitride phase
1. Introduction
Theoretical considerations indicate that dense carbon nitrides are candidates for producing new superhard materials [1-3]. As the thermodynamic effect of pressure is to stabilize dense crystal structures, we explore the use of ultrahigh pressures ( ;:;:; 10-100 GPa range) to create new carbon nitrides. Kinetic hindrances during synthesis are overcome through the simultaneous application of elevated temperatures ( ~ 2000-4000 K) and pressures, allowing us to quench the sample and retain it metastably at ambient conditions. This follows the approach successfully used for synthesizing several compositions within the cubic C-BN (diamond-borazan) solid solution (Fig. I) [4]. Our experiments are based on the laser-heated diamond cell, an instrument capable of taking samples to sustained pressures and temperatures exceeding 100 GPa and 4000 K, respectively [5]. Briefly, ~ 1-10 fig of sample material are compressed between the points of 2 gem-quality diamond crystals, and heated by means of a 20 watt, continuous-wave Nd:YAG laser beam that is focussed directly through the diamonds and onto the sample. A metal foil, acting as a gasket. contains the sample in place between the diamond anvils. Pressure is measured by the ruby-fluorescence method, and temperatures are determined by spectroradiometry. Our present system is identical to that described previously [6,7], except that we now use a CCD detector coupled to a fixed-grating imaging spectrograph in order to 0921-5093/96/$15.00 © 1996 SSDI 0921-5093(95)10129-2
Elsevier Science SA All rights reserved
determine temperature distributions across the sample [8]. We have carried out experiments on a variety of carbon + nitrogen mixtures, including the use of graphite, amorphous carbon and C 60 buckminsterfullerene as starting materials. The latter is particularly interesting because previous experiments demonstrate that C 611 transforms to a diamond-like amorphous car-
c
Diamond
\ "«.
~
'S:
Starting Materials
'\-
•o
Carbon + N 2
\\'-- J '\. ~
o
'\-
~
~
~ 'S:
'\ \
B
c-BN
N
Fig. 1. Synthesis of compounds within the B-C - N system using the laser-heated diamond cell. This study of compositions along the carbon-nitrogen binary (diagonal ruling: carbon + N 2 mixture as starting material) follows the approach used in previous syntheses across the cubic BN -C solid solution (open and stippled symbols refer to mixtures of graphite + hexagonal BN and to microcrystalline B,(CN)t _ , starting materials, respectively) [4].
24
J.H. Nguyen, R. Jeanla:. / Materials Science and Engineering A209 (1996) 23 -25
300
300,--,----,------,----,---,-------,
Carbon + Nitrogen Heated at - 30 GPa
250
.q '" ~
Carbon + Nitrogen Heated at -30 GPa
250
200
~
OJ
150
:>
.~
Ruby
;) ~
o'----_.L-_ _.L-_ _.L-_ _.L-_ _ 1~
15
LO 2.5 d-Spacing (A)
Iron
..l.-~"""""'"
3.0
o'----~"::_---:"::_---=-'::_---=-'_:_-~'::_""""'~
3.5
Fig. 2. Powder diffraction pattern (filtered Cu Kx radiation) of carbon-nitride sample quenched from heating a mixture of C 60 and nitrogen at 30 ( ± 5) GPa. The pattern was collected on film, scanned and analyzed as described elsewhere [9,15].
bon when decompressed from pressures exceeding 2530 GPa [9-11]. The nitrogen is loaded cryogenically, and less than 5 vol.% finely grained ruby is included with the sample for pressure calibration. Here, we focus on results obtained from heating a mixture of C 60 + N z, contained in a stainless steel gasket, up to 2000-2500 K at 30 (± 5) GPa. Microscopic observation of the sample inside the diamond cell reveals the nitrogen to be fluid during laser heating. Visually observed changes in texture and color of the carbon - nitrogen mixture suggest that these components react chemically at high pressures. Several grains of reacted sample, about 20 ,um in size, are observed to be reflective to light. All of the unreacted nitrogen escapes from the sample area at the end of the experiment, after turning off the laser and releasing the pressure. A Debye-Scherrer X-ray diffraction pattern of the remaining sample material confirms that a crystalline product is quenched to ambient conditions (Fig. 2). Comparisons with the diffraction patterns of ruby and iron show that there is no evidence of sample contamination by either the pressure calibrant or the gasket (Fig. 3). Similarly, we confirm that none of the known crystalline phases of carbon (e.g., hexagonal or cubic diamond, graphite, buckminsterfullerene) can explain our new diffraction pattern. Evidently, reaction of the carbon with nitrogen at high pressures and temperatures has produced a new carbon-nitride phase. Recent studies have suggested that dense carbon nitrides, possibly fJ -C 3N 4 , can be produced as thin films [12,13]. Diffraction patterns for two of these materials can be compared directly with our own results to show that the material we have produced is quite distinct from the thin"film samples (Fig. 4). Also, our diffraction pattern is not compatible with the theoretically expected pattern for fJ-C 3 N 4 [2,14].
1.0
1.5
2.0 25 d-Spacing (A)
3.0
35
Fig. 3. Comparison of the carbon-nitride diffraction pattern with diffraction patterns expected for possible ruby and gasket (iron) contaminants.
We have not yet deduced a unique structure from the X-ray diffraction patterns of our new carbon-nitride phase. For example, application of the crystallographic program TREOR to the d-spacings and intensities of the pattern shown in Fig. 2 yields cubic unit cells with dimensions of a = 7.699 (± 0.003) A (7/7 diffraction peaks matched) or a = 11.538 (± 0.001) A (6/7 diffraction peaks matched). Alternatively, a smaller cubic unit cell (a = 5.442 ± 0.004 A) matches 5 of the observed d -spacings. We have found the same, or similar, diffraction lines as in Fig. 2 when examining other C- N compounds that have been heated at 30-40 GPa. Additional diffraction lines may be present, suggesting the possibility of more than one phase being produced depending on the starting material. Nevertheless, the reproducibility in diffraction patterns obtained from different starting materials proves that at least one new 300,--,----,------,----,---,-------,
Carbon + Nitrogen Heated at -30 GPa
250
c-
'0;;
200
l::
OJ
:5 OJ
Yu. et af. (1994)
150
:>
.~
;) ~
oL--_...l-_ _- ' -_ _
..,-L-_ _- ' -_ _--'-:---""""""'"
1.0
1.5
2.0 2.5 d-Spacing (A)
3.5
Fig. 4. Comparison of the carbon-nitride diffraction pattern with diffraction patterns of the thin-film samples described by Niu et al. [12] and Yu et al. [13].
J.H. Nguyen, R. Jeanloc / Materials Science and Engineering A209 (/996) 23-25
carbon nitride phase is formed at the high pressuretemperature conditions of our experiments. Further work is required to determine the exact structure and composition of this phase. In addition, there is every reason to expect that additional phases may be obtained within the carbon-nitrogen binary as one varies pressure, temperature and composition over a wider range than we have explored so far.
Acknowledgements
This work was supported by NASA and ACS-PRF.
References [11 M.L. Cohen, Phys. Rev. B, 32 (1985) 7988. [2] A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841.
25
[3] M.L. Cohen. Mater. Sci. Eng., A, 209 (1996). [4] E, Knittle, R.B. Kaner, R. Jeanloz and M.L. Cohen, Phys. Rev. B, 51 (1995) 12149. [5] R. Jeanloz, Ann. Rev. Phys. Chon., 40 (1989) 237. [6] R. Jeanloz and D.L. Heinz, J. Phys. (Paris), 45 (C8) (1984) 83. [7] D.L. Heinz and R. Jeanloz, in M. Manghnani and Y. Syono (eds.), High-Pressure Research in Mineral Physics, Am. Geophys. Union, Washington, DC, 1987, pp. 113 - 127. [8] B. O'Neill and R. Jeanloz, EOS Trans. Am. Geophys. Union, 71 (1990) 1611. [9] J.H. Nguyen, M.B. Kruger and R. Jeanloz, Solid State Commun., 88 (1993) 719. [10] S.D. Kosowsky, e.H. Hsu, N.H. Chen, F. Moshary, P.S. Pershan and I.F. Silvera, Phys. Rev. B, 48 (1993) 8474. [11] H. Hirai, K. Kondo, N. Yoshizawa and M. Shiraishi, Appl. Phys. Lett., 64 (1994) 1797, Chem. Phys. Lett., 226 (1994) 595. [12] C, Niu, Y.Z. Liu and e.M. Lieber, Science, 261 (1993) 334. [13] K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu and I.e. Wu, Phys. Rec. B, 49 (1994) 5034. [14] A.Y. Liu and R.M. Wentzcovitch, Phys. Rev. B, 50 (1994) 10362. [15] lH. Nguyen and R. Jeanloz, Rev. Sci. lnstr., 64 (1993) 3456.
SCIEMCEIt EMCIMEERIMC ELSEVIER
A
Materials Science and Engineering A209 (1996) 23 25
Initial description of a new carbon-nitride phase synthesized at high pressures and temperatures Jeffrey H. Nguyena, Raymond Jeanloz b "Department oj Physics, UnilHSitl' of California, Berkeley, CA 94720, USA of CeoioKY and Ceophl'sic,\', Universitl' of California, Berkeley. C A 94720, USA
b Department
Abstract We describe a new crystalline phase synthesized from a mixture of carbon and nitrogen heated to 2000-2500 K at 30( ± 5) GPa, This phase, having an X-ray diffraction pattern compatible with cubic symmetry, is the first example of a dense carbon nitride formed in the bulk. Keywords: High pressures; High temperatures; Carbon-nitride phase
1. Introduction
Theoretical considerations indicate that dense carbon nitrides are candidates for producing new superhard materials [1-3]. As the thermodynamic effect of pressure is to stabilize dense crystal structures, we explore the use of ultrahigh pressures ( ;:;:; 10-100 GPa range) to create new carbon nitrides. Kinetic hindrances during synthesis are overcome through the simultaneous application of elevated temperatures ( ~ 2000-4000 K) and pressures, allowing us to quench the sample and retain it metastably at ambient conditions. This follows the approach successfully used for synthesizing several compositions within the cubic C-BN (diamond-borazan) solid solution (Fig. I) [4]. Our experiments are based on the laser-heated diamond cell, an instrument capable of taking samples to sustained pressures and temperatures exceeding 100 GPa and 4000 K, respectively [5]. Briefly, ~ 1-10 fig of sample material are compressed between the points of 2 gem-quality diamond crystals, and heated by means of a 20 watt, continuous-wave Nd:YAG laser beam that is focussed directly through the diamonds and onto the sample. A metal foil, acting as a gasket. contains the sample in place between the diamond anvils. Pressure is measured by the ruby-fluorescence method, and temperatures are determined by spectroradiometry. Our present system is identical to that described previously [6,7], except that we now use a CCD detector coupled to a fixed-grating imaging spectrograph in order to 0921-5093/96/$15.00 © 1996 SSDI 0921-5093(95)10129-2
Elsevier Science SA All rights reserved
determine temperature distributions across the sample [8]. We have carried out experiments on a variety of carbon + nitrogen mixtures, including the use of graphite, amorphous carbon and C 60 buckminsterfullerene as starting materials. The latter is particularly interesting because previous experiments demonstrate that C 611 transforms to a diamond-like amorphous car-
c
Diamond
\ "«.
~
'S:
Starting Materials
'\-
•o
Carbon + N 2
\\'-- J '\. ~
o
'\-
~
~
~ 'S:
'\ \
B
c-BN
N
Fig. 1. Synthesis of compounds within the B-C - N system using the laser-heated diamond cell. This study of compositions along the carbon-nitrogen binary (diagonal ruling: carbon + N 2 mixture as starting material) follows the approach used in previous syntheses across the cubic BN -C solid solution (open and stippled symbols refer to mixtures of graphite + hexagonal BN and to microcrystalline B,(CN)t _ , starting materials, respectively) [4].
24
J.H. Nguyen, R. Jeanla:. / Materials Science and Engineering A209 (1996) 23 -25
300
300,--,----,------,----,---,-------,
Carbon + Nitrogen Heated at - 30 GPa
250
.q '" ~
Carbon + Nitrogen Heated at -30 GPa
250
200
~
OJ
150
:>
.~
Ruby
;) ~
o'----_.L-_ _.L-_ _.L-_ _.L-_ _ 1~
15
LO 2.5 d-Spacing (A)
Iron
..l.-~"""""'"
3.0
o'----~"::_---:"::_---=-'::_---=-'_:_-~'::_""""'~
3.5
Fig. 2. Powder diffraction pattern (filtered Cu Kx radiation) of carbon-nitride sample quenched from heating a mixture of C 60 and nitrogen at 30 ( ± 5) GPa. The pattern was collected on film, scanned and analyzed as described elsewhere [9,15].
bon when decompressed from pressures exceeding 2530 GPa [9-11]. The nitrogen is loaded cryogenically, and less than 5 vol.% finely grained ruby is included with the sample for pressure calibration. Here, we focus on results obtained from heating a mixture of C 60 + N z, contained in a stainless steel gasket, up to 2000-2500 K at 30 (± 5) GPa. Microscopic observation of the sample inside the diamond cell reveals the nitrogen to be fluid during laser heating. Visually observed changes in texture and color of the carbon - nitrogen mixture suggest that these components react chemically at high pressures. Several grains of reacted sample, about 20 ,um in size, are observed to be reflective to light. All of the unreacted nitrogen escapes from the sample area at the end of the experiment, after turning off the laser and releasing the pressure. A Debye-Scherrer X-ray diffraction pattern of the remaining sample material confirms that a crystalline product is quenched to ambient conditions (Fig. 2). Comparisons with the diffraction patterns of ruby and iron show that there is no evidence of sample contamination by either the pressure calibrant or the gasket (Fig. 3). Similarly, we confirm that none of the known crystalline phases of carbon (e.g., hexagonal or cubic diamond, graphite, buckminsterfullerene) can explain our new diffraction pattern. Evidently, reaction of the carbon with nitrogen at high pressures and temperatures has produced a new carbon-nitride phase. Recent studies have suggested that dense carbon nitrides, possibly fJ -C 3N 4 , can be produced as thin films [12,13]. Diffraction patterns for two of these materials can be compared directly with our own results to show that the material we have produced is quite distinct from the thin"film samples (Fig. 4). Also, our diffraction pattern is not compatible with the theoretically expected pattern for fJ-C 3 N 4 [2,14].
1.0
1.5
2.0 25 d-Spacing (A)
3.0
35
Fig. 3. Comparison of the carbon-nitride diffraction pattern with diffraction patterns expected for possible ruby and gasket (iron) contaminants.
We have not yet deduced a unique structure from the X-ray diffraction patterns of our new carbon-nitride phase. For example, application of the crystallographic program TREOR to the d-spacings and intensities of the pattern shown in Fig. 2 yields cubic unit cells with dimensions of a = 7.699 (± 0.003) A (7/7 diffraction peaks matched) or a = 11.538 (± 0.001) A (6/7 diffraction peaks matched). Alternatively, a smaller cubic unit cell (a = 5.442 ± 0.004 A) matches 5 of the observed d -spacings. We have found the same, or similar, diffraction lines as in Fig. 2 when examining other C- N compounds that have been heated at 30-40 GPa. Additional diffraction lines may be present, suggesting the possibility of more than one phase being produced depending on the starting material. Nevertheless, the reproducibility in diffraction patterns obtained from different starting materials proves that at least one new 300,--,----,------,----,---,-------,
Carbon + Nitrogen Heated at -30 GPa
250
c-
'0;;
200
l::
OJ
:5 OJ
Yu. et af. (1994)
150
:>
.~
;) ~
oL--_...l-_ _- ' -_ _
..,-L-_ _- ' -_ _--'-:---""""""'"
1.0
1.5
2.0 2.5 d-Spacing (A)
3.5
Fig. 4. Comparison of the carbon-nitride diffraction pattern with diffraction patterns of the thin-film samples described by Niu et al. [12] and Yu et al. [13].
J.H. Nguyen, R. Jeanloc / Materials Science and Engineering A209 (/996) 23-25
carbon nitride phase is formed at the high pressuretemperature conditions of our experiments. Further work is required to determine the exact structure and composition of this phase. In addition, there is every reason to expect that additional phases may be obtained within the carbon-nitrogen binary as one varies pressure, temperature and composition over a wider range than we have explored so far.
Acknowledgements
This work was supported by NASA and ACS-PRF.
References [11 M.L. Cohen, Phys. Rev. B, 32 (1985) 7988. [2] A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841.
25
[3] M.L. Cohen. Mater. Sci. Eng., A, 209 (1996). [4] E, Knittle, R.B. Kaner, R. Jeanloz and M.L. Cohen, Phys. Rev. B, 51 (1995) 12149. [5] R. Jeanloz, Ann. Rev. Phys. Chon., 40 (1989) 237. [6] R. Jeanloz and D.L. Heinz, J. Phys. (Paris), 45 (C8) (1984) 83. [7] D.L. Heinz and R. Jeanloz, in M. Manghnani and Y. Syono (eds.), High-Pressure Research in Mineral Physics, Am. Geophys. Union, Washington, DC, 1987, pp. 113 - 127. [8] B. O'Neill and R. Jeanloz, EOS Trans. Am. Geophys. Union, 71 (1990) 1611. [9] J.H. Nguyen, M.B. Kruger and R. Jeanloz, Solid State Commun., 88 (1993) 719. [10] S.D. Kosowsky, e.H. Hsu, N.H. Chen, F. Moshary, P.S. Pershan and I.F. Silvera, Phys. Rev. B, 48 (1993) 8474. [11] H. Hirai, K. Kondo, N. Yoshizawa and M. Shiraishi, Appl. Phys. Lett., 64 (1994) 1797, Chem. Phys. Lett., 226 (1994) 595. [12] C, Niu, Y.Z. Liu and e.M. Lieber, Science, 261 (1993) 334. [13] K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu and I.e. Wu, Phys. Rec. B, 49 (1994) 5034. [14] A.Y. Liu and R.M. Wentzcovitch, Phys. Rev. B, 50 (1994) 10362. [15] lH. Nguyen and R. Jeanloz, Rev. Sci. lnstr., 64 (1993) 3456.