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Solvothermal synthesis of hexagonal B–C–N compound at low temperature conditions
Diamond & Related Materials 15 (2006) 1659 – 1662 www.elsevier.com/locate/diamond
Solvothermal synthesis of hexagonal B–C–N compound at low temperature conditions Guang Sun, ZhongYuan Liu, Zhongmin Zhou, Julong He, Dongli Yu, Yongjun Tian ⁎ Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Received 6 September 2005; received in revised form 11 January 2006; accepted 3 February 2006 Available online 6 March 2006
Abstract Hexagonal B–C–N crystals have been successfully synthesized by a solvothermal method from carbon tetrachloride and calcium boron nitride at 400 °C. The characterization of synthesized product was carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with electronic energy loss spectrometer. The composition of the hexagonal B–C–N single crystal is detected by electronic energy loss spectrometer as B0.52C0.11N0.37. © 2006 Elsevier B.V. All rights reserved. Keywords: B–C–N; Solvothermal synthesis; High pressure crystal growth
1. Introduction Ternary B–C–N compounds are of great research interest because of their potentially mechanical and electronic properties. Much attention has been paid to search for new and effective experimental methods to synthesize B–C–N compounds. A variety of extreme methods including chemical vapor deposition [1–3], physical vapor deposition [4–6], highpressure and high-temperature (HP/HT) synthesis [7–10] and solid phase pyrolysis [11] were developed. However, most of the synthesized products are of poor crystallinity and have turbostratic or amorphous structure. Only a few of them are crystalline. Badzian carried out the pioneer work to prepare crystalline diamond-like B–C–N compound at temperature of 3300 K and pressure of 14 GPa, the composition of the obtained compound was close to BC6N [7]. Knittle et al. [8] and Solozhenko et al. [9] reported the synthesis of diamond-like BCxN compounds using static pressure and laser-heated diamond cell from graphitic BC2N. We have successfully
⁎ Corresponding author. E-mail address: [email protected] (Y. Tian). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.02.001
synthesized two crystalline B–C–N compounds. One is a polycrystalline hexagonal B–C–N compound prepared by a chemical process [12]. The other is an orthorhombic B2CN crystal prepared under HP/HT condition [13]. Nevertheless, it is difficult to convince that the above products are true ternary B– C–N materials. Theoretical and experimental results revealed that the ternary B–C–N materials tend to segregate into C and BN [10,14–17]. Sasaki et al. observed that simultaneous crystallization of diamond and c-BN from the melt of cobalt– boron–carbon–nitrogen under 5.5 GPa and 1400∼1600 °C [10]. Sun et al. [14] studied seven possible diamond-structured BC2N phases by using ab initio pseudopotential density functional method. They indicated that diamond-structured BC2N energetically favor separation into diamond and c-BN. Therefore, the observed B–C–N materials may be the mixture of C and BN. It is needed to confirm that the single-crystal in the synthesized product consists of B, C and N elements. Recently, a solvothermal method was found to be an effective and moderate route to prepare metastable materials, such as successfully synthesized nano-diamond, c-BN, h-C3N4 and B4C etc. in recent years [18–21]. Huang et al. reported that a turbostratic boron carbonitride was prepared by solvothermal method [22]. In this work, we prepared crystalline B–C–N compounds through a solvothermal synthetic route by the
G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
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e
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Intensity(a.u.)
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reaction of calcium boron nitride (Ca3B2N4) and carbon tetrachloride (CCl4) at moderate temperature and pressure. 2. Experimental procedures In this experiment, 4 g Ca3B2N4 powder was put into 30 ml CCl4 solution in a stainless steel autoclave. Then the vessel was filled in high purity nitrogen gas (99.999%). The autoclave was heated to the reaction temperature, maintained for 12 h, and then cooled to room temperature naturally. The range of reaction temperature was 350–550 °C. The product, dark powder, was washed with absolute ethanol, dilute hydrochloric acid and distilled water in sequence, and then dried in vacuum at 100 °C for 5 h. The structure determination of the product was performed by X-ray diffraction (XRD, Rigaku D/Max-Rb) and transmission electron microscopy (TEM, JEM-2010). XRD pattern was carried out with Ni filtered Cu Kα radiation at a scanning rate of 5°/min. The average composition was determined under a scanning electron microscope (SEM, KYKY-2800) with energy dispersive X-ray (EDS, Kevex Sigma-32 Level-4). Electronic energy loss spectrometer (EELS, Gatan-ENFINA-776) was used to determine the composition and structural information of individual particle in the synthesized powder under TEM.
50
70
90
o
2θ ( )
2θ ( o ) Fig. 1. XRD patterns of sample obtained at (a) 350 °C; (b) 400 °C; (c) 450 °C; (d) 500 °C and (e) 550 °C.
30
Fig. 2. XRD pattern of the synthesized powder obtained at 400 °C.
Therefore, we can conclude that the optimum temperature for the formation of hexagonal phase is about 400 °C in our experiment. The average composition of the sample obtained at 400 °C was B0.36C0.35N0.29 detected by EDS. The ratio of B : C : N seems near to 1 : 1 : 1. To further investigate the structure and composition of the powder the sample was measured by TEM and EELS. The experimental results show that the composition in the powder is uneven. We found that amorphous carbon and several crystalline powders of graphite, h-BN and hexagonal B– C–N coexisted in the sample. The dominant phases in the product are h-BN and hexagonal B–C–N compounds. This phenomenon can be observed in the XRD result shown in Fig. 2. It can be seen that there are some short peaks with larger d spacing coexisted in the (002) peak of the sample, which result in the bottom of (002) peak becomes broad. This is because of the adjacent lattice parameters between graphite, h-BN and hexagonal B–C–N. Therefore, the XRD result indicates that there may be three kinds of hexagonal phases existed in the product. The X-ray diffraction data of the hexagonal compound from Fig. 2 are listed in Table 1. It is difficult to distinguish the two phases of h-BN and h-B–C–N in the XRD spectrum because of no obvious difference between their lattice parameters. Just from the XRD measurement itself, it is not possible to determine the existence of h-B–C–N compounds because h-BN and h-B–C–N phases are similar in structure and
3. Results and discussion The XRD spectra of the synthesized samples at different temperatures are shown in Fig. 1. The effect of reaction temperature on the formation of hexagonal phase was investigated in our experiment. We found that no chemical reactions happened below the temperature of 350 °C. When the reaction temperature reached 350 °C, a hexagonal graphite-like phase was formed. The main phase of the obtained product is amorphous (Fig. 1a). The obtained product at about 400 °C contains the least amorphous phase (Fig. 1b). The amount of the amorphous phase enhances in the product when the reaction temperature raised from 450 to 550 °C (Fig. 1c, d and e).
Table 1 The X-ray diffraction values of synthesized B–C–N compound obtained at 400 °C 2θ
d (nm)
(hkl)
26.781 41.621 43.963 50.182 55.085 71.560 76.000 82.258
0.3326 0.2168 0.2058 0.1817 0.1666 0.1318 0.1251 0.1171
(002) (100) (101) (102) (004) (104) (110) (112)
G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
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Fig. 3. TEM image (a) and SAED pattern (b) of a typical hexagonal B–C–N single crystal.
their peaks are generally overlapped with each other in the XRD spectrum. The existence of h-B–C–N crystals in our synthesized sample can be confirmed in the EELS measurements on a series of chosen particles. Fig. 3 shows a bright field image of a representative B–C–N single crystal (Fig. 3a) and its corresponding pattern of selected area electron diffraction (Fig. 3b). It is shown that the B–C–N single crystal has hexagonal structure. A typical EELS spectrum of the hexagonal B–C–N single crystal is displayed in Fig. 4. It exhibits the characteristic K-shell ionization edges of elemental boron, carbon and nitrogen at 188, 284 and 401 eV, respectively, which clearly shows the presence of sp2 bonding (π⁎ peaks). All three K edges (B, C, and N) appear simultaneously in the spectrum. This indicates that the single crystal is a true B–C–N ternary compound rather than segregated phases of graphite and h-BN. We quantitatively analyzed the EELS spectra to determine the chemical composition of the hexagonal B–C–N
B-K
Countes(a.u.)
σ∗
In summary, we have successfully synthesized a hexagonal B–C–N crystal by a solvothermal reaction between carbon tetrachloride and calcium boron nitride at the temperature range of 350–550 °C. The effect of reaction temperature on the formation of hexagonal phase was investigated in our experiment. Well crystallized B–C–N compound can be obtained at 400 °C. TEM and EELS measurements show that the synthesized B–C–N crystal has a hexagonal structure, which has the composition of B0.52C0.11N0.37. Due to the simple manipulation and cheaper reactants, such solvothermal synthesis technique has provided a facile and efficient method to get the attractive hexagonal B–C–N compounds.
This work was supported by National Natural Science Foundations of China (Grant Nos. 50225207, 50372055, 50472051 and 50532020) and the National Basic Research Program of China (Grant. No. 2005CB724405).
N-K
σ∗ π∗
200
4. Conclusions
Acknowledgements
π∗
C-K
150
single crystals. It was found that there are about 10 at.% carbon in the hexagonal B–C–N phase according to the EELS spectra. For example, the atomic percents are quantified with usual hydrogenic cross-sections [20] from Fig. 4 is B 52%, C 11% and N 37%, which can be formulated as B0.52C0.11N0.37. Owing to the relative low carbon content existed in the hexagonal B–C– N phase result in dispersive distribution of carbon atom in the lattice, the structure of C peak in the EELS spectrum exhibits the character of amorphous carbon.
250
300
σ∗ π∗
350
400
450
Energy Loss (eV) Fig. 4. The EELS spectrum of the typical hexagonal B–C–N single crystal showed in Fig. 3.
References [1] R.B. Kaner, J. Kouvetakis, C. Shen, C.E. Warble, M.L. Sattler, N. Bartlett, Mater. Res. Bull. 22 (1987) 399.
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G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
[2] A.W. Moore, S.L. Strong, G.L. Doll, M.S. Dresselhaus, I.L. Spain, C.W. Bowers, H.P. Issi, L. Piraux, J. Appl. Phys. 65 (1989) 5109. [3] D. Hegemann, R. Riedel, C. Oehr, Thin Solid Films 339 (1999) 154. [4] A. Dinescu, A. Perrone, L. Pcaricato, C. Mirenghi, C. Gerardi, L. Ghica, A. Frunza, Surf. Sci. 692 (1998) 127. [5] M.K. Lei, L.J. Yuan, Z.L. Zhang, T.C. Ma, Chin. Phys. Lett. 16 (1999) 71. [6] S. Ulrich, J. Scherer, J. Schwan, I. Barzen, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 4 (1995) 288. [7] A.R. Badzian, Mater. Res. Bull. 16 (1981) 1385. [8] E. Knittle, P.B. Kaner, R. Jeanloz, M.L. Cohen, Phys. Rev., B 51 (1995) 12149. [9] V.L. Solozhenko, D. Andrault, G. Fiquet, M. Mezouar, D.C. Rubie, Appl. Phys. Lett. 78 (2001) 1385. [10] T. Sasaki, M. Akaishi, S. Yamaoka, Y. Fujiki, T. Oikawa, Chem. Mater. 5 (1993) 695. [11] R. Riedel, J. Bill, G. Passing, Adv. Mater. 3 (1991) 551. [12] J.L. He, Y.J. Tian, D.L. Yu, T.S. Wang, F.R. Xiao, A.D. Li, D.C. Li, Y.G. Peng, L. Li, J. Mater. Sci. Lett. 19 (2000) 2061.
[13] J.L. He, Y.J. Tian, D.L. Yu, T.S. Wang, S.M. Liu, L.C. Guo, D.C. Li, X.P. Jia, L.X. Chen, G.T. Zou, O. Yanagisawa, Chem. Phys. Lett. 340 (2001) 431. [14] H. Sun, S.H. Jhi, D. Roundy, M.L. Cohen, S.G. Louie, Phys. Rev., B 64 (2001) 094108. [15] M. Mattesini, S.F. Matar, Comput. Mater. Sci. 20 (2001) 107. [16] M. Mattesini, S.F. Matar, J. Inorg. Mater. 3 (2001) 943. [17] R.Q. Zhang, K.S. Chan, H.F. Cheung, S.T. Lee, Appl. Phys. Lett. 75 (1999) 2259. [18] Y.D. Li, Y.T. Qian, H.W. Liao, Y. Ding, L. Yang, C.Y. Xu, F.Q. Li, G.E. Zhou, Science 281 (1998) 246. [19] M.Y. Yu, K. Lai, Z.F. Lai, D.L. Cui, X.P. Hao, M.H. Jiang, Q.L. Wang, J. Cryst. Growth 269 (2004) 570. [20] Y.J. Bai, B. Lv, Z.G. Liu, L. Li, J. Cryst. Growth 247 (2003) 505. [21] L. Shi, Y.L. Gu, L.Y. Chen, Y.T. Qian, Z.H. Yang, J.H. Ma, Solid State Commun. 128 (2003) 5. [22] F.L. Huang, C.B. Cao, X. Xiang, Diamond Relat. Mater. 13 (2004) 1757.
Solvothermal synthesis of hexagonal B–C–N compound at low temperature conditions Guang Sun, ZhongYuan Liu, Zhongmin Zhou, Julong He, Dongli Yu, Yongjun Tian ⁎ Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Received 6 September 2005; received in revised form 11 January 2006; accepted 3 February 2006 Available online 6 March 2006
Abstract Hexagonal B–C–N crystals have been successfully synthesized by a solvothermal method from carbon tetrachloride and calcium boron nitride at 400 °C. The characterization of synthesized product was carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with electronic energy loss spectrometer. The composition of the hexagonal B–C–N single crystal is detected by electronic energy loss spectrometer as B0.52C0.11N0.37. © 2006 Elsevier B.V. All rights reserved. Keywords: B–C–N; Solvothermal synthesis; High pressure crystal growth
1. Introduction Ternary B–C–N compounds are of great research interest because of their potentially mechanical and electronic properties. Much attention has been paid to search for new and effective experimental methods to synthesize B–C–N compounds. A variety of extreme methods including chemical vapor deposition [1–3], physical vapor deposition [4–6], highpressure and high-temperature (HP/HT) synthesis [7–10] and solid phase pyrolysis [11] were developed. However, most of the synthesized products are of poor crystallinity and have turbostratic or amorphous structure. Only a few of them are crystalline. Badzian carried out the pioneer work to prepare crystalline diamond-like B–C–N compound at temperature of 3300 K and pressure of 14 GPa, the composition of the obtained compound was close to BC6N [7]. Knittle et al. [8] and Solozhenko et al. [9] reported the synthesis of diamond-like BCxN compounds using static pressure and laser-heated diamond cell from graphitic BC2N. We have successfully
⁎ Corresponding author. E-mail address: [email protected] (Y. Tian). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.02.001
synthesized two crystalline B–C–N compounds. One is a polycrystalline hexagonal B–C–N compound prepared by a chemical process [12]. The other is an orthorhombic B2CN crystal prepared under HP/HT condition [13]. Nevertheless, it is difficult to convince that the above products are true ternary B– C–N materials. Theoretical and experimental results revealed that the ternary B–C–N materials tend to segregate into C and BN [10,14–17]. Sasaki et al. observed that simultaneous crystallization of diamond and c-BN from the melt of cobalt– boron–carbon–nitrogen under 5.5 GPa and 1400∼1600 °C [10]. Sun et al. [14] studied seven possible diamond-structured BC2N phases by using ab initio pseudopotential density functional method. They indicated that diamond-structured BC2N energetically favor separation into diamond and c-BN. Therefore, the observed B–C–N materials may be the mixture of C and BN. It is needed to confirm that the single-crystal in the synthesized product consists of B, C and N elements. Recently, a solvothermal method was found to be an effective and moderate route to prepare metastable materials, such as successfully synthesized nano-diamond, c-BN, h-C3N4 and B4C etc. in recent years [18–21]. Huang et al. reported that a turbostratic boron carbonitride was prepared by solvothermal method [22]. In this work, we prepared crystalline B–C–N compounds through a solvothermal synthetic route by the
G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
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reaction of calcium boron nitride (Ca3B2N4) and carbon tetrachloride (CCl4) at moderate temperature and pressure. 2. Experimental procedures In this experiment, 4 g Ca3B2N4 powder was put into 30 ml CCl4 solution in a stainless steel autoclave. Then the vessel was filled in high purity nitrogen gas (99.999%). The autoclave was heated to the reaction temperature, maintained for 12 h, and then cooled to room temperature naturally. The range of reaction temperature was 350–550 °C. The product, dark powder, was washed with absolute ethanol, dilute hydrochloric acid and distilled water in sequence, and then dried in vacuum at 100 °C for 5 h. The structure determination of the product was performed by X-ray diffraction (XRD, Rigaku D/Max-Rb) and transmission electron microscopy (TEM, JEM-2010). XRD pattern was carried out with Ni filtered Cu Kα radiation at a scanning rate of 5°/min. The average composition was determined under a scanning electron microscope (SEM, KYKY-2800) with energy dispersive X-ray (EDS, Kevex Sigma-32 Level-4). Electronic energy loss spectrometer (EELS, Gatan-ENFINA-776) was used to determine the composition and structural information of individual particle in the synthesized powder under TEM.
50
70
90
o
2θ ( )
2θ ( o ) Fig. 1. XRD patterns of sample obtained at (a) 350 °C; (b) 400 °C; (c) 450 °C; (d) 500 °C and (e) 550 °C.
30
Fig. 2. XRD pattern of the synthesized powder obtained at 400 °C.
Therefore, we can conclude that the optimum temperature for the formation of hexagonal phase is about 400 °C in our experiment. The average composition of the sample obtained at 400 °C was B0.36C0.35N0.29 detected by EDS. The ratio of B : C : N seems near to 1 : 1 : 1. To further investigate the structure and composition of the powder the sample was measured by TEM and EELS. The experimental results show that the composition in the powder is uneven. We found that amorphous carbon and several crystalline powders of graphite, h-BN and hexagonal B– C–N coexisted in the sample. The dominant phases in the product are h-BN and hexagonal B–C–N compounds. This phenomenon can be observed in the XRD result shown in Fig. 2. It can be seen that there are some short peaks with larger d spacing coexisted in the (002) peak of the sample, which result in the bottom of (002) peak becomes broad. This is because of the adjacent lattice parameters between graphite, h-BN and hexagonal B–C–N. Therefore, the XRD result indicates that there may be three kinds of hexagonal phases existed in the product. The X-ray diffraction data of the hexagonal compound from Fig. 2 are listed in Table 1. It is difficult to distinguish the two phases of h-BN and h-B–C–N in the XRD spectrum because of no obvious difference between their lattice parameters. Just from the XRD measurement itself, it is not possible to determine the existence of h-B–C–N compounds because h-BN and h-B–C–N phases are similar in structure and
3. Results and discussion The XRD spectra of the synthesized samples at different temperatures are shown in Fig. 1. The effect of reaction temperature on the formation of hexagonal phase was investigated in our experiment. We found that no chemical reactions happened below the temperature of 350 °C. When the reaction temperature reached 350 °C, a hexagonal graphite-like phase was formed. The main phase of the obtained product is amorphous (Fig. 1a). The obtained product at about 400 °C contains the least amorphous phase (Fig. 1b). The amount of the amorphous phase enhances in the product when the reaction temperature raised from 450 to 550 °C (Fig. 1c, d and e).
Table 1 The X-ray diffraction values of synthesized B–C–N compound obtained at 400 °C 2θ
d (nm)
(hkl)
26.781 41.621 43.963 50.182 55.085 71.560 76.000 82.258
0.3326 0.2168 0.2058 0.1817 0.1666 0.1318 0.1251 0.1171
(002) (100) (101) (102) (004) (104) (110) (112)
G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
1661
Fig. 3. TEM image (a) and SAED pattern (b) of a typical hexagonal B–C–N single crystal.
their peaks are generally overlapped with each other in the XRD spectrum. The existence of h-B–C–N crystals in our synthesized sample can be confirmed in the EELS measurements on a series of chosen particles. Fig. 3 shows a bright field image of a representative B–C–N single crystal (Fig. 3a) and its corresponding pattern of selected area electron diffraction (Fig. 3b). It is shown that the B–C–N single crystal has hexagonal structure. A typical EELS spectrum of the hexagonal B–C–N single crystal is displayed in Fig. 4. It exhibits the characteristic K-shell ionization edges of elemental boron, carbon and nitrogen at 188, 284 and 401 eV, respectively, which clearly shows the presence of sp2 bonding (π⁎ peaks). All three K edges (B, C, and N) appear simultaneously in the spectrum. This indicates that the single crystal is a true B–C–N ternary compound rather than segregated phases of graphite and h-BN. We quantitatively analyzed the EELS spectra to determine the chemical composition of the hexagonal B–C–N
B-K
Countes(a.u.)
σ∗
In summary, we have successfully synthesized a hexagonal B–C–N crystal by a solvothermal reaction between carbon tetrachloride and calcium boron nitride at the temperature range of 350–550 °C. The effect of reaction temperature on the formation of hexagonal phase was investigated in our experiment. Well crystallized B–C–N compound can be obtained at 400 °C. TEM and EELS measurements show that the synthesized B–C–N crystal has a hexagonal structure, which has the composition of B0.52C0.11N0.37. Due to the simple manipulation and cheaper reactants, such solvothermal synthesis technique has provided a facile and efficient method to get the attractive hexagonal B–C–N compounds.
This work was supported by National Natural Science Foundations of China (Grant Nos. 50225207, 50372055, 50472051 and 50532020) and the National Basic Research Program of China (Grant. No. 2005CB724405).
N-K
σ∗ π∗
200
4. Conclusions
Acknowledgements
π∗
C-K
150
single crystals. It was found that there are about 10 at.% carbon in the hexagonal B–C–N phase according to the EELS spectra. For example, the atomic percents are quantified with usual hydrogenic cross-sections [20] from Fig. 4 is B 52%, C 11% and N 37%, which can be formulated as B0.52C0.11N0.37. Owing to the relative low carbon content existed in the hexagonal B–C– N phase result in dispersive distribution of carbon atom in the lattice, the structure of C peak in the EELS spectrum exhibits the character of amorphous carbon.
250
300
σ∗ π∗
350
400
450
Energy Loss (eV) Fig. 4. The EELS spectrum of the typical hexagonal B–C–N single crystal showed in Fig. 3.
References [1] R.B. Kaner, J. Kouvetakis, C. Shen, C.E. Warble, M.L. Sattler, N. Bartlett, Mater. Res. Bull. 22 (1987) 399.
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G. Sun et al. / Diamond & Related Materials 15 (2006) 1659–1662
[2] A.W. Moore, S.L. Strong, G.L. Doll, M.S. Dresselhaus, I.L. Spain, C.W. Bowers, H.P. Issi, L. Piraux, J. Appl. Phys. 65 (1989) 5109. [3] D. Hegemann, R. Riedel, C. Oehr, Thin Solid Films 339 (1999) 154. [4] A. Dinescu, A. Perrone, L. Pcaricato, C. Mirenghi, C. Gerardi, L. Ghica, A. Frunza, Surf. Sci. 692 (1998) 127. [5] M.K. Lei, L.J. Yuan, Z.L. Zhang, T.C. Ma, Chin. Phys. Lett. 16 (1999) 71. [6] S. Ulrich, J. Scherer, J. Schwan, I. Barzen, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 4 (1995) 288. [7] A.R. Badzian, Mater. Res. Bull. 16 (1981) 1385. [8] E. Knittle, P.B. Kaner, R. Jeanloz, M.L. Cohen, Phys. Rev., B 51 (1995) 12149. [9] V.L. Solozhenko, D. Andrault, G. Fiquet, M. Mezouar, D.C. Rubie, Appl. Phys. Lett. 78 (2001) 1385. [10] T. Sasaki, M. Akaishi, S. Yamaoka, Y. Fujiki, T. Oikawa, Chem. Mater. 5 (1993) 695. [11] R. Riedel, J. Bill, G. Passing, Adv. Mater. 3 (1991) 551. [12] J.L. He, Y.J. Tian, D.L. Yu, T.S. Wang, F.R. Xiao, A.D. Li, D.C. Li, Y.G. Peng, L. Li, J. Mater. Sci. Lett. 19 (2000) 2061.
[13] J.L. He, Y.J. Tian, D.L. Yu, T.S. Wang, S.M. Liu, L.C. Guo, D.C. Li, X.P. Jia, L.X. Chen, G.T. Zou, O. Yanagisawa, Chem. Phys. Lett. 340 (2001) 431. [14] H. Sun, S.H. Jhi, D. Roundy, M.L. Cohen, S.G. Louie, Phys. Rev., B 64 (2001) 094108. [15] M. Mattesini, S.F. Matar, Comput. Mater. Sci. 20 (2001) 107. [16] M. Mattesini, S.F. Matar, J. Inorg. Mater. 3 (2001) 943. [17] R.Q. Zhang, K.S. Chan, H.F. Cheung, S.T. Lee, Appl. Phys. Lett. 75 (1999) 2259. [18] Y.D. Li, Y.T. Qian, H.W. Liao, Y. Ding, L. Yang, C.Y. Xu, F.Q. Li, G.E. Zhou, Science 281 (1998) 246. [19] M.Y. Yu, K. Lai, Z.F. Lai, D.L. Cui, X.P. Hao, M.H. Jiang, Q.L. Wang, J. Cryst. Growth 269 (2004) 570. [20] Y.J. Bai, B. Lv, Z.G. Liu, L. Li, J. Cryst. Growth 247 (2003) 505. [21] L. Shi, Y.L. Gu, L.Y. Chen, Y.T. Qian, Z.H. Yang, J.H. Ma, Solid State Commun. 128 (2003) 5. [22] F.L. Huang, C.B. Cao, X. Xiang, Diamond Relat. Mater. 13 (2004) 1757.