“BCN” Diamond under High Pressure and High Temperature Conditions

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Synthesis and Characterization of New “BCN” Diamond under High Pressure and High Temperature Conditions Xiaobing Liu, Xiaopeng Jia, Zhuangfei Zhang, Ming Zhao, Wei Guo, Guofeng Huang, and Hong-an Ma* State Key Lab of Superhard Materials, Jilin University, Changchun 130012, P. R. China ABSTRACT: In this paper, the graphitic mixtures of C and BN have been subjected to high pressure and high temperature (HPHT) conditions to study the crystallization of the cubic phase, and new diamond crystals doped with B and N atoms (BCxN) were successfully synthesized with iron and nickel as catalysts. The morphology and characterization of our obtained diamond changed significantly, which was attributed to the incorporation of the B and N atoms into the crystal structure. In addition, we detected that the cubic phases obtained in the C0.9(BN)0.1 system were separated because of the different B/N ratio, while in the C0.5(BN)0.5 system no phase separation was found and the obtained “BCN” diamond exhibited cuboctahedral shape, light yellow in color, and nearly transparent. According to our results, two possible reaction routes were introduced for the crystallization of diamond in the graphitic mixtures of C and BN. Moreover, X-ray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy were used to confirm their chemical composition and atomic-level hybrid qualities. Our results show that new B-C-N compounds can be synthesized by doping B and N atoms in diamond crystals under HPHT conditions, and it may become a new effective method in the future study of the preparation of cubic B-C-N alloys.

1. INTRODUCTION It is well-known that diamond and cubic boron nitride (c-BN) are very important materials for mechanical and electronic applications because of their superior properties of hardness, thermal conductivity, and chemical inactivity. Since Cohen proposed theoretically that covalently bonded carbon nitrogen compounds might be as hard as diamond with a relatively isotropic arrangement that is short (∼1.47 Å in length) and covalent (∼7% ionic),1-3 carbon nitride and related materials have been widely investigated and developed.4,5 These results suggest that such covalently bonded materials are important in high-performance engineering applications.6-9 Recently, hybrid compounds composed of three elements, boron, carbon, and nitrogen, have received special attention in demonstrating fundamental physics in materials and potential applications in new electronics with superior properties, and a large amount of research has been devoted to the synthesis and properties of B-C-N materials. In addition, crystalline B-C-N phases have been prepared using different techniques, such as chemical vapor deposition (CVD), shock compression, and high pressure and high temperature (HPHT) synthesis. And new hybrid composites have been successfully fabricated, showing interesting structural properties, such as a two-dimensional atomic film consisting of hybridized BN and C domains10 and hybrid composites involving BN nanotubes in carbon-based matrices.11 Furthermore, the synthesis of a diamond-like phase containing B, r 2011 American Chemical Society

C, and N elements is always a topic of interest in the expectation of intermediate or even superior properties because their band gaps are thought to be intermediate between diamond and c-BN, and can be determined by atomic composition and atomic arrangement.10,12-18 However, there is still a lack of general estimates for the formation of stable and reproducible cubic B-C-N alloy. Considering that the carbon and boron nitride have the same types of polymorphs, such as hexagonal forms (graphite and h-BN) and cubic ones (diamond and c-BN), a cubic B-C-N alloy would be expected to be prepared by transformation from HPHT conditions, by analogy with the preparation of diamond and c-BN from their hexagonal polymorphs.19-25 Previous studies have reported the synthesis of cubic B-C-N compounds under HPHT conditions.26-30 A cubic B-C-N phase has been successfully synthesized by Badzian at 14 GPa and 3300 K starting from the graphitic B-C-N,26 and Kakudate et al.31 have claimed the formation of a cubic B-C-N substance by a shock-wave technique. Knittle et al. have also reported the presence of cubic B-C-N both in the microcrystalline B-C-N compounds and the graphite-structured mixture of BN and C as starting materials, prepared at 30 GPa and 1500 K.27 Received: July 16, 2010 Revised: January 28, 2011 Published: March 14, 2011 1006

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Table 1. Experimental Results of Diamond Growth in the Mixture of Graphite and h-BN Compounds with Fe-Ni Alloy As Catalysta obtained cubic phase run

a

initial composition

pressure (GPa)

temperature (K)

time (min)

morphology

color

size (mm)

N-1

graphite

5.2

1500-1600

20

cuboctahedron

yellow

0.2-0.3

N-2

C0.98(BN)0.02 (g)

5.8-6.0

1550-1650

20

strip-shape

green

0.6-0.7

N-3 N-4

C0.9(BN)0.1 (g) C0.9(BN)0.1 (g) þ 1 wt% Al

6.3-6.4 6.1-6.2

1650-1880 1570-1690

20 20

irregular shape octahedron

dark green black

0.5-0.6 0.3-0.4

N-5

C0.5(BN)0.5 (g)

7.0-7.2

1800-2300

30-60

cuboctahedron

light yellow

0.02

(g) labeled in N-2-N-5 runs indicates a mechanical mixture of graphitic carbon and boron nitride.

Moreover, a mixture of diamond, c-BN, and cubic B-C-N substances is produced by Nakano et al. under 7.7 GPa in the temperature range from 2420 to 2570 K, but the cubic B-C-N substance tends to decompose into diamond and c-BN at higher temperatures, which suggests that the cubic B-C-N compound is not thermodynamically stable.28 Thus, it is difficult to prepare a reliable material, mainly due to the difficulty in distinguishing a B-C-N compound from a microcrystalline mixture of diamond and c-BN. However, some diamonds with minor boron and nitrogen elements are found in these systems under HPHT conditions.28,29 Besides, theoretical work has investigated that BCxN solids with higher carbon contents are more structurally stable and have higher elastic moduli, making them more attractive as potential superhard materials.32 Thus, the “BCN” diamond should be given more attention for the energetically favorable cubic structure and is expected to behave as electronic and optoelectronic devices for the incorporation of boron and nitrogen elements. In view of synthesizing a pure “BCN” diamond, the appropriate catalysts must be chosen carefully to avoid decomposition of B-C-N compounds and to grow crystals of considerable size under relatively mild conditions. In the present study, we examined the crystallization behavior for the conversion from the graphitic mixture of C and BN to cubic compounds in the presence of Fe-Ni alloys as catalysts. Previously, the hexagonal-to-cubic transformation with the help of cobalt metal as a catalyst under relatively mild conditions (e.g., 5.5 GPa and 1630 K) was also carried out; however, simultaneous crystallization of diamond and c-BN instead of a cubic B-C-N substance was obtained.29 While in this work, we have successfully synthesized the diamond with boron and nitrogen elements and found two possible reaction routes for the crystallization of diamond in the mixtures of graphite and h-BN. Our results suggest that, compared with the cubic B-C-N crystal, the “B-C-N” diamond is easier to be synthesized and more structurally stable. Thus, we suppose the “B-C-N” diamond will be more attractive in future studies on the cubic B-C-N alloys.

2. EXPERIMENTAL PROCEDURES Experiments on diamond crystallization were carried out using a china-type large volume cubic high-pressure apparatus (CHPA) (SPD6x1200). The temperature was measured in each experiment using a Pt30% RH/Pt-6% Rh thermocouple, whose junction was placed near the crystallization sample. The pressure was calibrated at room temperature by the change in resistance of standard substances and at high temperatures by the graphite-diamond equilibrium. The starting materials were three different mechanical mixtures of graphite-structured BN (h-BN) and C (graphite) with stoichiometries of C0.98(BN)0.02, C0.9(BN)0.1, and C0.5(BN)0.5. In order to avoid the

c-BN phase formation, the Fe70Ni30 alloy powders (200 mesh) were chosen as a catalyst. The starting materials were produced by mechanically milling a high purity mixture of h-BN (40 μm in size) and graphite (30 μm in size). The graphitic powders were mechanically milled for 10 h at ambient conditions and mixed with Fe-Ni alloy for 4 h. Then the mixtures were machined into a sample for synthesizing diamond. Samples were first subjected to pressures of 5.0-7.2 GPa, heated to temperatures of 1500-2300 K with different holding times, and then cooled down to room temperature. After the experiments, the composition and characterization of the synthesized products were characterized by an optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman spectrum. Then the products were cracked and dissolved in hot nitric acid to remove the remaining graphitic components and metal catalysts. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to confirm their chemical composition and atomic-level hybrid qualities.

3. RESULTS AND DISCUSSION 3.1. Crystallization of Diamond in the Mixtures of h-BN and Graphite with Stoichiometries of C0.98(BN)0.02 and C0.9(BN)0.1. Typical synthesis conditions for the cubic, high P-T

phases are pressures of 5.0-7.2 GPa and temperatures between 1500 and 2300 K, and the morphology of the obtained diamonds are summarized in Table 1. In experiments performed at 5.2 GPa, the diamond nucleation and growth are established in the FeNi-C system and the synthesized diamond, being greenish yellow in color, exhibits cubic, cuboctahedral, and octahedral shape at 1500 K, 1550 K, and 1600 K (N-1), respectively. The average size is about 0.2-0.3 mm in a 20 min run (Figure 1a). In run N-2, some diamond crystals are obtained and well dispersed as individual crystals in the C0.98(BN)0.02 system at 1550-1650 K and 5.8 GPa. Besides the normal cuboctahedral shape, the diamond crystals exhibit a strip shape which looks like stretched cuboctahedron along the {100} or {111} crystal surface (Figure 1b). The maximum size of the synthesized diamond is 600-700 μm in a 20 min growth process. The obtained diamonds are green or deep green, most of them being nearly opaque. With the pressure and temperature increasing to 6.7 GPa and 1850 K, respectively, the graphitic mixtures have fully changed to cubic phase in a 1 h run. The resulting products have been a mixture of diamond and catalyst (Figure 2a). In the mixtures of C0.9(BN)0.1, diamond crystals are obtained at 6.3 GPa and 1650-1880 K (N-3). These crystals have poor morphology and are dark green in color (Figure 1c). When the pressure increases to 7.0 GPa, besides the mixture of diamond and catalyst, some white materials are also found in the resulting products after treatment in a 2 h run (Figure 2b). The obtained 1007

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Figure 1. Optical images of diamond crystals obtained in the Fe-Ni-C system (a); in the compositions of graphite and h-BN with stoichiometries of C0.98(BN)0.02 (b); C0.9(BN)0.1 (c); and C0.9(BN)0.1 with 1 wt% Al additive (d) with Fe-Ni alloy as catalyst.

Figure 2. Optical images of the compositions of graphite and h-BN with stoichiometries of C0.98(BN)0.02 (a) and C0.9(BN)0.1 (b) after treatment at 6.7 and 7.0 GPa, respectively; the SEM image for the sample after treatment at 7.0 GPa in the C0.9(BN)0.1 system (c); panel (d) shows the Raman spectra of the white materials highlighted by circles in (b) in the resulting samples.

diamonds have fused into aggregate (Figure 2c). The Raman spectroscopy is known to be a very efficient technique for the investigation of chemical bonds of graphitic carbons.33,34 The typical Raman spectrum for these white materials is shown in Figure 2d. The Raman bond in h-BN is observed at 1364 cm-1, which indicates that the graphite-like sp2 B-N bonding still exists in the remaining materials.35 We notice the wavenumber to those found in graphite in which the band at 1580 cm-1 is shifted

to 1540 cm-1. This band shift is probably due to the doping of boron and nitrogen atoms in the graphite network. Furthermore, a small bond center at 1275 cm-1 is attributed to the decomposition of the h-BN in the system. To study the influence of the B and N elements on diamond crystallization, Al powders are added as nitrogen getter to synthesize diamond in the C0.9(BN)0.1 system (N-4). It is obvious that the obtained diamond exhibits octahedral morphology and is black in color (Figure 1d), 1008

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Figure 3. Powder XRD patterns of the cubic phases obtained in the graphite (a) and mixtures of graphite and h-BN with stoichiometries of C0.98(BN)0.02 (b), C0.9(BN)0.1 (c), and C0.9(BN)0.1 with 1 wt% Al additive (d). The top right insets are the XRD patterns for the corresponding {111} peak of the cubic phases.

which is the typical characteristic of boron-doped diamond.36 These results suggest that the special morphologies (Figure 1b,c) arise from the incorporation of nitrogen impurity. On the basis of the above results, we could conclude that the h-BN powders are partially decomposed to boron and nitrogen atoms, part of which could incorporate into the diamond crystal structures. Then, the remaining boron and nitrogen atoms in the C0.9(BN)0.1 system reform the h-BN again. Figure 3 shows the XRD spectra of the diamond prepared in different systems. It is found from Figure 3a that the diffraction pattern of obtained diamonds in the Fe-Ni-C system is mainly composed by the {111} and {220} lines of the cubic lines. The XRD patterns, as shown in Figure 3b, reveal the pure diamond phase without other cubic phases existing in resulting products in the C0.98(BN)0.02 system. However, note that the diffraction peaks assignable to the cubic products obtained in the C0.9(BN)0.1 system are rather broad and split (Figure 3c,d), indicating the multicomponent nature. As illustrated by the insets in Figure 3c,d, the resulting products in the C0.9(BN)0.1 system may be composed by two components labeled as A and B (see the inset in Figure 3c) and changed into three components (A, B, and C) with the nitrogen getter added in the system. All the lattice constants of the A, B, and C phases can be considered as that of diamond containing some amount of boron and nitrogen within the experimental error independent of the synthesis temperature, rather than that of the c-BN. This may be because Al acts only as a nitrogen getter in the diamond crystallization,37 instead of the catalyst for the nucleation of c-BN in this system. The broad and

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split diffraction peaks of the XRD patterns are attributed to the different B/N ratio incorporated into the diamond crystal structures. This is different from the previous report on the crystallization of the cubic phases from graphitic BC2N with Co additive.29 Their compressed product is thought to be consisting of c-BN and diamond, which may be due to the chosen Co catalyst. Co-Al alloy has also been reported as a catalyst for the phase transformation from h-BN to c-BN.38 In this paper, we believe that the Fe and Ni are not effective catalysts for the conversion from h-BN to c-BN. Thus, the Fe-Ni alloy as catalyst here would be suitable for the synthesis of the cubic B-C-N alloys. To further study the bonding state of the diamond doped with B and N (N-3), the XPS and FTIR spectrum are present in Figure 4. Figure 4a-c shows XPS spectra of B, N, and C, respectively. In Figure 4a, the deconvolution of B 1s spectrum gives four peaks centered at 188.6 eV, 190.1 eV, 191.4 eV, and 192.7 eV, respectively. The full width at half-maximum (fwhm) for the B 1s spectrum is 2.9 eV, which is larger than that of h-BN (0.92 eV) and BN films (1 eV), suggesting that B atoms are in different valence states. It has been reported that the B 1s spectra in B4C and BC3.4 have peaks at 188.4 and 189.4 eV, respectively.39 Thus, the resolved peak of 188.6 eV as shown in Figure 4a can be attributed to B-C bonding. Furthermore, since the peaks at 190.1 and 191.4 eV (Figure 4a) are very close to that of B 1s in h-BN (190.1 eV), the two peaks are due to B-N bonds. As previously reported,40 the peak energies due to B-O and B-B bonds are 192.0 and 187.9 eV, respectively. Thus, the peak energy of 192.7 eV is due to B-O bonds, and no obvious B-B bonding properties appear in the spectrum. Accordingly, boron atoms in our obtained diamond bond with carbon and nitrogen atoms. In Figure 4b, the results of the deconvolution demonstrate that there are two types of N chemical states in the diamond, and the peaks of their bonding energies are centered at 399.5 and 399.9 eV, respectively. The former can be assigned to N-B bonding and the latter to the sp2 NdC which is centered at 400 eV,41 indicating that N atoms bond with B and C atoms. Besides, the C 1s peak located at 284.7 eV should be attributed to the C-C bonding (Figure 4c). We know that the determination of the exact peak positions is affected by many factors including the deconvolution process of the spectra and charging during measurement, etc. Many reports assigned this C1s peak as sp2 CdC bonds,39,41,42 while others made a different assignment because of the similar C-C bonding (at 284.5 eV) observed in the cubic BC2N thin film.43 In this work the samples are dominated diamonds doping with B and N atoms; therefore, this C 1s peak should be due to the sp3 C-C bonding in diamonds. The small shoulder at the higher energy side is due to C-N bonds,5 whereas the other one at lower binding energies is from C-B bonds.43,44 Figure 4d shows the typical infrared absorption spectra of the diamond doping with N in the Fe-Ni-C system (curve A) and that simultaneously doping with B and N in C0.9(BN)0.1 (curve B). Generally, the absorption peaks at 1130 cm-1 and 1282 cm-1 show that the N impurity in the diamond is composed of the A-form (pairs of nearest neighboring substitutional N atoms) and the C-form of N (single substitutional N atoms).45 It is clear from Figure 4d that the FTIR spectra of the diamond obtained in the C0.9(BN)0.1 system (curve B) are obviously different from that in Fe-Ni-C system, and a very wide absorption band centered at 1398 cm-1 is observed in curve B. Considering the higher content of N atoms in the obtained diamond, the wide 1009

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Figure 4. Evidence for synthesis of the diamond doping with B and N elements in the C0.9(BN)0.1 system: panels (a), (b), and (c) are the XPS spectra for B, N, and C 1s core levels, respectively. The spectrum curves are deconvoluted (green curves) by Gaussian fitting (red curves), indicating possible multibonding information. Panel (d) shows the typical FTIR spectra recorded for diamond crystals (A) only doping with nitrogen synthesized in the Fe-Ni-C system and (B) simultaneously doping with boron and nitrogen elements obtained in the mixtures of graphite and h-BN. Spectra (curves A and B) have been displaced vertically for comparison.

absorption band centered at 1398 cm-1 should be attributed to the B-N bonds similar to the in-plane stretching of sp2 B-N bonds (centered at 1380 cm-1)42 and the C-N bonds detected at 1200-1265 cm-1 in carbon nitride films.46 It indicates that the main B-N bonding configuration in our diamond is similar to that of h-BN, where three N atoms surround one B atom. Accordingly, we suggest that some C-C pairs are replaced by B-N pairs during the mixtures of graphite-structured BN (hBN) and C (graphite) to form the cubic phase. The FTIR results also indicate that C atoms bond with both B and N atoms in our obtained diamonds, which is consistent with the XPS results. 3.2. Diamond Crystallization in the Mixtures of h-BN and Graphite with Stoichiometries of C0.5(BN)0.5. Figure 5 illustrates the optical and SEM images for the materials in C0.5(BN)0.5 system (N-5) before and after HPHT treatment. The original sample is a mixture composition of graphite and h-BN (Figure 5a), which are black and white in color, respectively. Figure 5b shows that the sample has a flat surface. The sample gradually changes to polycrystalline phase and it is uniformly gray in the processing pressure at 7.0 GPa and temperature at 1800 K (see Figure 5c). It is clearly illustrated from Figure 5d that some small particles appear on the surface of the sample with a size of about 1-3 μm. With increasing pressure to 7.2 GPa and temperature to 1800-2250 K, some new phases are established in the C0.5(BN)0.5 system (N-5), shown in Figure 5e,f. Note that

some well-faceted crystals with a columnar shape, 4-5 μm in size, are obtained in the samples, which is similar to the hexagonal morphologies of C3N4.4,5 Besides, diamonds are also found in this system (Figure 5g,h). The diamond exhibits cuboctahedral shape and is about 20 μm in size. It is clearly seen from Figure 6 that the obtained crystals in this system, light yellow and nearly transparent, are quite distinguished from that obtained in the systems of C0.98(BN)0.02 and C0.9(BN)0.1. Therefore, it is important to identify the formation of the cubic products in the C0.5(BN)0.5 system. Figure 7 shows the XRD patterns of the samples treated at 7.2 GPa and 1500-2300 K for 30 min. The pattern of the sample obtained at 1500 K is almost identical with that of the starting graphitic mixtures of C and BN (see Figure 7a). With the increase of temperature, new graphitic phases are found in the resulting materials, as shown in Figure 7b,c. Moreover, the XRD results definitely demonstrate that a cubic phase is synthesized in the graphitic compounds. With increasing temperature to 2300 K, the cubic phase disappears gradually (Figure 7d). Figure 7e shows the typical XRD pattern of the resulted sample at 7.2 GPa and 2050 K in the C0.5(BN)0.5 system, which is after treatment by hot nitric acid to remove the remaining graphitic components and metal catalysts. The appearance of the weak {200} diffraction peak implies that there are some B-C-N materials with a graphite-like sp2 structure existing in the system.15,47 1010

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Figure 5. Optical images (left panels) and SEM micrographs (right panels) for the mixtures of graphite and h-BN with stoichiometries of C0.5(BN)0.5 before (a, b), and after treated under 7.0 GPa at 1800 K (c, d); at 7.2 GPa and 2050 K (e, f); typical diamond obtained at 7.2 GPa and 2250 K (g, h).

Figure 6. Optical images of diamond in the C0.5(BN)0.5 system.

Considering this peak still appears after an acid treatment, it may suggest that the substance is trapped in the cubic crystals. The

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diffraction pattern of the samples with strong {111}, {220}, and {311} lines of the cubic lines with no splitting indicates that the sample is a single cubic phase without phase separation in the C0.5(BN)0.5 system. Table 2 gives the lattice parameters for each cubic phase obtained in this work and previously reported.27 As shown, the lattice structure of our obtained “BCN” diamond expands with the incorporation of boron and nitrogen into the cubic diamond lattice and the lattice parameters are the ones between that of the diamond and cubic C0.6(BN)0.4 compounds. The Raman spectra of the resulting samples are also shown in Figure 8. Compared with that of the starting materials, new peaks appear as the temperature increases to 1800 K (Figure 8b). These new phases may be attributed to a compressed graphitic substance in the resulting materials, just like graphitic B-C-N compounds. With increasing pressure to 7.2 GPa and temperature to 2050 K, some cubic phases are also synthesized in the system (Figure 8c). Combined with the above results and analysis, we therefore assume that the high-pressure phase is composed by the starting material, graphite-like B-C-N compounds, and cubic diamond doping with B and N atoms. The Raman spectrum of obtained samples has only one first-order optic mode in all the systems (Figure 8d). Generally, the Raman spectrum of diamond has one first-order optic mode at 1332 cm-1, whereas c-BN has both a longitudinal optical (LO) and a transverse optical (TO) mode in its first-order Raman spectrum at 1305 and 1056 cm-1, respectively. As shown in Figure 8d, a relatively strong 1330.2 cm-1 Raman peak of the synthesized diamond is found in the C0.9(BN)0.1 system (curve B), while the Raman peak is found to shift to 1328 cm-1 in the C0.5(BN)0.5 system (shown in curve C). The position of the Raman band of the cubic phase obtained in this work locates between the Raman peak of diamond (1332 cm-1) and that of c-BN (1305 cm-1). Thus, the Raman spectrum indicates that the lattice structure of the cubic crystals should still keep stable as that of diamond rather than c-BN. On the basis of recent reports on B-C-N compound formation, it seems reasonable to assume that the shift of the observed Raman peak is most likely due to substitution of C atoms by B and N atoms, which leads to the increase in ionic character of the B-N bond relative to the C-C bond in diamond lattice. A similar suggestion has been made by Knittle et al. who found a Raman peak at 1323 cm-1 in the study of the cubic C0.3(BN)0.7,27 and Raman peaks at 1325.7 and 1326.3 cm-1 were also detected by Hubble et al.48 XPS spectra indicate that the diamond samples (N-5) contain boron, carbon, and nitrogen elements in the crystal structure. The deconvolution of B 1s spectrum (Figure 9a) for the sample presents three peaks centered at 189.1 eV, 189.8 eV, and 190.5 eV, respectively. The resolved peaks of 189.1 eV and the 189.8 eV can be attributed to B-C bonding39,43 and the peak at 190.5 eV is due to B-N bonds. Thus, most of the B atoms in the diamond bond with C and N atoms. In Figure 9b, the results of the deconvolution show that there are two types of N chemical states in the diamond and the peaks of their bonding energies are centered at 397.8 and 398.8 eV, respectively. The resolved peak of 397.8 eV can be attributed to B-N bonding,43 while the peak at binding of 398.8 eV is due to the contribution of the N-C sp3 binding.5,49 Nevertheless, compared with the above XPS data in the C0.9(BN)0.1 system (Figure 4b), an obvious distinction is the absence of the sp2 NdC which is centered at 400 eV in the C0.5(BN)0.5 system. Deconvolution of the C1s peak (Figure 9c) for the sample illustrates that two peaks at 284.3 and 286.0 eV can be attributed to the C-C and C-N bonds, respectively.43 1011

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Figure 7. Powder XRD patterns of the samples after treated at 7.2 GPa and (a) 1500 K, (b) 1800 K, (c) 2050 K, and (d) 2300 K for 20 min, and the patterns in panel (e) show the cubic phases obtained in C0.5(BN)0.5 system at 7.2 GPa and 2050 K.

Table 2. Lattice Parameters of Cubic Compounds from Previous Experiments27 and Our Present Experimental Results composition

synthesis pressure (GPa)

lattice parameter (Å)

hkl

refs

C (diamond)

5.2

3.560

111, 220

this study

“BCN” diamond

7.2

3.575

110, 111, 200, 220, 311

this study

C0.6(BN)0.4

30 ( 5, 40 ( 7

3.596

111, 200, 220

ref 27

C0.5(BN)0.5

35 ( 4

3.602

111, 220

ref 27

C0.3-0.33(BN)0.67-0.7 BN (borazon)

30 ( 3, 50 ( 10 5.3

3.613 3.617

111, 200, 20 111, 200, 220

ref 27 this study

Figure 8. Raman spectra of the samples in the C0.5(BN)0.5 system before (a) and after treatment at 7.2 GPa, 1800 K (b) and 2050 K (c). Panel (d) shows the Raman spectra of the cubic phases obtained in the systems of graphite, C0.9(BN)0.1, and C0.5(BN)0.5 with Fe-Ni alloy as catalyst.

Figure 9d shows the typical infrared absorption spectra of the diamond samples in C0.5(BN)0.5 system. Besides the absorption band attributed to the B-N bonds (centered at 1398 cm-1), the absorption peaks also exist at 1130 cm-1 and 1282 cm-1. This fact indicates that a large number of N atoms are incorporated into the diamond crystal structure by A and C forms, while some C-C pairs are replaced by B-N pairs in the sample. The nitrogen concentration can be determined using the FTIR

spectra.50-52 In our obtained diamond crystals, the nitrogen concentrations for the A- and C-forms exceed 2000 ppm. 3.3. Possible Reaction Routes for the Crystallization of Diamond in the Mixtures of Graphite and h-BN. On the basis of the above investigations, we suggest that there are two possible reaction routes for the crystallization of diamond in the graphitic mixtures of graphite and h-BN caused by the compression as described below: First, in the system of C0.98(BN)0.02 and 1012

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Figure 9. Evidence for synthesis of the diamond doping with B and N elements obtained in the C0.5(BN)0.5 system: panels (a), (b), and (c) are the XPS spectra for B, N, and C 1s core levels, respectively. Panel (d) illustrates the typical FTIR spectrum recorded for diamond crystals.

C0.9(BN)0.1, the h-BN powders are partially decomposed into B and N atoms, which incorporate into the diamond crystal structure during the diamond crystallization process (eq 1). Then, certain C-C pairs are replaced by B-N pairs for the high growth rate. Both morphology and color of the diamond change significantly because of the B and N incorporation. However, it is difficult to control the additives of B and N atoms, which lead to the phase separation for the different B/N ratio in the diamond crystal structures. According to the XPS and FTIR results, the B, C, and N elements are all found in the crystal structure. The results also indicate that C atoms have bonded with both B and N atoms, and most of the B-N bonds are very similar to that of the in-plane stretching of sp2 B-N bonds. T g 1550 K

h - BN sf B þ N

)

ð1Þ

P g 5:8 GPa

graphite þ FeNi sf diamond

P g 7 GPa

h - BN þ graphite þ FeNi sf BCN ðgraphiticÞ P g 7:2 GPa

BCN ðgraphiticÞ þ FeNi sf diamond ðB þ NÞ

)

ð2Þ

In the C0.5(BN)0.5 system, the h-BN component could not decompose absolutely before the diamond crystallization, and some new graphitic phases are found after treatment under HPHT conditions. Furthermore, besides the B-N pairs, the

N-C sp3 bonding are also found in the crystal structure. We thus conclude that the other route for the diamond crystallization is that the mixtures of h-BN and graphite are first transformed to some graphitic B-C-N compounds under HPHT treatment, and then the diamond crystallize directly from graphitic B-C-N compounds in C0.5(BN)0.5 system (eq 2). Therefore, no phase separation in diamond crystals is found in this system. In addition, the obtained diamond crystals, with good morphology, are nearly transparent because of the uniform distribution of B and N atoms in the crystal structure.

4. CONCLUSIONS In conclusion, the mixtures of graphite and h-BN powders with iron and nickel as catalysts have been subjected to high pressure and high temperature conditions to study the diamond crystallization with their duration from 20 min to 2 h. Diamond crystals including a few of B and N elements are obtained in the mechanical mixtures of graphite-structured BN (h-BN) and C (graphite) with stoichiometries of C0.98(BN)0.02, C0.9(BN)0.1, and C0.5(BN)0.5 at a temperature of 1500-2300 K and pressure of 5.0-7.2 GPa. Our results suggest that, compared with the cubic B-C-N crystals, the “BCN” diamonds doped with B and N elements are easier to obtain and more structurally stable, and thus we believe that the “BCN” diamond will be more attractive in future study on the new behaviors for electronic and optoelectronic devices. 1013

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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