Synthesis of Semimetallic BC3. 3N with Orthorhombic Structure at

Synthesis of Semimetallic BC3.3N with Orthorhombic Structure at High Pressure and Temperature Dongxu Li, Dongli Yu, Bo Xu, Julong He, Zhongyuan Liu, Peng Wang, and Yongjun Tian*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2096–2100

State Key Laboratory of Metastable Materials Science & Technology, Yanshan UniVersity, Qinhuangdao 066004, P.R. China ReceiVed December 8, 2007; ReVised Manuscript ReceiVed April 13, 2008

ABSTRACT: A new BC3.3N crystalline compound with orthorhombic structure has been synthesized using an amorphous B-C-N precursor at 6 GPa and 1773 K. Results of energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) experiments show that the compound has a 1.04:3.27:1 B:C:N chemical stoichiometry. The lattice parameters of BC3.3N crystal were obtained to be a ) 6.610 (( 0.004) Å, b ) 4.977 (( 0.006) Å, and c ) 8.509 (( 0.008) Å by X-ray diffraction (XRD) and select area electron diffraction (SAED). The measurement of resistivity shows that this BC3.3N compound is semimetallic conductor with resistivity about 2.99 × 10-3 Ω cm at 300 K. On the basis of the analyses of XRD refinement and EELS results about the bonding situation, one possible conducting BC3.3N (B3C10N3) model with a space group of Pmma (No. 51) is proposed. The band structure and density of state of this model were calculated by CASTEP. The atomic positions, bond lengths, and bond angles are given. Dense B-C-N ternary phases are expected to be excellent alternative materials for diamond and cubic boron nitride (c-BN), which have some intrinsic limitations. For example, diamond is not stable in the presence of oxygen at high temperatures, whereas c-BN, with an improved thermal stability, has a smaller hardness. In the search for new superhard materials, theoretical predictions 1–6 and syntheses7–11 of B-C-N compounds have attracted a lot of attention, especially for BC2N. Solozhenko et al.12 reported that a superhard cubic BC2N was synthesized under 18 GPa and 2200 K. The hardness of this cubic BC2N (76 GPa) was between those of diamond and c-BN. According to the semiempirical hardness evaluation method for covalent crystal, Gao et al.13 predicted a value of 78 GPa for β-BC2N hardness. Riedel et al. 14 synthesized three-dimensional ordered graphitelike BC2N ternary crystal under 3-5 GPa and 1200-1500 K. The most probable structure of g-BC2N with ABAB stacking order was proposed, whose symmetry reduced to orthorhombic Cmcm because of the unidirectional C-C bonds. Although the research and simulation results suggest that different compositions of B-C-N compounds lead to different physical properties, for other BCxN (x > 2) compounds, the studied were mostly focused on BCxN (x ) 4, 6).15–17 Liu et al. 18 reported BC3N thin films deposited by radio frequency magnetron sputtering using h-BN and graphite. Kawaguchi et al. 15,17 prepared turbostratic BCxN(H) (x ) 1, 3) by the reaction of acetonitrile and boron trichloride. The conductivity of 88.5 Ω-1 cm-1 semiconductor behavior was given by the results of the four-probe basal-plane conductivity. However, the synthesis of crystalline BCxN (x ≈ 3) was seldom published. As for the theoretical models, Saalfrank et al. 7 presented an intralayer of BNC3.33 by substituted of graphite and hexagonal boron nitride, which is semiconducting with energy gap of 5.70 eV. Melamine is an excellent raw material to prepare B-C-N precursors because of the s-triazine rings in the molecule. Popov et al.19 reported the syntheses of nitrogen-rich B-C-N materials from melamine and boron trichloride at 673 and 873 K, respectively. However, melamine molecules start to decompose at 623 K 20–22 and release ammonia. It seems easier to obtain boron nitride than to form B-C-N compounds under this condition. In this paper, we used boron trichloride and melamine pyrolysate as raw materials to prepare B-C-N precursors, because the hydrogen content of melamine pyrolysate was lower than that of melamine. BCxN (x ≈ * Corresponding author. E-mail: [email protected].

Figure 1. XRD pattern of the B-C-N sample synthesized from 6 GPa and 1773 K: hexagonal BN (0), silicon (O), and B-C-N phase (9).

3.3) compounds were synthesized after treating the precursors under high-temperature and high-pressure (HTHP). The melamine pyrolysate was prepared by two melamine pyrolysis cycles (650 K for 2 h, 720 K for 1 h, and 770 K for 30 min in a vacuum). This pyrolysis possesses a turbostratic structure with the interlayer distance of 3.216 (( 0.0004) Å. Its composition is about 32.3 at % C, 50.8 at % N, and 16.9 at % H. The melamine pyrolysate was put in a quartz tube and heated to 800 K in Ar atmosphere with about 5 Pa pressure. Boron trichloride (BCl3) was then introduced into the quartz tube at a flow rate of 10 sccm for 15 min. The produced B-C-N powder was then heated up to 1773 K for 30 min in another vacuum furnace under 5 × 10-1 Pa N2 atmosphere in order to eliminate the volatile matter. The powder color changed from brown to black after this heat treatment. HTHP experiments were performed in a cubic-anvil-type apparatus at 1773 K and 6 GPa for 15 min. The phase structure and composition of the B-C-N precursor powder were analyzed by X-ray diffraction (XRD) (Rigaku, D/max-2500/PC) with a Cu KR radiation and energy-dispersive X-ray spectrometer (EDS) (Kevex Lever4) attached to the scanning electron microscopy (KYKY2800). The sample obtained after HTHP experiment was treated with diluted acids for purification. Both of them were grinded in agate mortar, dispersed in ethanol by ultrasonic, spread on the germanium-coated microgrid, and mounted on a standard TEM grid. The crystal

10.1021/cg701206a CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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Crystal Growth & Design, Vol. 8, No. 7, 2008 2097

Figure 4. Temperature dependence of resistivity for as-prepared sample (a) and B-C-N phase (b) at zero field.

Figure 2. (a) Typical TEM micrograph of graphite (A) and h-BN (B). (b) TEM image of B-C-N sheet and its SAED patterns (c-e) along the three different crystalline axes.

Figure 5. Model of orthorhombic BC3.3N (B3C10N3). B, C, and N atoms are white, gray, and black, respectively.

Figure 3. EELS spectra taken from the B-C-N and BN sheets in the sample. (a) The K-shell excitations in 1s-π* and 1s-σ* regions of B, C, and N come from the B-C-N sheet shown in Figure 2. (b) The 1s-π* and 1s-σ* K-shell excitations of B and N in a typical BN sheet.

structure, composition, and chemical bonding state of the sample were characterized by XRD, TEM (JEM-2010) with SAED, EELS, and EDS. Resistivity measurements were performed on both of asprepared and treated B-C-N samples (3.06 mm2 × 5.14 mm and 2.96 mm2 × 4.08 mm) using the conventional four-wire technique by PPMS-9 (physical properties measurement system, Quantum Design company). The refinement of XRD was completed by Reflex code of Materials Studio software. The first principle calculations were performed using ultrasoft pseudopotentials and local density approximation (LDA) in the CASTEP code. The kinetic energy cutoff is 310 eV and the sets of k-points is 4 × 5 × 3 with a k-point separation of 0.04 Å-1.

Figure 6. Comparison between the experimental pattern and the simulated pattern obtained by Rietveld refinement.

The melamine pyrolysate with layer structure was used as the starting material, in which there still remain some H atoms. Heating the melamine pyrolysate above 800 K would cause further polymerization with the volatilization of H2 or NH3. When BCl3 gas is introduced and decomposed during this process, Cl atoms will react with H2 or NH3 to product HCl or NH4Cl, whereas B atoms will diffuse into the carbon nitride fragments to form the B-C-N powder with layer structure. In this original B-C-N

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Table 1. Refined Data for B-C-N Crystal molecular formula space group lattice parameters (Å) volume of cell (Å3) density (g/m3) Rwp (%) Rp (%)

B3C10N3 Pmma (No. 51) a ) 6.611, b ) 4.927, c ) 8.478 276.149 2.339 11.48 9.81

powder, the content of B is only about 6.2 at %. After being heated at 1773 K for half an hour, the powder lost about 70-80 wt %. EDS result shows that the contents for B, C, and N in this B-C-N precursor are 19.3, 59.1, and 21.6 at %, respectively, which correspond to a 1:3.04:1.12 B:C:N ratio. This phenomenon indicated a further decomposing of the melamine pyrolysate as well as a reaction of B atoms with C and N atoms. It should be noticed that the content of B rise from 6.2 a t% to 19.3 at %. The XRD results illuminate that the B-C-N precursor possesses an amorphous structure, which is different from the turbostratic structure of the original B-C-N powder. The XRD pattern of the HTHP B-C-N sample is showed in Figure 1. In order to calibrate the peak position, crystalline Si powder was introduced into the sample as standard crystal. In Figure 1, seven peaks of Si (JCPDS 65-1060) were clearly presented, as well as 10 peaks of h-BN (JCPDS 34-0421). It is noted, except for the peak at about 2θ ) 20.860°, there exists reflection peak adjacent to almost every h-BN peak. Moreover, the shape of standard (002) peak of h-BN is not completely homologous with the peak observed in our experiment. Using profile fitting, this peak can be fitted into two peaks at 26.779 and 26.954°, respectively. The first peak corresponds to (002) of BN. It is well-known that the B-C-N precursor would easily decompose into crystalline BN and C under high temperature and pressure.23–25 Hence, it is plausible that the other unknown peaks come from graphite. However, less than two peaks were identical after comparing our

Figure 7. (a) Band structure (partly) of BC3.3N calculated by CASTEP. (b) Total and partial density of states of BC3.3N. In partial DOS of B, C, and N atoms, dotted and solid lines represent s and p orbits, respectively.

experiment pattern with known XRD data of graphite. Furthermore, the (002) line of graphite does not appear in our XRD patterns at all. On the basis of the XRD result, it is doubtful that there is crystalline graphite phase presented in our sample. The unknown peaks likely come from a new phase with a structure similar to that of h-BN. The results of EDS analysis reveal, except for the C and BN rich regions, there exists ternary B-C-N phase with 1.1: 3.4:1 B:C:N ratio, which suggests a new B-C-N phase. To further confirm this B-C-N phase, we performed TEM measurements coupled with EDS, SAED, and EELS for some particles with different morphology. EDS and SAED analyses show that the floccules (marked A in Figure 2a) are amorphous graphite, whereas the nummular flakes (marked B in Figure 2a) are h-BN crystal. Some strip-shaped slices shown in Figure 2b are ternary B-C-N single crystal with an 1:3.2:1 B:C:N ratio by EDS. So we confirm that the peak at about 26.954° should not be imputed to the disorder in interlayer distances of graphite or h-BN, but from B-C-N phase. The electron energy loss spectroscopy (EELS) spectrums taken from a number of different points on the B-C-N crystal were consistent with each other, and one of them is presented in Figure 3a. The EELS spectrum shows the simultaneous presence of three elements B, C, and N and is similar to those reported in the literature.26–28 The EELS of h-BN crystal coexisting in the same sample is also presented in Figure 3b. Comparing the fine structures of B and N K-edge from B-C-N crystal with those from h-BN, we found there are only three clear peaks present in 1s-σ* region of B K-edge for the B-C-N crystal, whereas there are at least four clear peaks in 1s-σ* region of B K-edge for h-BN. The difference in the 1s-σ* region of B K-edge indicates that bonding surroundings of B atoms in the B-C-N crystal may be influenced by C and N atoms simultaneously. The σ* region of N K-edge for h-BN shows two distinct peaks, whereas for B-C-N, the two peaks tend to become blurred and are not well-resolved. This difference in N K-edge π* resonance suggests a different bonding configuration in B-C-N.29–33 The N (1s-σ*) core loss features look similar to that of N in the C-N material.32 The 1s-π* and 1s-σ* transitions in the C 1s K-edge, on the other hand, are not consistent completely with those in graphite, showing coexistence of influences from B or N with C atoms to C bonding surroundings in the sample. The average composition of B-C-N crystal calculated from the EELS spectra is about 19.54 ( 2.4 at% B, 61.64 ( 7.4 at% C, and 18.83 ( 2.3 at% N. The ratio of three elements is about B:C:N ) 1.04:3.27:1, which is consistent with the result from EDS. The resistivity of as-prepared sample from 2 to 300 K at zero field was measured by PPMS, as shown in Figure 4a). We found that the resistivity depending on temperatures is very similar to those of HOPG (high oriented pyrolytic graphite).34–36 On the basis of above analyses, we know that there are three phases included in the as-prepared sample: h-BN, amorphous carbon, and the B-C-N phase. To explore the conductivity of pure B-C-N phase, we

Table 2. Atomic Positions, Interatomic Distances and Bond Angles of B3C10N3 Model atomic positions atom

x

y

z

bond

bond length (Å)

bonds

angle (deg)

C(1) C(2) C(3) C(4) C(5) C(6) C(7) N(1) N(2) B(1) B(2) B(3)

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

0.2545 0.7589 0.7587 0 0 0 0.5 0.2512 0.5 0 0.5 0.5

0.5951 0.0888 0.2526 0.8366 0.3336 0.0034 0.8360 0.7545 0.5181 0.5072 0.3438 0.0043

B(1)s2 × C(1) B(2)s1 × N(2) B(2)s2 × C(3) B(3)s2 × C(2) C(1)s1 × N(1) C(2)s1 × C(3) C(4)s1 × C(6) C(4)s2 × N(1) C(5)s1 × B(1) C(5)s2 × C(3) C(6)s2 × C(2) C(7)s1 × B(3) C(7)s2 × N(1) N(2)s2 × C(1)

1.4585(5) 1.4773(3) 1.4910(6) 1.4627(2) 1.3511(8) 1.3890(7) 1.4133(2) 1.4201(8) 1.4725(2) 1.3727(6) 1.3911(2) 1.4272(8) 1.4071(0) 1.3746(5)

CsCsC CsCsB CsCsN CsBsC CsBsN CsNsB CsNsC BsCsN NsCsN

121.39 118.75 119.37 118.56 121.25 118.36 120.06 120.91 121.19

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Figure 8. Total energy as a function of volume for the B3C10N3 model.

eliminated amorphous carbon from the as-prepared sample using mixed acids. The volume fraction of h-BN in the treated sample was estimated to be 36% by semiquantitative analysis of XRD. After subtracting the influence of h-BN phase from the result for the resistance measurement of the treated sample, the resistivity of the B-C-N phase (Figure 4b) is obviously lower than that of the as-prepared sample and about 2.99 × 10-3 Ω cm at 300 K. It indicates that this B-C-N phase possess a semimetallic conductivity. It should be noticed that there appear three shoulders in the temperature ranges of 48-55, 80-110, and 280-300 K, which need to be further studied in the future. To identify the structure of the BC3.3N crystal, full-profile Rietveld refinement was carried out for the XRD spectrum (11 peaks) of B-C-N phase over the angular range from 15 to 95° using Materials Studio software. The initial BC3.3N (B3C10N3) models used in the Rietveld refinement consisted of two BC3.3N layers. There exist many possible ways to build the B3C10N3 models. In this work, melamine pyrolysate with the turbostratic structures like the polymer of melem or melam was used as raw materials. Considering the structural transmissibility and C-N structure metastability under high temperature and pressure,37 we maintained one C-N ring in the B-C-N layer. At the same time, we notice the appearance of considerable crystalline h-BN in the high pressure product. However, the amount of amorphous carbon is less than h-BN in the product (from XRD result). These suggest it may not be a simple phase separation. The position of B atoms in the B-C-N precursor may affect the process of phase separation. When B atoms occupy the position of C in the C-N network, it

will bond with three N atoms and form the h-BN phase. On the basis of the above thought, some constraint conditions were used in building models, such as eliminating the case of B atom bonded with three N atoms. The B-B and N-N bonds should not be included in the structure, because these bonds would increase the total energy comparatively. Then, according to the 3:10:3 B:C:N ratio, all 32 possible kinds of B3C10N3 single layers were constructed. Up to now, 22 kinds of B-C-N models with AB stacking order were studied. The unit cell contains 32 atoms and four kinds of chemical bonds such as B-C, B-N, C-N, and C-C bonds. The impurity is an important influencing factor for the sample conductivity. In our sample, the main impurity is amorphous carbon, which led to the resistivity depending on a temperature similar to HOPG, although the relation between the resistivity and temperature for the treated sample is obviously different from that of the asprepared sample. By the analysis of XRD and TEM results, h-BN and B-C-N phase are included in the treated sample. This shows that the B-C-N phase should mainly contributed to the conductivity in the treated sample. So we searched the conductivity for all our BC3.3N models and found that six of them are conductive. The results of XRD refinement showed that only one of the six models satisfied the conditions of Rwp < 15% and Rp < 12%, with Rwp) 11.48% and Rp) 9.81%; this is shown in Figure 5. Because of the constraints for atomic position in the models, not every possible model of BC3.3N was mentioned. Here, we only explored a part of possible BC3.3N crystal models and proposed one probable model constructed by above constraint conditions, which corresponds well with the results of XRD and the measured conductivity. Other possible B3C10N3 models will be studied in detailed in the succeeding work. Figure 6 presents the Rietveld refinement plots for the BC3.3N phase. Rietveld refinement parameters include lattice parameters, atom positions, temperature factor and so on. The BC3.3N can be indexed as orthorhombic structure (space group of Pmma, No. 51) with the lattice parameters listed in Table 1. The intensity difference of peaks implies that there exist preferred orientations in the BC3.3N crystal. According as the results of band structure calculated by CASTEP in Figure 7a there appear orbits crossing the Fermi level, also implying the dominative hole-conductivity. As shown in Figure 7b, the partial densities of states for each atom, as well as the total density of states in the BC3.3N unit cell, have been calculated. The calculations were repeated with higher K-points mesh such as 8 × 10 × 6 and 10 × 12 × 7, and the results are unchanging. The valence bands between -23 and -19 eV are mainly from N s orbits, but C s orbits also make small contributions. The states in the Fermi level consist mostly of p orbits of C atoms, in which C atoms contribute 0.14 electrons/eV. The rest from B and N atoms are 0.06

Table 3. Comparing Data Obtained from XRD, SAED, and B3C10N3 Model XRD

model

2θ (deg)

d (Å)

intensity (%)

d (Å)

intensity (%)

∆d (Å) (dXRD - dmodel)

Miller indices h k l

20.860 26.954 42.039 44.301 50.621 55.561 60.121 72.001 76.981 83.102 93.143

4.2548 3.3051 2.1475 2.0430 1.8017 1.6527 1.5378 1.3105 1.2376 1.1613 1.0607

6 100 2 21 2 5 3 1 2 3 1

4.2390 3.3055 2.1299 2.0273 1.7904 1.6528 1.5315 1.3058 1.2257 1.1492 1.0514

4 100 3 17 2 5 3 1 3 4 1

0.0158 -0.0004 0.0176 0.0157 0.0113 -0.0001 0.0063 0.0047 0.0119 0.0121 0.0093

002 200 022 122 222 400 322 422 026 226 144

Lattice Parameters

a (Å) b (Å) c (Å)

XRD

model

SAED

6.610 (( 0.004) 4.977 (( 0.006) 8.509 (( 0.008)

6.611 4.927 8.478

6.6 (( 0.1) 4.9 (( 0.1) 8.5 (( 0.1)

SAED d (Å)

2.1 2.0 1.8

(022j) (1j22) (2j22)

1.5 (3j22)

2100 Crystal Growth & Design, Vol. 8, No. 7, 2008 and 0.03 electrons/eV severally. Table 2 lists the atomic positions, bond lengths, and bond angles of the refined BC3.3N structure. Because of the unequal bond lengths such as the shortest C-N bond for 1.3511 Å and the longest B-C bond for 1.4910 Å, the bond angles are not exactly equal to 120°. The energy-volume curve was calculated and showed in Figure 8. The lowest energy of BC3.3N structure occurs at about the volume of 276 Å3, indicating the most stable state at zero pressure. The average total energy of each atom in the BC3.3N unit cell calculated by CASTEP, Eaverage ) -162.56 eV/atom, is higher than that of h-BN (-176.23 eV/ atom). This higher energy may contribute to the B-N phase segregation mentioned previously. Because of the layer structure, the bulk modulus of this model is lower with value of 15.8 GPa. SAED patterns obtained from three different diffraction axes of the B-C-N crystal shown in Figure 2c-e are identified to [211], [411], and [451] crystal axes of the orthorhombic BC3.3N crystal. The experimental data of XRD, SAED, and calculated data according to BC3.3N structure are listed in Table 3, which illuminates the good agreement between the experimental data and calculated results. According to the analysis results, crystalline BC3.3N, h-BN and amorphous carbon were found in the product, but there is no pure B or B-C phase observed in the sample. We speculated that the BC3.3N phases formed through introducing B atoms to the C-N networks or substituting C or N atoms for B atoms during melamine pyrolysate decomposed and reacted with boron chichloride. If N was substituted, networks including the B-C bonds appear, and N combined with other B to form BN possibly. On the other hand, if C was substituted, B bonded with N to compose B-N networks. However, when the enrichments of B-N appear, it might lead to BN phase separating from the B-C-N ternary system because of its higher stability and result in the formation of some amorphous carbon. Great loss of precursor after heating at 1773 K comes mainly from decomposition of melamine pyrolysate. Generally, B-C-N ternary phase tends to separate into BN and C because of their thermal stabilities. In our sample, the possible reasons for the stable crystalline BC3.3N are the synthesis condition of HTHP and the atomic distribution for B, C, and N in the precursor. These factors may block the phase separation process and led to formation of the well-ordered crystalline BC3.3N phase.

Conclusions. A new BC3.3N phase was found in the product obtained by treating the amorphous B-C-N precursor at 6 GPa and 1773 K. In situ SAED and EELS further confirm that the phase is a ternary crystalline compound with the average composition of about 19.54 ( 2.4 at % B, 61.64 ( 7.4 at % C, and 18.83 ( 2.3 at % N. The result of resistivity measurement shows the BC3.3N is semimetallic compound with room-temperature resistivity about 2.99 × 10-3 Ω cm. The structure is indexed as orthorhombic structure by SAED and XRD, with the following lattice parameters: with lattice parameters of a ) 6.610 (( 0.004) Å, b ) 4.977 (( 0.006) Å, and c ) 8.509 (( 0.008) Å. On the basis of analyses for the B3C10N3 layers and crystal models, one possible conducting BC3.3N (B3C10N3) model is built by overlapping the B3C10N3 layers. The full-profile Rietveld refinement using the XRD data showed that the BC3.3N model has an orthorhombic structure with space group of Pmma (No. 51). The R factors of Rwp) 11.48% and Rp) 9.81% indicted that the structural parameters of the model are in good agreement with the experimental data. Band structure and density of states of this model were calculated by CASTEP, implying the majority of hole-conductivity. More physical properties of BC3.3N phase will be investigated in the future. Acknowledgment. The authors acknowledge financial support from the National Science Foundation of China (Grant 10474083,

Communications 50532020, 50472051, 50672081), Science Foundation of Hebei Province (Grant E2005000353), NBRPC (Grant 2005CB724400), and PCSIRT (Grant IRT0650).

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