ATEM-EELS study of new diamond-like phases in the B–C–N

system

Falko Langenhorsta and Vladimir L. Solozhenkob

a Bayerisches Geoinstitut, Universitat Bayreuth, 95440 Bayreuth, Germany.E-mail: Falko.Langenhorst@uni-bayreuth.de

b Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev 04074,Ukraine

Received 9th July 2002, Accepted 16th August 2002First published as an Advance Article on the web 17th September 2002

Novel ternary diamond-like phases of the B–C–N system were synthesized at pressures up to 30 GPa andtemperatures up to 3500 K by static and dynamic compression of graphite-like BN–C solid solutions. Thestructure and properties of these new phases were studied using analytical transmission electron microscopy(ATEM), electron energy loss spectroscopy (EELS) and powder X-ray diffraction. Our results reveal that B, Cand N atoms are homogeneously distributed over the crystal lattices of the B–C–N phases under study withoutforming superstructures. All ternary diamond-like B–C–N phases indicate a clear tendency to decompose athigh temperatures because of supersaturation of diamond in the BN component.

Introduction

The similar atom sizes and the similar structures of boronnitride and carbon polymorphs (i.e. cubic BN (cBN) and dia-mond, or hexagonal graphite-like BN (hBN) and graphite)suggest that it might be possible to synthesize phases contain-ing all three elements. While several low-density graphite-likephases containing B, C, and N have been reported (see e.g.,ref. 1), there is less information on the synthesis of densephases containing all three elements.2–6 Such diamond-liketernary phases are highly desirable as they might combinethe best properties of the elemental or binary compounds ofthe B–C–N system. For example, a cBN–diamond alloy shouldbe thermally more stable and resistant to oxidation, less reac-tive with iron than pure diamond, and harder than cubic boronnitride.However, the data on the synthesis of diamond-like B–C–N

phases reported in ref. 2–6 are rather contradictory. Despitethe experimental efforts, the physical properties of these mate-rials have not been well characterized, nor is their crystal struc-ture known. To date it is unclear whether the synthesisproducts are diamond-like solid solutions between carbonand boron nitride or just mechanical mixtures of highly dis-persed diamond and cBN.Very recently the cubic BC2N, a new superhard phase, and

diamond-like BN–C solid solutions have been synthesized athigh pressures and temperatures by Solozhenko et al.7,8 Here,we report the results of detailed ATEM-EELS studies of somenew diamond-like ternary phases in the B–C–N system.

Experimental

Starting materials

A nanopowder of turbostratic graphite-like (BN)0.48C0.52 solidsolution was used as the starting material for the synthesis ofdiamond-like B–C–N phases. The precursor was synthesizedby simultaneous nitridation of boric acid and carbonizationof saccharose in molten urea followed by annealing in nitrogenat 1770 K.9 Since the stoichiometry of the starting material is

very close to (BN)0.5C0.5 we will later refer to this phase as gra-phite-like gBC2N.

Synthesis at static pressures

Diamond-like B–C–N ternary phases were synthesized accord-ing to Solozhenko et al.7 by direct solid-state phase transitionof graphite-like BC2N at 25 GPa and temperatures of �2100 Kand �2300 K (annealing times: 60 and 30 s, respectively) usinga large-volume multianvil system (MA) and a Sumitomo 1200ton press at the Bayerisches Geoinstitut. The 10/4 mm cellassembly,10 composed of eight WC cubes with 4 mm trunca-tions, a MgO octahedron of 10 mm edge length, and a LaCrO3

furnace, was used. The sample was placed into a Pt capsule.Temperature was measured using a W3%Re–W25%Re ther-mocouple without corrections for the pressure effect on thethermocouple emf. Particularly at the high temperaturesachieved in this study, the uncertainty in the exact temperaturevalue is considerable due to the substantial temperature gradi-ent along the sample axis. The thermocouple measures at oneend of the sample, whereas the observations are done in thecenter of the sample, where temperatures are usually 100 to200 K higher. This temperature correction has been taken intoaccount in Table 1. The sample pressure at high temperatureswas calibrated as a function of hydraulic oil pressure usingphase diagrams of MgSiO3 and Mg2SiO4 .High-pressure phase transitions of gBC2N at temperatures

above 2500 K were studied using a laser-heated diamond anvilcell (DAC) at beamline ID30, European Synchrotron Radia-tion Facility.

Shock–compression synthesis

The isoentropic shock–compression (SC) synthesis of dia-mond-like BN–C solid solution was carried out at 30 GPausing gBC2N in cylindrical recovery containers with a ringgap11 that allowed concentration of explosion energy in agiven direction and multiple reflections of shock waves at thewalls of the container. The incident shock pressure on the sam-ple is governed by the type of high-explosive (RDX–ANFO or

DOI: 10.1039/b206691b Phys. Chem. Chem. Phys., 2002, 4, 5183–5188 5183

This journal is # The Owner Societies 2002

PCCP

RDX–Nobelit 100 compositions). The use of the special addi-tive that is characterized by a high shock temperature and ahigh compressibility, allowed heating of the sample up to3500 K, and its abrupt cooling (�108 K s�1) on decompres-sion. The recovered samples were immersed in hydrochloricacid to dissolve an additive, then heated in concentratedHClO4 to remove unchanged starting material.

X-ray diffraction

Samples synthesized in multianvil experiments were studiedusing a Rigaku powder microdiffractometer (Cr-Ka-radiation,l ¼ 2.2910 A) and energy-dispersive X-ray diffraction with syn-chrotron radiation at beamline F2.1, HASYLAB (see ref. 12).Powder X-ray diffraction patterns of recovered samples pro-

duced in shock–compression experiments were taken in theback-reflection mode using a D5000 SIEMENS diffractometer(Cu-Ka-radiation, l ¼ 1.54184 A). Data were collected from20 to 100� 2y in steps of 0.02�; the counting time was 10 s ateach step. A high purity silicon standard was used to adjustthe goniometer. The diffraction patterns were evaluated usingthe GSAS software package.13

Analytical transmission electron microscopy (ATEM)

The as-synthesized samples of diamond-like B–C–N phaseswere sufficiently brittle to be crushed between two glass slides.

The powdered samples were dispersed in a droplet of ethanoland then loaded on copper grids coated with a holey carbonfilm. For comparison, we also prepared samples of startinggraphite-like material, diamond and cBN in a similar way.The samples were studied using a PHILIPS CM20 field

emission gun transmission electron microscope (FEG-TEM)operating at 200 kV. The microstructure of the samples wascharacterized by bright-field (BF) and high-resolution trans-mission electron microscopy (HRTEM) as well as by selectedarea electron diffraction (SAED) (Figs. 1 and 2). To obtaininterplanar spacings of the cubic B–C–N compounds, SAEDring patterns were scanned and quantitatively evaluated usingthe Process Diffraction program.14 The three strongest diffrac-tion rings 111, 220, and 311 were then used for the refinementof the lattice constant a (Fig. 3). The camera constant was cali-brated with diamond and cBN standards, operating the TEMunder the same diffraction conditions as employed for B–C–Ncompounds. Errors in the lattice constant a are estimated to be0.01 A.The electron energy loss spectra (EELS) of the B K, C K,

and N K edges were detected with the GATAN parallel elec-tron energy-loss spectrometer (PEELS 666). Measurementswere performed in diffraction mode at a camera length of210 mm, using a 2 mm entrance aperture. The energy resolu-tion defined as full width at half maximum height of thezero-loss peak was 0.8 eV. EELS measurements were per-formed with two energy dispersions: 0.1 eV per channel and

Fig. 1 Bright-field TEM images of (a) graphite-like BC2N precursor and nanocrystalline diamond-like B–C–N phases synthesized in (b) multi-anvil, (c) diamond-anvil cell and (d) shock–compression experiments.

Table 1 Synthesis conditions, compositions and lattice parameters a of the diamond-like cubic B–C–N phases and the graphite-like BC2N pre-

cursor. Most lattice parameters are based on electron diffraction patterns

Sample Experimenta P,T,t-conditions B (atom %) C (atom %) N (atom %) a (A)

gBC2N — — 24.9 51.7 23.4 —

cBC2N SC 30 GPa, �3500 K, 1ms 26.8 46.6 26.6 3.600(10) 3.593(4)b

cBC3N MA 20 GPa, �2300 K, 30 s 21.9 57.2 20.9 3.603(10) 3.611(6)b

cBC3N MA 20 GPa, �2100 K, 60 s 20.5 59.4 20.1 3.582(10)

cBC8N DAC 30 GPa, �3000 K 180 s 9.5 79.5 11.0 3.572(10)

a SC ¼ Shock–compression; MA ¼ Multi-anvil; DAC ¼ Diamond anvil cell. b X-ray diffraction data.

5184 Phys. Chem. Chem. Phys., 2002, 4, 5183–5188

0.3 eV per channel. The latter energy dispersion allowed us tosimultaneously detect all three edges and to determine the che-mical composition of compounds according to the equation

CA

CB¼ sBðb;DEÞIAðb;DEÞ

sAðb;DEÞIBðb;DEÞ

where CA and CB are the atomic concentrations of the ele-ments A and B.15 s and I are the partial ionization cross sec-tions and intensities of the K ionization edges, respectively.They were determined for a collection semi-angle b ¼ 2.7mrad and an energy window DE ¼ 50 eV. The partial ioniza-tion cross sections were experimentally calibrated using cBNand gBC2N as standards. Especially in the case of light ele-ments, the use of standards for quantifying compositions issuperior to the theoretical calculation of partial ionizationcross sections. From charge balance considerations the num-ber of boron and nitrogen atoms should be identical in the

B–C–N compounds, which provides a means to estimate theerrors in analyses. The data in Table 1 suggest that the relativeerrors in boron and nitrogen concentrations are up to 10%.The 0.3 eV per channel spectra were also used to determine

the exact energy loss positions of B K and N K edges for B–C–N phases synthesized (Fig. 4). The energy position of the s*peak maximum in the C K spectra was assumed to be at291.7 eV. The exact energy dispersion of the spectrometerwas determined using the calibration routine in the GATANEL/P software.The energy dispersion of 0.1 eV per channel was selected to

reveal, with good energy resolution, the energy-loss near-edge structure (ELNES) of the B K, C K, and N K edges(Figs. 5–7). In each series of measurements, six ELNES spectrawere acquired and summed. Furthermore, data reductioninvolved the correction for dark current and channel-to-channel gain variation, the subtraction of an inverse powerlaw background, and the removal of plural scattering contri-butions by the Fourier-ratio technique.15 For comparison,the ELNES spectra of all samples were normalized to unity.

Results

Graphite-like BC2N precursor

The X-ray diffraction pattern of the precursor material, gra-phite-like (BN)0.48C0.52 , exhibits diffuse 002, 100, 101 and004 lines, which is indicative of the very disordered, poorlycrystalline state of the material. The lattice parametera ¼ 2.48(2) A is intermediate between the correspondingvalues for graphite (2.46 A) and BN (2.51 A) while the inter-layer spacing d002 ¼ 3.64(2) A is much greater than those inturbostratic carbon (3.44 A) and graphite-like BN (3.43 A).9

At the TEM scale, the phase shows a turbostratic structure(Fig. 1a). It is impossible to define grains or grain boundariesbecause the material is almost amorphous. This is also evidentfrom the selected area electron diffraction pattern (Fig. 2a),which displays broad diffuse rings similar to those of turbo-stratic graphite. The C K ELNES spectrum of graphite-likeBC2N is also in line with this interpretation. It contains the

Fig. 3 Variation in lattice parameter a of synthesized diamond-likeB–C–N phases (cBCN) as a function of the carbon concentration. Lat-tice parameters of the end-members of the BN–C solid solution, dia-mond and cBN, are shown for comparison. Circles representelectron diffraction data, the triangle represents the X-ray diffractiondata.

Fig. 4 Electron energy-loss spectra of various diamond-like B–C–Nphases and graphite-like BC2N precursor showing the B K, C K,and N K ionization edges. The spectra were acquired with an energydispersion of 0.3 eV per channel. The background was, in part,removed by fitting an inverse power law function to the B K pre-edgeregion. MA refers to multi-anvil experiments, DAC to diamond anvilcell and SC to shock–compression experiments.

Fig. 2 Selected area electron diffraction patterns of (a) graphite-likeBC2N precursor and nanocrystalline diamond-like B–C–N phasessynthesized in (b) multianvil, (c) diamond-anvil and (d) shock–com-pression experiments.

Phys. Chem. Chem. Phys., 2002, 4, 5183–5188 5185

p* peak at 285 eV followed by a relatively featureless, broads* peak (Fig. 6). These spectral characteristics are similar tothose for amorphous carbon but quite different to those ofwell-crystallized graphite, displaying a complex fine structurewith a number of sharp peaks (Fig. 6).

Diamond-like BC3N synthesized in a multianvil press

According to the quantification of the EELS spectra, the stoi-chiometries of the two B–C–N samples synthesized in different

multianvil (MA) experiments are (BN)0.43C0.57 and(BN)0.41C0.59 (Table 1, Fig. 4). Considering the errors in thedetermination of the light elements and the similar synthesisconditions, both samples may have an almost identical compo-sition, with an average stoichiometry (BN)0.42C0.58 (or simpler,approximately BC3N). They seem hence to contain 6% less BNcomponent than the graphite-like BC2N precursor material.The X-ray diffraction pattern of one of the BC3N samples

exhibits only 111, 220, and 311 lines of the cubic lattice(Fd3m space group) with the lattice parameter of 3.611(6) A,which is larger than that of diamond (3.5667 A) and close tothat of cBN (3.6158 A) (JCPDS NN. 6-0675 and 35-1365,respectively). The absence of the 200 line indicates that theatomic scattering factors of the two zinc blende-type fcc sub-lattices in cBC3N are equal, which is possible only if B, Cand N atoms are uniformly distributed over both sublattices.According to the TEM observations, the produced cubic

phases occur as nanocrystalline aggregates with clearly visiblebut very small (average size: 20–30 nm) grains (Fig. 1b). Thelargest grains show regular crystal shapes with cubic forms(octahedra and cubes), while smaller grains appear to beround. In line with X-ray results, SAED patterns are fullycompatible with the diamond structure and exhibit sharp ringswith numerous discrete spots corresponding to the 111, 220and 311 reflections of the cubic phase (Fig. 2b). The latticeparameters a of the two samples, derived from electron diffrac-tion data, are 3.58(1) A and 3.60(1) A (Table 1). These latticeparameters are intermediate between those of diamond (3.5667A; JCPDS NN. 6-0675) and cBN (3.6158 A, space group:F43m; JCPDS NN. 35-1365), as expected from ideal mixingbetween these two endmembers of the BN–C solid solution(Fig. 3). Superstructures could not be observed, pointing alsoto a statistical uniform distribution of B, C, and N atoms inthe crystal lattice.B K, C K, and N K ELNES spectra of the two cBC3N

samples resemble very much those of diamond and cBN(Figs. 5–7). The p* peak is almost completely absent and thefine structure of the ELNES spectra fits very well to a phasewith almost pure sp3 type atomic bonding and tetrahedralcoordination of B, C, and N atoms. The only clear difference

Fig. 6 C K ELNES spectra of various diamond-like B–C–N phasesand the graphite-like BC2N precursor in comparison to the spectraof diamond, graphite and amorphous carbon.

Fig. 7 N K ELNES spectra of various diamond-like B–C–N phasesin comparison with the spectra of cubic BN and graphite-like BC2Nprecursor.

Fig. 5 B K ELNES spectra of various diamond-like B–C–N phasesin comparison with the spectra of cubic BN and graphite-like BC2Nprecursor.

5186 Phys. Chem. Chem. Phys., 2002, 4, 5183–5188

between the ELNES spectra of diamond and cBN is the mar-ginal broadening of peaks.

Diamond-like BC8N phase synthesized in a DAC

In situ X-ray studies of the graphite-like BC2N in the 25–32GPa pressure range at temperatures above 2900 K usinglaser-heated DAC show complex X-ray diagrams with overlap-ping peaks of cubic phases. The data suggest that the precursormaterial has decomposed into diamond, cubic boron nitrideand another cubic phase.To further inspect this decomposition reaction and to char-

acterize the new phase assemblage we used the TEM. TheDAC sample is composed of separate, about 1 mm large poly-crystalline aggregates of cBN, diamond, and a cubic B–C–Ncompound, which is distinctly depleted in the BN component(Fig. 4). The evaluation of the EELS spectrum yields a compo-sition of (BN)0.21C0.79 , i.e. approximately BC8N (Table 1).This indicates a decomposition reaction of the gBC2N precur-sor into the assemblage cBN+C (diamond)+ cBC8N.The grain size of the idiomorphic BC8N crystallites within

the aggregates is of the order of 20–30 nm (Fig. 1c). The cor-responding SAED pattern fits to the diamond structure andshows sharp 111, 220, and 311 diffraction rings with discretespots (Fig. 2c). The lattice constant a ¼ 3.57(1) A is in agree-ment with the linear variation of the a parameter as a functionof carbon content (Fig. 3).The B K, C K, and N K ELNES spectra are fully compatible

with a diamond-structured material but are slightly less sharpthan those of diamond and cBN (Figs. 5–7).

Diamond-like BC2N synthesized by shock compression

According to the EELS results (Table 1, Fig. 4), the stoichio-metry of the sample recovered from the shock compression(SC) experiment is (BN)0.53C0.47 (or simpler, approximatelyBC2N). The BN content is slightly higher than that of the gra-phite-like BC2N starting material. It is unlikely that this smalldifference is due to a preferential loss of carbon but it possiblyresults from heterogeneities in the precursor material.The X-ray diffraction pattern of the sample exhibits broad

111, 220 and 311 lines, while the 200 line is missing. B, Cand N atoms are hence statistically distributed over the crystallattice. A profile analysis of the 220 and 311 lines did not revealoverlapping or asymmetric peaks. It follows thus that theexperimentally observed diffraction patterns are solely attribu-table to a diamond-like BN–C uniform solid solution, and notto a mechanical mixture of diamond and cBN. The latticeparameter of c(BN)0.53C0.47 has been found to be 3.593(4) Aand is in a good agreement with the corresponding value(3.591 A) that should be expected from ideal mixing betweendiamond and cBN (Fig. 3).Under TEM, the sample is polycrystalline with an average

grain size of about 5 nm (Fig. 1d). In contrast to the B–C–Ncompounds synthesized at static pressures, the grains ofshock-synthesized BC2N are irregularly shaped and are poorlycrystallized, as is obvious from the corresponding electron dif-fraction pattern displaying broadened lines with very few dis-crete spots (Fig. 2d). Similarly broad lines are also observedfor the graphite-like precursor (Fig. 2a). Besides the linebroadening, the SAED pattern of shock-synthesized cubicBC2N is fully consistent with the diamond structure. No extrapeaks or streaks appeared that would indicate the presence of asuperstructure with an ordered stacking sequence.Overall, the B K, C K, and N K ELNES spectra are similar

to those of diamond and cBN but the fine structure of the edgesis distinctly less sharp (Figs. 5–7). The p* peak is clearly visiblein the C K spectrum but seems to be absent in the B K and N Kspectra. Among all synthesized cubic B–C–N compounds, the

shock-produced sample is also the most beam-sensitive. Afterabout 1 min of electron irradiation the EELS spectrum ofshock-synthesized BC2N completely changed into a spectrum,which resembles that of the precursor material (Fig. 8). Thethree B K, C K, and N K edges contain a strong p* pre-peakand the following edge is featureless, indicating the structuralbreakdown of the material under the beam.

Discussion

BN solubility in the diamond structure

Our EELS data show that almost all synthesized diamond-likeB–C–N phases have changed their compositions in comparisonto the graphite-like BC2N precursor (Table 1, Fig. 4). The onlyexception is the shock-synthesized BC2N sample, which due tothe short duration of the shock pulse (only about 1 ms) couldnot change composition. Additionally, the shock-synthesizedcubic BC2N apparently inherited some microstructural proper-ties of the graphite-like BC2N precursor, such as the poor crys-tallinity and irregular shape of grains. For the other samples,there is a clear tendency that higher temperatures and/orlonger annealing result in a lower BN concentration in the dia-mond structure. This effect is most obvious for the DAC-synthesized sample, which contains separate aggregates ofcBN and diamond that are associated with a new cubic B–C–N phase with the approximate stoichiometry BC8N. Thisobservation demonstrates that cBN and diamond have segre-gated out of the BC2N precursor, forming consequently thedepleted cBC8N phase.At the synthesis conditions, the diamond-structured phases

are obviously supersaturated in BN and therefore tend todecompose. Equilibrium was not reached in the experiments,possibly because of the low diffusivity of B and N in the dia-mond structure. An additional kinetic barrier could be thatB and N can only be expelled from the diamond structure bya coupled diffusion mechanism, fulfilling the requirement ofthe charge balance. For a better understanding of the diffusionmechanism, it is however mandatory to measure concentrationprofiles across cubic B–C–N grains, as has been successfullydemonstrated for graphitic BC2N nanotubes.16,17

Based on our observations, one can expect that, under equili-brium conditions, the BN solubility in diamond is quite limited.These conclusions are strengthened by the observation reportedin ref. 18, that cubic B–C–N phases could not be synthesizedfrom very fine-grained mechanical mixtures of hBN and gra-phite. In order to grow cubic ternary phases with a high BN

Fig. 8 Electron energy-loss spectra of shock-synthesized BC2Nbefore and after electron irradiation. The appearance of p* peaks atall three edges reveals the breakdown of the cubic structure to anamorphous state with sp2 hybridization.

Phys. Chem. Chem. Phys., 2002, 4, 5183–5188 5187

concentration, hence, it is necessary to use precursor materialsthat are perfect chemical B–C–N mixtures with large grains.

Short-range structure of cubic B–C–N compounds

X-ray and electron diffraction data show that the synthesizedB–C–N phases possess the diamond structure with space groupFd3m (Fig. 2). The absence of extra reflections, such as the 200line, indicates that superstructures are not formed under synth-esis conditions. The lattice parameter a shows a linear depen-dence on composition (Fig. 3), which is characteristic for idealmixing between diamond and cBN, the end-members of theBN–C solid solution. These results reveal that the BN compo-nent is homogeneously dissolved in the diamond crystal lattice.Further hints as to the incorporation and short-range struc-

tural effect of BN in diamond-structured phases is provided bythe ELNES spectra, which are a fingerprint of the local envir-onment of the excited atom. For a better understanding of thespectral characteristics, we compare and discuss here theELNES spectra of synthesized diamond-like B–C–N phaseswith those of diamond, cBN, and graphite-like BC2N precur-sor (Figs. 5–7).In terms of edge shape and energy positions, the B K, C K

and N K ELNES spectra of the diamond-like B–C–N phasesare similar to those of cBN and diamond (Figs. 5–7). The posi-tions of the onset peaks in the B K, C K, and N K spectra ofthe cubic B–C–N phases are at 197.7, 291.7, and 406.3 eV,respectively. These energy positions significantly differ fromthose reported in ref. 19 for cubic BC2N. The onset peaks inthe B K and N K spectra of cBN seem to be only marginallyshifted to 197.4 and 406.6 eV, respectively (Figs. 5 and 7).These values are in reasonable agreement with the datareported in ref. 20–22 for cBN but differ by more than 2 eVfrom the measurements given in ref. 23.A detailed comparison between the different cubic phases of

the B–C–N concentration triangle reveals, however, a clear dif-ference in the sharpness of the ELNES spectra. Diamond andcBN clearly yield the best-resolved ELNES spectra with sharppeaks. The ELNES spectra of all cubic B–C–N phases are lesssharp, particularly at the edge onset. The shock-synthesized,most beam-sensitive cBC2N phase shows the strongest broad-ening of the ELNES signals. Additionally, one notes the pre-sence of a pre-peak in the C K spectra of cubic B–C–Ncompounds at about 4 eV before the edge onset, i.e. at the posi-tion of the p* peak for the graphite-like BC2N. One could arguethat this pre-peak is due to contamination under the electronbeam but then we would expect to detect this peak for diamondand cBN, as well. The p* pre-peak at the C K edge seems ratherto be related to the BN content in the cubic B–C–N phases,since its intensity increases with BN concentration (Fig. 6).Recently, the spectral features of the B K and N K ELNES

spectra were related to structural elements in cubic and hexa-gonal (wurtzite-structured) BN, using first-principles MO cal-culations.24 According to these calculations, the onset peakin the B K and N K spectra is strongly dominated by the corehole effect and may provide information on the regularity ofthe tetrahedra, whereas peaks at higher energy losses provideinformation on the tetrahedral stacking, i.e. on the second-nearest neighbours. The broadening of the onset peaks in theB K, C K and N K ELNES spectra may therefore indicate thatthe tetrahedra in the cubic B–C–N phases are irregularly andvariably distorted and that this short-range disorder increaseswith BN content.Regarding the weak p* signal in the pre-edge region of the C

K ELNES spectra, it has been previously demonstrated thatthis peak may result from dislocations or other types ofdefects.15,25–27 We could, hence, interpret the weak p* peaksin our spectra of cubic B–C–N as the result of transitions tolocalized defect states. The fact that only the C K spectrumof B–C–N compounds contains this pre-edge signal may

indicate that the defects are located at carbon sites in the dia-mond structure. Since dislocations are absent in all our sam-ples, we assume that point defects or surface defects could beresponsible for such localized defect states. Indeed, theshock-synthesized cBC2N phase, which is the most imperfectand disordered sample with large surface area, shows thestrongest pre-edge signal.

Acknowledgements

The authors thank D. Andrault, D. Frost, M. Mezouar, T.Peun and D.C. Rubie for their assistance with synthesizingsamples and A.V. Kurdyumov for helpful discussions. Multi-anvil synthesis was performed at the Bayerisches Geoinstitutunder the EU ‘‘IHP – Access to Research Infrastructures ’’Program (Contract No. HPRI-1999-CT-00004 to D.C. Rubie).The DAC experiments were carried out during beamtime allo-cated to proposals HS-918 and HS-1693 at ID30, EuropeanSynchrotron Radiation Facility. Shock–compression experi-ments were supported by the Dynamit Nobel GmbH.

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