Structure of an Amorphous Boron Carbide Film: An Experimental and

Article pubs.acs.org/cm

Structure of an Amorphous Boron Carbide Film: An Experimental and Computational Approach Camille Pallier,† Jean-Marc Leyssale,† Lionel A. Truflandier,‡ Anh Thy Bui,† Patrick Weisbecker,† Christel Gervais,§ Henry E. Fischer,# Fausto Sirotti,∇ Francis Teyssandier,† and Georges Chollon*,† †

CNRS, Laboratoire des Composites ThermoStructuraux, UMR 5801 CNRS/Herakles/CEA/Univ. Bordeaux 1, 3, allée de La Boétie, 33600 Pessac, France ‡ Institut des Sciences Moléculaires, Univ. Bordeaux 1, CNRS UMR 5255, 351 cours de la libération, 33405 Talence, France § Univ. Paris 06, Laboratoire de Chimie de la Matière Condensée de Paris, UPMC Université Paris 06, UMR 7574, Collège de France, 11, place Marcelin Berthelot, 75231 Paris Cedex 05, France # Institut Laue-Langevin, BP 156, 6, rue Jules Horowitz, 38042 Grenoble Cedex 9, France ∇ Synchrotron SOLEIL, L’Orme les Merisiers, Saint-Aubin − BP 48, 91192 Gif-sur-Yvette, France S Supporting Information *

ABSTRACT: An amorphous boron carbide ceramic is prepared via hot wall chemical vapor deposition at 1000 °C using a BCl3/CH4/H2 mixture. Its elemental composition is assessed by electron probe microanalysis (EPMA) and its structure studied by Raman spectroscopy, transmission electron microscopy (TEM), both X-ray diffraction (XRD) and neutron diffraction, 11B magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy (XAS), and ab initio modeling. The atomic structure factor and pair distribution function derived from neutron diffraction data are compared to those deduced from an atomistic model obtained by a liquid quench ab initio molecular dynamics simulation. The good agreement between experimental data and simulation shows that the asprepared material is essentially made of a random arrangement of icosahedra (B12, B11C, and B10C2) embedded in an amorphous matrix rich in trigonal (BC3 or BC2B) and tetrahedral (CB4) sites. The existence of trigonal boron environments is clearly confirmed by a peak at 50 ppm in both the experimental and simulated 11B MAS NMR spectra, as well as a 190.0 eV component in the XANES-B(1s) spectrum. The intericosahedral linear C−B−C chains observed in crystalline B4C are absent in the as-processed material. Free hexagonal carbon and B4C crystallites appear in the ceramic when heat-treated at 1300 °C/2 h/Ar, as evidenced by high-resolution TEM and Raman spectroscopy. Comparing the pair distribution functions of the heat-treated material with the crystalline B4C model allows confirming the apparition of C−B−C chains in the material. Indeed, two new peaks located at 1.42 and 2.35 Å can only be attributed to a first-neighbor distance between the B and C atoms in the chain and a second-neighbor distance between a chain-boron atom and an icosahedron-boron atom, respectively. KEYWORDS: amorphous ceramics, boron carbide, chemical vapor deposition, local structure, solid-state NMR, neutron diffraction, X-ray absorption, ab initio molecular dynamics

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INTRODUCTION Boron carbide (B4C) is an exceptionally hard, lightweight, and refractory material. This unique set of thermomechanical properties has led to many uses as cutting tools, ballistic armor, low-wear bearings, and abrasion-resistant dies and nozzles.1,2 It also is of great interest for nuclear applications, because of its high neutron absorption without activity and its self-healing capacities.3 Boron carbide ceramics can be obtained using various processing routes, such as carbothermal reduction,2,4 powder sintering,2 polymer pyrolysis,5,6 or gasphase reactions.7 Physical and chemical vapor deposition processes have allowed the deposition of pure and homogeneous boron carbide coatings with amorphous or polycrystal© XXXX American Chemical Society

line structures and a wide stoichiometry range (symbolized by B1−xCx or BxC).7−19 These high-quality coatings might find applications for wear resistance20,21 or as optical and electronic materials. The optical band gap of BxC films and the properties of BxC/Si diodes can indeed be varied by simply changing the B/C ratio.11,13,22 Some of these benefits, however, can be strongly limited by the poor oxidation resistance of boron carbide in air beyond 600 °C. In the specific case of self-healing matrices the Received: March 14, 2013 Revised: June 3, 2013

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dx.doi.org/10.1021/cm400847t | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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subject of the present study such a high oxidability can actually become an advantage. The boron carbide layers, introduced in this last generation of silicon carbide-based composites, are intentionally aimed at promoting matrix oxidation. A borosilicate glass is formed, sealing the matrix cracks and protecting the fiber reinforcement against oxidation.23−25 This particular feature provides self-healing matrix composites with an excellent durability under load in air, which is very attractive for aeronautic applications. Significant weight and noise reductions could, in this way, be achieved by replacing metal parts of aircraft engines (e.g., rear bodies or secondary flaps).26 These ceramics are generally obtained from the pyrolysis of gas precursor mixtures using an isothermal chemical vapor deposition (CVD) process under reduced pressure.16−27 They are amorphous and their stoichiometry BxC varies with the CVD conditions (2 < x < 4). Although their composition belongs to the B4C + C domain of the phase diagram, graphitic carbon and crystalline boron carbide are both absent in the asdeposited coatings. These amorphous materials, which can be considered as a homogeneous phase, are referred to as a-BxC in the following. Self-healing matrix composites revealed primary creep stage28 or viscoelastic behavior29 at 1100−1200 °C, which was related to the structural instability of the a-BxC layers and their crystallization into B4C.30 These results suggest that the metastability of the a-BxC ceramics might lead to a density change and a transient viscous behavior beyond the mentioned temperature range. This instability and its consequences on the mechanical behavior might be detrimental to the performances of ceramic matrix composites during severe aeronautic uses, e.g., as engine turbine blades or primary flaps. Consequently, there is a crucial need to characterize more precisely the local chemical structure of self-healing BxC ceramics, in their asprocessed state as well as after annealing, to better understand and predict this phenomenon. It is long-time known that B4C crystallizes within a rhombohedral lattice R3̅m space group) with 12-atom icosahedra at the vertices and 3-atom linear chains connecting them along the (111) axis.31,32 However, if, in these early years, the structure was considered to be made of B12 icosahedra and C−C−C linear chains, a long controversy (see the recent review of Domnich et al.33 on this topic) recently went to an end, thanks to density functional theory (DFT) calculations. Indeed, by comparing the formation energies of different structures, as well as their theoretical Raman and 11B NMR features to experimental spectra, Vast and co-workers34−36 were able to unambiguously show that B4C is mostly made of B11C icosahedra, with the carbon atom located in one of the six polar sites (the polar sites being directly linked to a neighboring icosahedron while the equatorial sites are bonded to the linear chains) and a C−B−C chain (see the picture of an orthorhombic supercell made of 12 rhombohedral unit cells displayed in Figure 1). These authors have also proposed that up to 5% of B12 and B10C2 icosahedra must be present in the structure to fully reproduce the experimental 11B NMR spectra.35 Only a few studies describe the short-range order of amorphous boron carbide (a-BxC) films, vibrational and photoemission spectroscopies being often used to characterize the coatings.19,37−42 Yet, these techniques are relatively imprecise due to the broad distribution of phonon frequencies in the IR/Raman spectra (close to density of states) or to the

Figure 1. Atomistic model of crystalline boron carbide (c-B4C) made of 12 primitive unit cells (B11C + C−B−C) arranged in an orthorhombic supercell. Small spheres are B atoms (polar and equatorial atoms are displayed in blue and red, respectively; chain atoms are shown in green); large spheres are C atoms (polar and chain atoms are displayed in orange and purple, respectively).

uncertain assignment of the photoelectron binding energies, that usually results from surface contamination or charge effects. Recently, some authors have completed their investigations of the local structure by pair distribution function (PDF) analyses derived from electron diffraction patterns.39,41 Although being a valuable way of describing short-range order in amorphous solids, the PDF analysis based on electron diffraction dramatically suffers from large experimental broadening. For instance, a g(r) function reported by Bao et al.41 has a maximum intensity that is only very slightly larger than unity at the first peak, which is not so relevant for interatomic distance distribution in a solid, even when very disordered. Finally, it is important noting that most of the latter studies have been performed on materials prepared by physical vapor deposition techniques37−42 usually providing much more disordered structures than those obtained by thermally activated CVD. Liquid quench molecular dynamics simulations using ab initio (AIMD) total energy calculations have emerged in the past years as a powerful technique to investigate the local structure of amorphous materials prepared by physical vapor deposition (PVD) techniques.43,44 It has been recently applied by Ivashchenko et al.45 to an amorphous boron carbide material. These authors have studied systems of 120 and 135 atoms, at the density of the crystal, with a rather fast quench rate (840 K/ps), which is typical of PVD processes such as ion implantation techniques.46 In the present work, the short-range order of an a-BxC ceramic prepared by CVD is studied using PDF data as obtained from neutron diffraction, which is a more accurate technique than electron diffraction. The local chemical structure of the coating is further assessed by 11B NMR and X-ray absorption at B(1s) and C(1s) edges (XANES). These experimental methods have already been applied to bulk B4C,3,47,48 Si−B−C−N ceramics,49,50 to an amorphous hydrogenated boron carbide51 and to amorphous boron carbide B



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coatings obtained by PVD,17 but have never been combined to ascertain independently a common structure. In addition, an atomistic representation of the material is build using an AIMD liquid quench simulation. Direct comparisons between computed and experimental PDF and 11B MAS NMR spectra allow us proposing a model of a CVD a-BxC coating. Finally, in the last part of the paper, we discuss the structural changes that occur in the material after heat treatment in an inert atmosphere at 1300 °C.

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Raman Spectroscopy. Raman microspectroscopy analyses were performed using a Horiba Jobin−Yvon, Labram HR spectrometer (λHe−Ne = 632.8 nm). The lateral resolution is ∼1−2 μm, while the thickness probed typically ranges from a few tens of nanometers to several hundreds of nanometers (depending on the light absorption of the material), i.e., in any case below the thickness of the coating. The power of the incident laser beam was limited to a few tenths of mW, to avoid any local heating of the specimen. The same hot-pressed B4C standard as that used for the EPMA was also examined. Neutron Diffraction. Neutron diffraction measurements were carried out on the D4 diffractometer at Institut Laue-Langevin (ILL).53 Cylindrical vanadium containers (5 mm in diameter) were filled with the powder specimens (∼100 mg) and the total scattered intensities were measured over a period of 6 h. The incident neutron wavelength of 0.497 Å was determined using a nickel sample; the scattering vector (Q) was in the range of 0.3−23.6 Å−1. Background scans were performed for the empty vanadium container, for the container filled with boron powder (10B) as an absorbing sample, and for the empty instrument (no container present). A standard vanadium solid cylindrical rod was also run for intensity calibration. The CORRECT program54 was used to correct the measured scattered intensity for background, attenuation, and multiple scattering, and to normalize the intensity of the resulting total-scattering structure factors S(Q). 11 B Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) Spectroscopy. The chemical environment of B atoms was studied by 11B MAS NMR. The spectra were recorded at 11.75 T on a Bruker Avance Model 500 wide-bore spectrometer operating at νL = 160.5 MHz, using a Bruker 4 mm probe and a spinning frequency of 14 kHz for the rotor. The spectra were acquired using a spin−echo θ−τ−2θ pulse sequence (with θ = 90°) to overcome problems due to the probe signal. The τ delay was synchronized with the spinning frequency and a recycle delay of 1 s was used. The chemical shifts were referenced to BF3(OEt)2 (δ = 0 ppm). A commercial B4C powder (98%, 200 mesh, from Sigma−Aldrich Chimie SARL, Lille, France) was also analyzed under the same conditions. X-ray Photoabsorption Spectroscopy. The bonding structure was studied by X-ray absorption near-edge spectroscopy (XANES) at the TEMPO55 beamline of SOLEIL synchrotron. The B(1s) and C(1s) core photoabsorption spectra were recorded at room temperature under ultrahigh vacuum (P < 10−8 Pa). In the total yield mode, the XANES signal corresponds to a depth of ∼10 nm (the secondary electron path). The absolute error is ∼0.1−0.2 eV (the photon energies are not adjusted to standard values) but the relative error is less than a few meV. XANES experiments were also performed on reference B4C (the same as for EPMA) and highly oriented pyrolytic graphite (HOPG, from Mersen, Gennevilliers, France) under the same conditions to compare and assign the main features in the B(1s) and C(1s) spectra. Computer Simulation. An amorphous model containing 154 B atoms and 62 C atoms with a density of 2.47 g/cm3 (i.e., representative of the a-BxC material, see Results and Discussion below), was produced by a liquid quench AIMD simulation using plane-wave/pseudo-potential implementation of density functional theory (DFT). Car−Parrinello56 MD simulations were performed using the CPMD package57 and the BLYP58,59 functional was used in conjunction with Goedecker norm conserving pseudo-potentials.60 Plane-waves were expanded up to an energy cutoff of 80 Ry and the Brillouin zone sampling was limited to the Γ-point. The system was enclosed in a cubic cell with periodic boundary conditions and the quench was performed using a procedure very similar to those adopted by other authors.43−45 First, a simple cubic crystal with a random assignment of B and C atoms on the lattice was simulated at 4000 K using a constant number of atoms (N) and volume (V) MD algorithm at 4000 K, until complete melting was reached (in a few picoseconds). In a second step, the resulting liquid was quenched at constant N and V but with a predefined ramping of the temperature from 4000 K to 0 K, by modifying the target temperature of the thermostat at each time step. To save computer time while obtaining a sufficiently converged structural model, a nonuniform cooling rate was used,61,62 giving

METHODS

Sample Preparation. High-purity a-BxC coatings were obtained by the decomposition of a mixture of BCl3, CH4 and H2 in a hot-wall CVD reactor operating at 1000 °C under a total pressure of 10 kPa. The reactor consists of a silica tube (100 mm inner diameter) heated in its central part with a radio frequency generator. The quasiisothermal deposition zone (±5 °C) is ∼100 mm long (a diagram of the CVD setup is shown in ref 52). In reference to earlier works,16 the initial composition of the gas phase was defined by two ratios of the gas flow rates: δ = Q(BCl3)/Q(CH4) = 2 and γ = Q(H2)/Q(BCl3) = 5. The total gas flow rate was equal to 390 sccm (no further carrier gas was added in the mixture). [Note: 1 sccm = 1 standard cm3/min.] These conditions lead to a deposition rate of ∼1.5 μm/h. Most of the samples were deposited on a few blocks (∼1 cm × 1 cm × 1 cm) of open-cell vitreous carbon foams (from Ultramet, Pacoima, CA, USA) with a pore size of 100 ppi, an apparent density of 0.05 g/cm3, and a specific surface of ∼55 cm−1). These substrates were used for bulk characterization (X-ray diffraction, neutron diffraction, 11B solid-state NMR, density measurements, etc.), after grinding and purification of the sample. The deposition time was typically 6 h, leading to a thickness of the coating of ∼9 μm and a net weight of ∼0.6 g. A few distinct particles from the same powder samples were also used for local analyses by Raman microspectroscopy and transmission electron microscopy (TEM). Prior to analysis, the infiltrated foams were carefully crushed and the carbon substrate was removed by oxidation in dry air at 430 °C, the a-BxC ceramic being only slightly oxidized at this temperature. The powders were thoroughly weighed and examined by Raman spectroscopy to confirm the complete elimination of the substrate. The specimens were finally washed in hot water to remove B2O3 traces and dried. Approximately 0.5 g of pure a-BxC ceramic powder was obtained at the end of the whole procedure. The same coatings were simultaneously deposited on silicon wafers (100 surface from Neyco, Paris, France) specifically for surface characterization such as X-ray absorption near edge structure (XANES) spectroscopy and electron probe microanalysis (EPMA). A part of the a-BxC powder specimen was heat-treated during 2 h at 1300 °C under a pure argon flow (Alphagaz 2, 99.9999% from Air Liquide, 105 Pa) in order to investigate the resulting structural changes. Electron Probe Microanalysis (EPMA). The elemental composition of the as-prepared coatings deposited on silicon wafers was estimated using a Cameca SX100 electron microprobe working at 7 kV. High-purity B4C (polycrystalline, hot-pressed, from GoodFellow SARL, Lille, France) was used as a standard for both B and C elements, and pure silica glass for oxygen. Density Measurement. The density of the a-BxC powder was measured by helium pycnometry (Model Accupyc 1330, Micromeretics). X-ray Diffraction (XRD). The long-range structure of the same powder specimen, as deposited and heat-treated, was studied by X-ray diffraction (XRD) (Bruker D8, λCu Kα1 = 0.154056 nm) in the Bragg− Brentano θ−θ mode. Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) (Philips, Model CM30ST, accelerating voltage = 300 kV) was used to identify the crystalline phases in the coatings in both selected-area electron diffraction (SAED) and highresolution imaging modes. Thin particles from the ceramic powders spread on a grid were observed without preparation. C



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including PAW (GIPAW) extension70 for the electric field gradient (EFG) and shielding tensor, respectively. The 11B quadrupolar parameters are derived from EFG tensor principal values (Vxx,Vyy,Vzz) using standard conventions: |Vxx| ≤ |Vyy| ≤ |Vzz|, Cq = eQVzz/h and ηq = (Vxx − Vyy)/Vzz, where Cq and ηq are the quadrupolar coupling constant and asymmetry parameters, respectively. The recently compiled71 boron quadupolar moment Q = 40.59 × 10−31m2 has been used. Calculated shielding tensor principal values (σxx,σyy,σzz) are ordered using |σyy − σiso| ≤ |σxx − σiso| ≤ |σzz − σiso|, with the isotropic component defined as σiso = 1/3(σxx + σyy + σzz). Using the Haeberlen convention, this yields to define the anisotropic and asymmetry shielding parameters as σaniso = σiso − σzz and η = (σyy − σxx)/σaniso, respectively. Boron isotropic chemical shifts (CS) are referenced with respect to the boron trifluoride etherate (BTE) isotropic shielding, using

priority to the temperature range at which the amorphous solid is formed. Accordingly, a cooling rate of 165 K/ps was used in both the [4000 K: 2500 K] and [1500 K: 0 K] temperature ranges and a 41.25 K/fs rate in the [2500 K: 1500 K] domain (i.e., 5−20 times slower than in ref 45). The upper limit of the latter domain is close to the melting line of the boron carbide phase diagram (∼2245 °C at our working composition63), while the lower limit corresponds to the freezing of translational mobility in the system, characterized by vanishing self-diffusion coefficients. Energetics and Wannier function centers (WFCs) were calculated on the resulting quenched configuration (0 K). Other properties (structure factors, pair distribution functions) were computed as statistical averages over 10 ps NVT MD runs performed at 300 K (preceded by 3 ps thermalization runs). In addition, some simulations were also performed on the model of rhombohedral B4C (c-B4C) displayed in Figure 1. It consists of a 180atom orthorhombic supercell made of B11C icosahedra with C atoms on the polar sites and C−B−C linear chains, in agreement with the current view of the B4C structure.33−35 One C atom has been randomly assigned to a polar site of every icosahedron with the constraint that no C−C bond is formed, because there is no experimental evidence for these types of features in c-B4C. The experimental parameters of the hexagonal unit cell31 (a = 5.60 Å, c = 12.07 Å, d = 2.51 g/cm3) were used. A time step of 2.5 atomic unit (1 atu ≈ 0.024 fs) was used for all simulations. The constant temperature runs were performed with a Nosé−Hoover thermostat,64 while a Berendsen thermostat65 was used for the quench simulations. Pair Distribution Functions. The normalized total scattering structure factors S(Q) were obtained from the differential scattering cross-section per atom I(Q) measured during the neutron diffraction experiment, according to n

n −2

S(Q ) = (∑ cibi̅ ) [I(Q ) − i=1

CS δiso = − (σiso − σBTE)

We have used σBTE = 96.9 ppm. [Using a secondary reference such as [B12H12]2−, we have deduced the reference shielding σBTE from eq 5, using the experimental 11B chemical shift (δ = −14.9 ppm) for [B12H12]2−, with respect to BTE, and the calculated shielding (σB = 11.8 ppm).] We recall that the chemical shift anisotropy δaniso is related to σaniso by δaniso = −σaniso. Because of the strong quadrupolar coupling observed for some boron environments (vide infra), second-order quadrupolar effects must be also included in order for the calculated 11 B resonance shift to be directly comparable to experiment. Isotropic quadrupolar isotropic shift (in ppm) is evaluated72 using: Q δiso =−

i=1

∫0

Q max

Q [S(Q ) − 1] sin(Qr ) dQ

CS Q δ = δiso + δiso

(1)

n i=1

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RESULTS AND DISCUSSION Structure of the As-Deposited Material. For the processing conditions given above, the atomic concentration of the ceramic, as measured by EPMA, was 70.5 at. % boron, 28.6 at. % carbon, and 0.9 at. % oxygen. The related stoichiometry (a-B2.5C) reveals a significant excess of carbon in the material with respect to B4C. Under these conditions, the equilibrium phase diagram suggests the coexistence of rhombohedral B4C and graphitic carbon. The density of the material measured by helium pycnometry is 2.47 (±0.01) g/ cm3, which is a value very close to the density of crystalline B4C (2.52 g/cm3). Figure 2 presents a TEM image of the material in the high-resolution mode (HRTEM). It does not reveal the presence of any crystalline order or graphene sheets. The selected-area electron diffraction (SAED) pattern (Figure 2, inset) shows a limited number of complete and diffuse rings, as usually found for isotropic and amorphous material. The X-ray diffraction (XRD) pattern (see the Supporting Information) and Raman spectrum (Figure 3) of the material confirm the absence of both pure carbon and crystalline boron carbide phases, in contrast to the thermodynamic predictions. Indeed, the Raman spectrum of the as-processed material shown in Figure 3 is composed of two very broad bands, at ∼400−700 cm−1 and 850−1350 cm−1, which are typical of a

(2)

(3)

where gij(r) designate the partial pair distribution functions extracted from MD simulations. The theoretical reduced pair distribution functions is then deduced from eq 3, using G PDF(r ) = 4πρ0 r[g (r ) − 1]

(7)

B powder NMR spectra were simulated with the fpNMR program.73,74 For our study, a single optimized configuration extracted from the AIMD performed at 300 K, combined with a Gaussian convolution of half-width 1.5 ppm, was sufficient to simulate the experimental NMR parameter distribution without compromising the assignment of the principal resonances.

n i=1

11

11

The theoretical total pair distribution functions g(r) were computed using

g (r ) = (∑ cibi̅ )−2 ∑ cicjbi̅ bj̅ gij(r )

(6)

where Cq and νL are given in MHz and I = /2 for B. The total isotropic shift δ, as observed in NMR experiment, is then given by

where Q is the scattering vector, and ci and bi̅ are the concentration and the coherent bound neutron scattering length, respectively, of 2 species i (averaged over the different isotopes), and where 4πbi = 4π(b2i,coh + b2i,incoh) is the total bound scattering cross section for species i. Standard bi̅ values of 5.30 and 6.65 fm were adopted for the B and C atoms, respectively. The experimental reduced pair distribution function GPDF(r) is subsequently obtained by Fourier transform:

2 G PDF(r ) = π

2 η2 ⎞ 3 ⎛ Cq ⎞ I(I + 1) − 3/4 ⎛ 1 + ⎟ × 106 ⎜ ⎟ 2 2 ⎜ 40 ⎝ vL ⎠ I (2I − 1) ⎝ 3⎠ 3

2

∑ cibi̅ ] + 1

(5)

(4)

where ρ0 is the atomic number density of the material. Eventually, the total atomic structure factors of the models are obtained through the inverse transform of eq 2. 11 B NMR Parameters: Calculation and Spectra Simulation. NMR parameters calculations were performed at the DFT level of theory, using a plane-wave basis set and norm-conserving pseudopotentials, as implemented in the Quantum-ESPRESSO suite of programs.66 The generalized gradient PBE67 functional was employed to approximate the exchange correlation potential. Explicit periodic conditions were taken into account for NMR calculations through the projector augmented-wave (PAW) approach68,69 and the gaugeD



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Figure 4. Structure factor S(Q) (panel a) and pair distribution function GPDF(r) (panel b) of amorphous boron carbide. Empty circles: neutron diffraction data obtained from the a-B2.5C material; line represents the AIMD B154C62 model.

Figure 2. High-resolution transmission electron microscopy (HRTEM) image of the a-B2.5C material. The corresponding selected area electron diffraction (SAED) pattern is shown in the inset.

experimental and modeled S(Q) and GPDF(r) is good. Every peak of the experimental functions is well-reproduced by the model, both in terms of position and intensity, the model peaks being only very slightly less intense than their experimental counterparts. The atomistic model can thus be considered as a possible nanostructural model for the material, even though probably slightly more disordered (possibly due to a still high cooling rate). Looking more closely at Figure 4b, four main peaks located at 1.7, 2.9, 4.3, and 5.5 Å can be distinguished, revealing the local structure of the material. A snapshot of the model is given in Figure 5, where the three 12-atom icosahedral units are clearly identified. One has the B11C stoichiometry, which is, by far, the most prevalent in cB4C. The two others have the B12 and B10C2 compositions, identified as the two main substitutions of B and C atoms in the icosahedra of c-B4C.35 In our model, these icosahedral units appear to be embedded in an amorphous matrix made of incomplete icosahedra and lower coordinated atoms. Figure 6 shows an enlargement of Figure 5, where the maximally localized Wannier function centers (WFCs), which are a practical way of locating chemical bonds, are displayed,77 in addition to B and C atoms. The coordination numbers, as determined using the atom−WFC pair distribution functions, are given in Table 1. Among all atoms, 66 B atoms (42.9%) and 3 C atoms (4.8%) are 6-fold coordinated. As can be seen in Figure 6, these atoms form many three-centered bonds, identified by a WFC lying almost exactly in the middle of a three-atom triangle. The occurrences of every possible boron and carbon atomic environments are respectively given in Tables 2 (boron) and 3 (carbon). Among the 66 hexacoordinated B atoms, 13, 31, and 17 respectively show BB6, BB5C, and BB4C2 environments, in similar proportions to that in c-B4C (respectively, 31 (BB6), 58 (BB5C), and 43 (BB4C2) environments are found among the 132 BX6 atoms of the model crystal displayed Figure 1). Also, as in c-B4C, the three hexacoordi-

Figure 3. Raman spectra of the as-prepared (a-B2.5C) and heat-treated (HT-BxC) coating (1300 °C/2 h/Ar). The 200−1200 cm−1 region of the HT-B2.5C spectrum is highlighted in the inset and the spectrum of hot-pressed B4C is also shown for comparison.

nearly amorphous state. We note that the steep slope on the left side of the spectrum is due to the Notch filter cutoff. The more intense band centered around 1050 cm−1 could be attributed to breathing modes of some icosahedron-like units, by analogy with the c-B4C features.75 The absence of D (1350 cm−1) and G (1580 cm−1) bands that are characteristic of graphite-like carbon76 confirms the absence of free sp2 carbon domains in the material. The atomic structure factor (Figure 4a) and the reduced pair distribution function (Figure 4b) obtained by neutron diffraction are compared to those calculated from the atomistic B154C62 model in Figure 4. The agreement between the E



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the intericosahedral linear chains in c-B4C, these atoms form four two-center covalent bonds, essentially with B atoms (respectively, 24 CB4, 10 CB3C, and 4 CB2C2 environments). However, no 2-fold B atom, characteristic of the C−B−C chains in c-B4C, could be detected (see Table 1). Instead, 23 (15%) 3-fold (sp2) B atoms are found, essentially in BC3 (8) and BC2B (11) environments. It is worth noting that only three of the 62 carbon atoms are 3-fold, indicating the absence of free hexagonal carbon in the model. Tables 1−3 show that the model also contains many 4-fold boron atoms, as well as 5-fold B and C atoms, although in lower amounts than in the models of Ivashchenko et al.45 A closer view of the first two peaks of the total pair distribution function g(r) is plotted in Figure 7, together with its various ij-pair components (see eq 3). The first peak corresponding to first-neighbor distances, ∼1.7 Å, is essentially a combination of two distinct components: a 1.75 Å B−B peak and a 1.57 Å B−C peak. The intensity of the C−C first peak is very low, because of the small number of C−C bonds existing in the material. This peak is centered at 1.54 Å, which is a value characteristic of sp3 bonds. The second-neighbor peak is rather broad and contains multiple components resulting from a wide distribution of atomic pair distances. It finally appears that the overlap between the first two peaks of the global g(r) is attributed to the B−B pairs. The 11B NMR spectra recorded on the as deposited a-B2.5C coating and heat-treated HT-B2.5C deposit are compared to the hot-pressed crystalline c-B4C in Figure 8. Only one broad peak centered at −5 ppm is observed for c-B4C. This feature, which is assigned to icosahedral B atoms,35 is still present in the amorphous material but broadened and up-shifted (0 ppm). Another well-distinct component can be observed at ∼50 ppm in the case of a-B2.5C. This chemical shift is close to the values reported for polymer-derived Si−B−C−N ceramics.50 It was attributed to BC3 environments by comparison with the spectrum of trimethylborane (B(CH3)3).78 The 11B MAS NMR spectrum computed for the B154C62 model is plotted in Figure 9, along with the deconvolution as a function of the total coordination numbers of B atoms (see Table 1). Whereas only two distinct contributions are identified on the experimental line shape, the theoretical spectrum presents intermediate components ranging from 0 to 50 ppm. The deconvolution proposed in Figure 9b, along with the numerical results collected in Table 4, demonstrate that these extra features are due to the intermediate coordinations BX4 and BX5. These environments can be viewed as “defects” in our model and are probably due to the high quench rate employed during the simulation, which is far from experimental reality. This also outlines the limits in interpreting the one-dimensional structure factor as measured by neutron diffraction experiment. Despite the strong overlapping of the intermediate components with BX3, we are able to confirm that the main contributions to the resonances observed at 0 and 50 ppm are due to the icosahedral (hexacoordinated) BCxB6−x and trigonal (trivalent) BC3−xBx environments, respectively. This is supported by Figure 9c, whereas only the BX3 and BX6 features have been retained for simulating the theoretical spectra. Considering the MAS conditions used for the experiments and the low values of chemical shift anisotropy for BX6 (see Table 4), the shoulder at 25 ppm arises from the distribution of isotropic chemical shifts, which have been identified to boron in icosahedral units, as depicted in Figure 9c.

Figure 5. Snapshot of the atomistic B154C62 model showing three icosahedra (B12, B11C, and B10C2) in an amorphous matrix. (Large white spheres represent B atoms, small black spheres represent C atoms, blue spheres represent B atoms in icosahedra, and red spheres represent C atoms in icosahedra; orange denotes bonds between atoms belonging to icosahedra.)

Figure 6. Enlargement of Figure 5 with superimposed Wannier function centers (WFCs) highlighting the different bond structures in the model. (The color code is the same as that for Figure 5, with WFCs displayed using small green spheres.)

Table 1. Total Coordination Numbers of Boron and Carbon Atoms in the B154C62 Model 1st neighbor number of B atoms B (at. %) number of C atoms C (at. %)

2

3

4

5

6

7

0

23

29

33

66

3

(0)

(15.0)

(18.8)

(21.4)

(42.9)

(1.9)

0

3

38

18

3

0

(0)

(4.9)

(61.3)

(29.0)

(4.8)

(0)

nated C atoms are exclusively found in CB6 environments. This confirms the low probability of finding C−C bonds in the icosahedral units, despite the higher carbon concentration in aB2.5C. Apart from these highly coordinated atoms, Table 1 reveals many 4-fold C atoms (sp3 hybridization). They represent 61.3% of the C atoms in the model. As the C atoms at the two ends of F



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Table 2. Local Environments of B Atoms in the B154C52 Model Given by the Number of Atoms Having NB (Columns) Boron Neighbors and NC (Rows) Carbon Neighborsa NC NC NC NC NC a

= = = = =

0 1 2 3 4

NB = 0

NB = 1

NB = 2

NB = 3

NB = 4

NB = 5

NB = 6

NB = 7

0 0 0 8 1

0 0 11 5 1

0 3 15 6 0

1 8 11 5 0

0 11 17 0 0

4 31 1 0 0

13 0 0 0 0

2 0 0 0 0

BX6 and BX3 environments are highlighted in bold and italic fonts, respectively.

Table 3. Local Environments of C Atoms in the B154C52 Model Given by the Number of Atoms Having NB (Columns) Boron Neighbors and NC (Rows) Carbon Neighborsa NC NC NC NC

= = = =

0 1 2 3

NB = 0

NB = 1

NB = 2

NB = 3

NB = 4

NB = 5

NB = 6

0 0 0 1

0 0 1 0

0 0 4 0

1 10 0 0

24 3 0 0

15 0 0 0

3 0 0 0

a

CX6 and CX4 environments are highlighted, in italic and bold font, respectively. Arrangement is equivalent to that shown in Table 2, but for C atoms.

Figure 7. Total pair distribution function g(r) of the B154C62 model (thin red line) and the individual contributions of boron−boron (black straight line), boron−carbon (black dashed line), and carbon−carbon (black dotted line) pairs (see eq 3).

From the detailed analysis of the theoretical 11B NMR parameters obtained for the amorphous model given in Table 4, we can state that the shielding of the 11B nucleus increases as a function of the coordination number of boron (from 47 ppm to −11 ppm for the chemical shift) and is accompanied by the decrease of the quadrupolar coupling (from 4.7 MHz to 1.2 MHz for the coupling constant). The second observation explains the dependence of the 11B resonance shift with respect to the quadrupolar isotropic shift, which can reach −23 ppm for trigonal BX3 environment. The strong impact of second-order quadrupolar effect onto the spectral line shape is shown in Figure 9b. To complete our theoretical investigation, a set of representative NMR parameters for crystalline B4C was calculated. We have focused mainly on two of the most stable models previously proposed by Mauri et al.,35 namely the “equatorial” and “polar” configurations. Both consist of a cell containing 15 atoms with one B11C icosahedron and one linear C−B−C chain.34 From the results collected in Table 4, we observe the overall upshift of the central resonance (assigned to

Figure 8. 11B MAS NMR spectra of the as-prepared (panel b, a-B2.5C) and heat-treated (panel a, HT-B2.5C) coating (1300 °C/2 h/Ar). The spectrum of hot-pressed crystalline B4C is also shown as panel c, for the sake of comparison.

the BX6 environments) when going from an amorphous composition (⟨δ⟩ ≈ 1 ppm) to a crystalline composition (⟨δ⟩ ≈ −4.6 ppm). The NMR parameters calculated for the C−B−C chains indicate that the experimental 11B resonance shift, which must be found at 12 and 16 ppm, respectively, fall within the range covered by the hexacoordinated boron. The corresponding BC2 environments are characterized by a large quadrupolar constant of ∼7 MHz, leading to a strong quadrupolar G



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model, respectively. Our present atomistic model for a-B2.5C does not contain BC 2 environments. As a result, an unambiguous discussion of the influence of the chain boron atoms contributions to the 11B NMR spectra requires further investigation, which is beyond the scope of this article. It should be mentioned that a few 13C MAS NMR analyses of the a-B2.5C coating were also attempted. They failed due to a poor signal/noise ratio (because of the low 13C natural abundance). From the theoretical point of view, the simulated spectra show a very broad distribution of chemical shifts covering a range of 200 ppm, which is mainly due to the presence of BX4 and BX5 defects. An extended analysis of 13C chemical shifts along with the influence of the C and B speciation in the BXn environments NMR parameters will be presented in a forthcoming paper. The XANES B(1s) and C(1s) core level absorption spectra of the a-B2.5C coating are shown in Figure 10. The reference

Figure 9. (a) Full theoretical 11B MAS NMR spectrum of the B154C62 model (theoretical) within the range [−300, 300] ppm. The experimental line shape obtained for a-B2.5C is also presented. Both spectra have been normalized. (b) Decomposition of the full theoretical spectrum within the range [−150, 150] ppm, as a function of the boron coordination numbers BXn. The dotted line shapes are obtained when second-order quadrupolar effects are disregarded in the simulation. (c) Comparison between the experimental and theoretical spectra. The latter includes only the hexacoordinated and tricoordinated boron contributions (BX6+BX3). The signature of B atoms from the three icosahedra is also represented.

Figure 10. XANES spectra of the a-B2.5C material ((left) B(1s) edge and (right) C(1s) edge); the spectra of reference materials (B4C and HOPG (this work) and B2O3, diamond, and B0.333C film (taken from ref 17)) are given for the sake of comparison.

deshielding above −45 ppm. This contrasts with the value of the chemical shift found at 62.3 and 62.5 ppm, but emphasizes the strong anisotropy of the shielding tensor, which reaches values of −155.9 and −165.9 ppm for the equatorial and polar

spectra of pure elements (graphite, diamond) and compounds (B4C), as well as a carbon-rich thin film of composition B0.333C

Table 4. Distribution of Theoretical 11B NMR Parameters in the Amorphous B154C62 and Crystalline B4C Models, as a Function of the Boron Coordinationa NMR parameters

Coord.

average resonance shift, ⟨δ⟩ (ppm)

chemical shift, ⟨δCS iso ⟩ (ppm)

second-order quadrupolar shift, ⟨δQiso⟩ (ppm)

⟨δaniso⟩

⟨Cq⟩ (MHz)

⟨η⟩

⟨ηq⟩

B154C62 Model ± ± ± ± ±

BX3 BX4 BX5 BX6 BX7

47 37 23 1 −11

BX6 BC2 BX6 BC2

−4.4 ± 2.5 12.0 −4.8 ± 4.1 16.1

18 26 22 16 8

−23 −14 −6 −3 −2

70 51 29 4 −9

−37 −8 0 −4 −30

± ± ± ± ±

61 57 55 38 34

0.6 0.5 0.7 0.6 0.6

± ± ± ± ±

0.2 0.2 0.2 0.2 0.1

4.7 3.5 2.2 1.5 1.2

± ± ± ± ±

0.7 0.8 0.8 0.7 0.3

0.2 0.5 0.6 0.5 0.7

± ± ± ± ±

0.2 0.2 0.2 0.2 0.2

B4C Models equatorial equatorial polar polar

−2.6 62.3 −3.6 63.3

−1.8 −50.2 −1.2 −47.2

−3 ± 20 −155.9 −4 ± 15 −165.9

0.3 ± 0.2 0.03 0.3 ± 0.3 0.01

1.2 ± 0.5 7.2 1.0 ± 0.4 6.9

0.5 ± 0.3 0.07 0.6 ± 0.4 0.05

b Q The value of the average resonance shift (⟨δ⟩) is decomposed as a sum over the ⟨δCS iso ⟩ and the ⟨δiso⟩ calculated at νL = 160.5 MHz. Only the two B4C models, called “polar” and “equatorial”, as proposed by Mauri et al., have been considered here. Description of the crystal structures can be found in ref 35.

a

H



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% to 75 at. % and why it finally vanishes at 90 at. % of C, in favor of the 192.4 eV peak, while some hexagonal character of the material eventually appears.17 The C(1s) spectrum of the a-B2.5C coating shows a main peak at 284.9 eV, a small bump at 288.2 eV, and a broad band at ∼292 eV (Figure 10). The 284.9 eV component is very similar to the π* transition of c-B4C (284.9 eV), but also close to the π* peak from graphite (285.2 eV). Jimenez et al. assigned this c-B4C main feature to the end-chain C atoms.14 It probably has the same origin here in the a-B2.5C coating. The material is indeed free of sp2 carbon as already indicated by the Raman analysis and confirmed by the absence of a sharp σ* feature at ∼291.5 eV. Similar sp3 CB4 sites are also present in a-B2.5C between the icosahedral units and the trigonal BC3−xBx sites (in place of the C−B−C chains in c-B4C), as suggested by the model. Structure of the Heat-Treated Material. In this section, we briefly report on the structural changes in the B2.5C coating resulting from a 2-h heat treatment at 1300 °C in an argon atmosphere. As expected, the bulk chemical composition at the micrometer scale (as determined by EPMA) is unaffected by the heat treatment. Figure 11 shows a HR-TEM image and a

prepared by coevaporation of B and C atoms,17 are also displayed for comparison. The B(1s) and C(1s) spectra recorded from the a-BxC coating are rather complex and undefined. The absence of sharp and intense σ* and π* features suggests intermediate or multiple hybridizations of both the B and C atoms in the material.14,17 In contrast, the C(1s) spectrum of HOPG shows very distinct π* and σ* peaks (at ∼285 and ∼291.5 eV, respectively) that are typical of the sp2 hexagonal structure, while the diamond spectrum is characterized by a sharp σ* absorption edge at 288.5 eV related to the sp3 sites (Figure 10). The B(1s) spectrum recorded for a-B2.5C is also significantly different from the one of hexagonal-BN, which shows a strong π* peak at ∼192 eV and a more complex σ* contribution around 197−202 eV, both assigned to a sp2 hybridization of boron atoms.14 Four distinct components can be observed in the photon energy range 189−195 eV of the B(1s) absorption edge for aB2.5C. These features appear at 190.0, 191.3, 192.4, and 194.1 eV, i.e., at very similar energies as those reported by Caretti et al.17 for carbon-rich coevaporated BxC1−x thin films (0.25 < x < 0.8, see the B(1s) and C(1s) spectra, for x = 0.25, in Figure 10). Following their notation, peaks B0, B1, B3, and B4 were indeed observed in their BxC1−x films at 189.7, 191.0, 192.4, and 194.0 eV, respectively. B1 is obviously the most intense peak obtained on c-B4C (Figure 10). Jiménez et al.14 suggested that this peak (labeled “A” in their work) originates, for c-B4C, from icosahedral B atoms linked to an end-chain carbon or a boron in the chain itself. This contribution could correspond, as suggested by the atomistic model, to a hexacoordinated B atom in an icosahedron (BCxB6‑x) bonded to a sp3 carbon atom (CB4) of the surrounding amorphous matrix. Peak B4 can be definitely attributed to boron oxide sites (BO3) by comparison to the B2O3 spectra, considering that XANES is a surface analysis and that the a-B2.5C coating is probably covered with native oxide layer. Similarly, according to Caretti et al.,17 peak B3 would be a signature of superficial boron suboxyde environments (BO3−xCx). Interestingly, peak B0 at 190.0 eV is clearly visible in the a-B2.5C spectrum, whereas it is totally absent in the spectrum of c-B4C reference. The same component was observed by Caretti et al.17 for coevaporated BxC1−x films, its intensity increasing with the carbon concentration in the range of 20−75 at. %. Caretti et al. attributed the B0 peak to a hexagonal BC3 environment by comparison with electron energy-loss spectroscopy (EELS) analyses of a CVD coating of composition BC3 assumed to be hexagonal at long range.79 This 190 eV component, however, was not clearly evidenced by a more recent EELS analysis of a similar “BC3” material and of a B-doped graphitic carbon (containing 6 at. % boron).80 This was confirmed by a XANES investigation of highly oriented boron-doped graphite.81 In these materials, where the B atoms substitute for C atoms, the B(1s) spectrum is characterized by a strong π* peak at ∼192 eV (as in h-BN) assigned to the sp2 trigonal BC3 environments in the nearly ideal hexagonal carbon structure. These results indicate that the B0 peak might actually arise from trigonal BC3 (or mixed: BC3−xBx) environments in an amorphous structure (i.e., with no long-range hexagonal order), as suggested by the atomistic model. Its attribution, still in good agreement with the 11 B MAS NMR analysis,50 seems more compatible with the amorphous structure of the present a-B2.5C coating and the highly disordered ex-PVD BxC1−x films analyzed by Caretti et al.17 This would also explain why its intensity gradually increases when the carbon concentration is increased from 5 at.

Figure 11. HRTEM image of the HT-B2.5C coating (1300 °C/2 h/ Ar); the corresponding SAED pattern is shown in the inset. Lattice fringes of crystalline boron carbide and turbostratic carbon domains are indicated with arrows.

SAED pattern of the heat-treated material (HT-B2.5C). From the abundant and typical diffraction spots and lattice fringes (mostly 003, 012, 104, 021), we conclude that the rhombohedral B4C phase is present in a highly crystalline state. The crystallization of the boron carbide phase is confirmed by XRD analysis (see the HT-B2.5C pattern in the Supporting Information). The Scherrer equation, applied to the (021) diffraction peak, leads to a mean size of the B4C crystallites of 44 nm. A few turbostratic carbon domains with curved layers (002 fringes) are observed in the HR-TEM image. The presence of free graphite-like carbon is also obvious in the Raman spectrum (see Figure 3) on which most of the signal is I


Chemistry of Materials dominated by the G and D carbon bands.76 A close examination of the low-wavelength region of the spectrum (see inset) reveals the two sharp 480 and 535 cm−1 peaks, as well as the overlapping bands in the 600−1200 and 900−1200 cm−1 regions, characteristic of crystalline B4C.3,34,75 The 50 ppm peak has vanished in the 11B MAS NMR spectrum of the HT-B2.5C coating, indicating the disappearance of the trigonal BC3−xBx sites (Figure 8). The main band, initially at 0 ppm, is also shifted to −5 ppm, as a result of the crystallization of B4C at high temperature.3 While no obvious feature could be revealed in the 13C MAS NMR spectrum of aB2.5C, the spectrum of HT-B2.5C is characteristic of the B4C phase, with two sharp peaks at 0.5 and 80 ppm (not shown), assigned respectively to the carbon atoms located at the end of the C−B−C chains or in the icosahedral (polar) sites.35,48 Figure 12 shows the pair distribution function, GPDF(r), as measured by neutron diffraction from the HT-B2.5C material.

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CONCLUSION

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ASSOCIATED CONTENT

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The structural properties of an amorphous boron carbide coating prepared by chemical vapor deposition (CVD) have been assessed by complementary techniques that are able to probe the short-range (11B MAS NMR, XANES, Raman) and medium-range (neutron diffraction) order in the material. In addition, a liquid quench ab initio molecular dynamics simulation was used to produce an atomistic model of the material. This model was thoroughly validated against experiments and allows concluding that the as-deposited a-B2.5C coating contains a high amount of complete 12-atom boronrich icosahedral units with compositions B12, B11C, or B10C2. In opposition to c-B4C, in which the icosahedra are directly bound to each other through their polar sites or the C−B−C intericosahedral linear chains, in the amorphous state, the icosahedra are randomly located in an amorphous matrix consisting essentially of sp3 carbon atoms (with a CB4 environment, also found in B4C) and sp2 boron atoms (with mainly BC3 and BC2B environments). Although tricoordinated boron atoms do not exist in the crystal, clear signatures of their presence in the amorphous material have been evidenced in this paper. First, by comparison between experimental and computational data, we have shown that these BC3−xBx environments are responsible for the ∼50 ppm peak in the 11 B MAS NMR spectra. Also, they have been identified (190 eV peak) on the B(1s) edge of the XANES spectra and assigned, by comparison with amorphous and hexagonal B−C materials, to trigonal sites in a disordered structure. This amorphous intericosahedral phase, containing mostly sp3 carbon and sp2 boron (with only a few boron−boron bonds and almost no carbon−carbon bonds) can be viewed as an amorphous variety of the hypothetical B4C3 polymorphs, derived from the Si3N4 structures and predicted by DFT calculations.82,83 The comparison between the experimental 11B NMR spectrum and the simulated spectrum deduced from the model showed that the 4- and 5-fold B atoms, as well as the 5-fold C atoms observed in the model do not exist in the material. After heat treatment at 1300 °C, the amorphous a-B2.5C coating crystallizes into rhombohedral boron carbide. The crystallization of B4C is accompanied by the formation of free turbostratic carbon domains, as evidenced by TEM and Raman spectroscopy. Together with the apparition of B4C crystallites in HT-B2.5C, two small peaks appeared in the PDF, as measured by neutron diffraction. By comparison with the data computed from the B4C model, we have shown that these distances unambiguously correspond to the C−B−C linear chains. This distinction in the PDFs of a-B2.5C and HT-B2.5C confirms that linear chains do not exist in the amorphous state. These results suggest that some carbon atoms from the amorphous phase likely gain enough mobility at elevated temperature to coalesce into free turbostratic carbon (possibly partially boron-substituted). Besides, the intericosahedral BC3 sites disappear in favor of the linear C−B−C chains typical of rhombohedral B4C.

Figure 12. Pair distribution function GPDF(r) of the HT-B2.5C coating as obtained from neutron diffraction. A comparison of the short distance part of the experimental function with the GPDF(r) curve calculated from the model B4C crystal (see Figure 1), is shown in the inset (the red arrows indicate two peaks, absent in the a-B2.5C material and characteristic of the C−B−C linear chains).

Well-defined peaks are clearly visible up to 2 nm, confirming the high order state of the material. Looking at the inset, where GPDF(r) is superimposed to the function computed from the cB4C model in Figure 1, HT-B2.5C appears as almost crystalline at short interatomic distances. It is difficult to assess whether the slightly broader peaks obtained in the neutron experiments are due to some structural disorder or to experimental broadening due to the finite range of measurable Q values. Of particular interest in these GPDF(r) functions are the two small peaks (indicated by the arrows in Figure 12) at 1.42 and 2.35 Å. These two peaks correspond respectively to the firstneighbor distance between boron and carbon atoms in the C− B−C chains (1.42 Å) and to the second-neighbor distance between an icosahedral (equatorial) boron atom and a middlechain boron atom (2.35 Å). These two peaks are thus clear signatures of the presence of linear C−B−C chains in the heattreated material. It is important to notice that none of them could be identified in the amorphous material, confirming the absence of linear chains in the as-prepared material.

S Supporting Information *

X-ray diffraction pattern of the as-prepared (a-B2.5C) and heattreated (HT-B2.5C) coating (1300 °C/2 h/Ar), in TIFF format. This material is available free of charge via the Internet at http://pubs.acs.org. J



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to CNRS and Herakles (Safran group) for providing a grant to C.P. They are grateful to Synchrotron SOLEIL for supporting the X-ray photoabsorption experiments on TEMPO (Proposal No. 20110047). They also acknowledge A. Delcamp (Herakles), for discussions and his constant interest in this work. Most of the calculations reported in this paper have been performed using the computer resources of the Mésocentre de Calcul Intensif en Aquitaine (MCIA). Prof. N. A. Marks is gratefully acknowledged by J.M.L. for useful discussions on the calculation of WFCs using CPMD. L.A.T. is grateful to Dr. T. Charpentier for providing the last version of fpNMR.

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Structure of an Amorphous Boron Carbide Film: An Experimental and

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