Structure of an amorphous boron carbide film: An experimental and computational approach

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 as-prepared material is essentially made of a random arrangement of icosahedra (B₁₂, B₁₁C, and B ₁₀C₂) 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.