Substitution of silicon within the rhombohedral boron carbide (B4C) crystal lattice through high-energy ball-millingElectronic supplementary information (ESI) available: Fig. S1 depicts the structure of the lowest energy configuration of Si in B11Cp-CBC, Fig. S2 shows the XRD spectra of the as-received B4C after MeOH : HCl (95 : 5) washing and of the B4C:Si wherein the initial Si:B4C unit cell ratio is 1 : 1, and Fig. S3 shows the XRD spectrum of the as-received B4C sample after ball-milling und

Boron carbide (B 4 C) is a ceramic with a structure composed of B 12 or B 11 C icosahedra bonded to each other and to three (C and/or B)-atom chains. Despite its excellent hardness, B 4 C fails catastrophically under shock loading, but substituting other elements into lattice sites may change and possibly improve its mechanical properties. Density functional theory calculations of elemental inclusions in the most abundant polytypes of boron carbide, B 12 -CCC, B 12 -CBC, and B 11 C p -CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the chain center atom and that for non-metallic elements (N, P and S) it is generally the chain end atom of the three-atom chain in B 4 C's rhombohedral crystal lattice. However, Si, a semi-metal, seems to prefer the chain center in B 12 -CCC and icosahedral polar sites in both B 12 -CBC and B 11 C p -CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B 4 C at low temperatures (∼200-400 °C) by high-energy ball-milling. High-resolution transmission electron microscopy showed that the Si atoms were uniformly dispersed in the product, and the magnitude of the lattice expansion and Rietveld analysis of the X-ray diffraction data were analyzed to determine the likely sites of Si substitution in B 4 C. Further corroborative evidence was obtained from electron spin resonance spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy characterizations of the samples. Thus, a simple, top-down approach to manipulating the chemistry of B 4 C is presented with potential for generating materials with tailored properties for a broad range of applications. Substitution of silicon within the rhombohedral boron carbide (B 4 C) crystal lattice at moderate temperatures (∼200-400 °C) achieved through high-energy ball-milling.