A classical central force field model is applied to a boron carbide lattice assumed to consist of B11C icosahedra and C‐B‐C chains. The icosahedral carbon is positioned in a polar triangle and is puckered inward by ∼10%. Interaction constants are fitted to account for the observed Raman and IR spectral frequencies in terms of q=0 phonons; isotopic shifts are included as input data. Analysis is aided through consideration of a fictitious lattice with primitive cells consisting of B12 icosahedra and C‐B‐C chains; such a lattice retains D3d symmetry. We find the central force constant for the chain C‐B interaction to be 5.4 mdyne/Å. Intraicosahedral B‐B bonds have a force constant of only 0.7 mdyne/Å, much weaker than in other related borides.

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