Estimation of the upper limit of carbon concentration in boron carbide crystals

The existence of a boron carbide phase with similar to 25 at % carbon was proven experimentally. To evaluate the maximum possible concentration of C atoms in boron carbide (B(12 - x) C (x) )(BC(2)) crystals, we performed quantum-chemical calculations of (B(12 - x) C (x) )(BH(2))(6)(CH(3))(6) model compounds (x = 0-4; the goal of calculations was to determine the upper limiting number of C atoms in the B(12 - x) C (x) icosahedron) by the density functional theory method (B3LYP, 6-31G** basis set, full geometry optimization). A comparison of the experimental and calculated data showed that the calculations of the model compounds reproduced the experimental dependences of the structural parameters of the icosahedron (mean bond length and volume) on the number of C atoms in it. The icosahedra were found to be stable at x a parts per thousand currency sign 3. According to the results of the quantum-chemical calculations, the maximum carbon concentration in boron carbide was 33 at %, which corresponded to the composition B(10)C(5) and the structural formula (B(9)C(3))(BC(2)).