Ab initio study of defective chains in icosahedral boron carbide B4C

Two kinds of chain defects have been studied in icosahedral boron carbide, with the density functional theory: (i) The boron chain vacancy at the center of the chain, modeled by (B11Cp)C–V–C, is found to be metastable at low pressure, and to transform to (B11Cp) C–C with a discontinuous variation of volume at high pressure. (ii) The formation of a carbon–carbon bond in the defective chain, modeled by (B11Cp)C–C, is found to be metastable both at low and high pressure. The energy barrier for the formation of a carbon–carbon bond in presence of an interstitial boron atom is shown to be very high. [Display omitted] ►Monovacancies in icosahedral boron carbide are studied within the density functional theory. ► Vacancies are boron vacancies located at the center of triatomic chains. ► In defective chains carbon--carbon bonds are formed under pressure. ► This mechanism is identified as the reason for the loss of mechanical strength under extreme conditions. ► The hypothesis of a chain bending in boron carbide under pressure is discarded.