Boron carbide under torsional deformation: evidence of the formation of chain vacancies in the plastic regime.

We report a combined experimental and theoretical study of boron carbide under stress/deformation. A special rotating anvil press, the rotating tomography Paris Edinburgh cell (RotoPEC), has been used to apply torsional deformation to boron carbide under a pressure of 5~GPa at ambient temperature. Subsequent damages and point defects have been analysed at ambient pressure by energy dispersive X-ray microdiffraction at the synchrotron and by Raman spectroscopy, combined with calculations based on the density functional theory (DFT). We show that apart from the signals due to B$_4$C, new peaks appear in both characterisation methods. The DFT calculations of atomic structures and phonon frequencies enable us to attribute most of the new peaks to boron vacancies in the intericosahedral chains of boron carbide. Some of the Raman spectra also show three peaks that have been attributed to amorphous boron carbide in the literature. Deformed boron carbide thus shows small inclusions of clusters of boron carbide with chain vacancies, and/or small zones interpreted as amorphous zones.