Atomic-Level Understanding of "Asymmetric Twins" in Boron Carbide
Kelvin Y. Xie, Qi An, M. Fatih Toksoy, James W. McCauley, Richard A. Haber, William A. Goddard III, Kevin J. Hemker
Abstract
Recent observations of planar defects in boron carbide have been shown to deviate from perfect mirror symmetry and are referred to as "asymmetric twins." Here, we demonstrate that these asymmetric twins are really phase boundaries that form in stoichiometric B_4C (i.e., B_(12)C_3) but not in B_(13)C_2. TEM observations and ab initio simulations have been coupled to show that these planar defects result from an interplay of stoichiometry, atomic positioning, icosahedral twinning, and structural hierarchy. The composition of icosahedra in B_4C is B_(11)C and translation of the carbon atom from a polar to equatorial site leads to a shift in bonding and a slight distortion of the lattice. No such distortion is observed in boron-rich B_(13)C_2 because the icosahedra do not contain carbon. Implications for tailoring boron carbide with stoichiometry and extrapolations to other hierarchical crystalline materials are discussed.
Group Members
Xie, K. Y., An, Q., Toksoy, M. F., McCauley, J. W., Haber, R. A., III, W. A. G., & Hemker, K. J. (2015). Atomic-Level Understanding of "Asymmetric Twins" in Boron Carbide. *Phys. Rev. Lett.*, *115*(17), Art. No. 175501. https://doi.org/10.1103/PhysRevLett.115.175501