Atomic-Level Understanding of "Asymmetric Twins" in Boron Carbide
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.
© 2015 American Physical Society. Received 8 July 2015; published 20 October 2015. This research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement No. W911NF-12-2-0022. In addition, Q. A. and W. A. G. also received support from the Defense Advanced Research Projects Agency (W31P4Q-13-1-0010, program manager, Judah Goldwasser), and the National Science Foundation (DMR-1436985). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. "The U.S. Government retains and the publisher, by accepting the paper for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes."
Published - PhysRevLett.115.175501.pdf