title: Atomic-Level Understanding of "Asymmetric Twins" in Boron Carbide.
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(4)C (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(4)C 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.
authors.affiliation: Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Kelvin Y
authors.initials: KY
authors.lastname: Xie
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Qi
authors.initials: Q
authors.lastname: An
authors.affiliation: Ceramic and Composite Materials Center, Rutgers University, New Brunswick, New Jersey 08854, USA.
authors.forename: M Fatih
authors.initials: MF
authors.lastname: Toksoy
authors.affiliation: Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.|U.S. Army Research Lab, Aberdeen Proving Ground, Maryland 21005, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: McCauley
authors.affiliation: Ceramic and Composite Materials Center, Rutgers University, New Brunswick, New Jersey 08854, USA.
authors.forename: Richard A
authors.initials: RA
authors.lastname: Haber
authors.affiliation: Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.affiliation: Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Hemker