Nanotwinned Boron Suboxide (B_6O): New Ground State of B_6O

Abstract

Nanotwinned structures in superhard ceramics rhombohedral boron suboxide (R-B_6O) have been examined using a combination of transmission electron microscopy (TEM) and quantum mechanics (QM). QM predicts negative relative energies to R-B_6O for various twinned R-B_6O (denoted as τ-B_6O, 2τ-B_6O, and 4τ-B_6O), consistent with the recently predicted B_6O structure with Cmcm space group (τ-B_6O) which has an energy 1.1 meV/B_6O lower than R-B_6O. We report here TEM observations of this τ-B_6O structure, confirming the QM predictions. QM studies under pure shear deformation and indentation conditions are used to determine the deformation mechanisms of the new τ-B_6O phase which are compared to R-B_6O and 2τ-B_6O. The lowest stress slip system of τ-B_6O is (010)/⟨001⟩ which transforms τ-B_6O to R-B_6O under pure shear deformation. However, under indentation conditions, the lowest stress slip system changes to (001)/⟨110⟩, leading to icosahedra disintegration and hence amorphous band formation.

Additional Information

© 2016 American Chemical Society. Received: March 21, 2016. Revised: May 23, 2016. Published: June 2, 2016. This work was supported by the Defense Advanced Research Projects Agency (W31P4Q-13-1-0010 and W31P4Q1210008 program manager, John Paschkewitz), by the Army Research Laboratory under Cooperative Agreement Number W911NF-12-2-0022, and by the National Science Foundation (DMR-1436985, program manager, John Schlueter). The TEM experimental work was carried at WPI-AIMR, Tohoku University, Japan with research supported by the "World Premier International (WPI) Center Initiative Atoms, Molecules and Materials", MEXT, Japan. We thank Professor Takashi Goto of Institute for Materials Research, Tohoku University for providing the B_6O sample for experimental observations. The authors declare no competing financial interest.

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