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

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.