Nanotwinned Boron Suboxide (B₆O): New Ground State of B₆O

Nanotwinned structures in superhard ceramics rhombohedral boron suboxide (R-B₆O) have been examined using a combination of transmission electron microscopy (TEM) and quantum mechanics (QM). QM predicts negative relative energies to R-B₆O for various twinned R-B₆O (denoted as -B₆O, 2-B₆O, and 4-B₆O), consistent with the recently predicted B₆O structure with Cmcm space group (-B₆O) which has an energy 1.1 meV/B₆O lower than R-B₆O. We report here TEM observations of this -B₆O structure, confirming the QM predictions. QM studies under pure shear deformation and indentation conditions are used to determine the deformation mechanisms of the new -B₆O phase which are compared to R-B₆O and 2-B₆O. The lowest stress slip system of -B₆O is (010)/«001» which transforms -B₆O to R-B₆O 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.