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

Abstract

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 τ-B6O, 2τ-B6O, and 4τ-B6O), consistent with the recently predicted B6O structure with Cmcm space group (τ-B6O) which has an energy 1.1 meV/B6O lower than R-B6O. We report here TEM observations of this τ-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 τ-B6O phase which are compared to R-B6O and 2τ-B6O. The lowest stress slip system of τ-B6O is (010)/⟨001⟩ which transforms τ-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.

Boron oxides under pressure: Prediction of the hardest oxides

“…The enthalpy difference between these two structures is small (at ambient pressure it is only 1.8 meV per formula unit, and increases slightly with pressure). This prediction was published [12] and then our predicted Cmcm-B 6 O phase was experimentally confirmed [34].…”

Amorphous boron suboxide

“…Shear modulus of the B 6 O crystal is projected to be 227 GPa, agreeing with the available results of 207‐218 GPa in the literature. For the amorphous state the modulus is computed to be 86 GPa.…”

“…Therefore, the crystal shows more compact arrangement than the amorphous network. The K value for the crystal is projected to be 231 GPa, which reasonably agrees with the other results of 230‐232 GPa while it is computed to be ~106 GPa for the glassy B 6 O.…”

“…Form these three equations, the Vickers hardness is projected to be about 34 GPa (Teter), 43 GPa (Chen), and 41 GPa (Tian) for the crystal structure, comparable with the experimental predictions of 38‐45 GPa . For the amorphous phase, on the other hand, we compute the Vickers hardness to be 13 GPa (Teter), 18 GPa (Chen) and 17 GPa (Tian).…”

Prediction of a new ground state of superhard compound B6O at ambient conditions

“…1; phonon calculations have confirmed its dynamical stability. Transmission electron microscopy recently confirmed its existence15. Structural parameters and some of the physical properties of β-B 6 O are shown in Table 1, in comparison with α-B 6 O and two related forms of pure boron.…”