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

TL;DR

This paper predicts a new, more stable ground state of superhard B6O at ambient conditions, which could explain unexplained experimental Raman spectra and has potential for synthesis.

Contribution

The study identifies a new stable crystal structure of B6O (Cmcm-B6O) using quantum-mechanical methods, expanding understanding of its phase stability.

Findings

New stable ground state of B6O identified

Cmcm-B6O is more stable than the known structure

Potential for synthesis and explanation of Raman spectra

Abstract

Boron suboxide B6O, the hardest known oxide, has an R-3m crystal structure ({\alpha}-B6O) that can be described as an oxygen-intercalated structure of {\alpha}-boron, or, equivalently, as a cubic close packing of B12 icosahedra with two oxygen atoms occupying all octahedral voids in it. Here we show a new ground state of this compound at ambient conditions, Cmcm-B6O (\b{eta}-B6O), which in all quantum-mechanical treatments that we tested (GGA, LDA, and hybrid functional HSE06) comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilize it with respect to the known {\alpha}-B6O structure. \b{eta}-B6O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that \b{eta}-B6O is present in mixture with {\alpha}-B6O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.