In situ pressure Raman spectroscopy and mechanical stability of superhard boron suboxide

In situ Raman spectroscopy was used to explore the mechanical stability and electronic properties of superhard boron suboxide $(B6O)$ under nonhydrostatic pressure. $B6O$ stabilizes in a rhombohedral structure $(R3¯m)$ to the peak pressure of $90GPa$⁠. The icosahedra-chain (B–O) phonon at $274cm−1$ exhibits a negative pressure dependent profile of $−0.21cm−1∕GPa$⁠, while the intraicosahedral vibrations (B–B) between 400 and $1100cm−1$ and the intericosahedral phonon at $1141cm−1$ display 0.7–2.3 and $3.8cm−1∕GPa$⁠, respectively. The small pressure dependencies of intraicosahedral modes resulting from the crystal lattice are significant indications of the low compressibility of $B6O$⁠. Upon decompression to $3.3GPa$⁠, crystalline $B6O$ transforms to the amorphous boron oxide and glassy boron. The structure instability of superhard $B6O$ upon release of pressure implies its critical weakness for applications in harsh environments of dynamic impacts and high stress concentrations.