Using radial x-ray diffraction techniques together with lattice strain theory, the behavior of boron suboxide $\left({\mathrm{B}}_6\mathrm{O}\right)$ was investigated under nonhydrostatic compression to $65.3\mathrm{GPa}$ in a diamond-anvil cell. The apparent bulk modulus derived from nonhydrostatic compression data varies from $363\mathrm{GPa}\text{to}124\mathrm{GPa}$ depending on the orientation of the diffraction planes with respect to the loading axis. Measurement of the variation of lattice spacing with angle, $\psi$, from the loading axis allows the $d$ spacings corresponding to hydrostatic compression to be obtained. The hydrostatic $d$ spacing obtained from a linear fitting to data at 0° and 90° is consistent with direct measurements at the appropriate angle $(\psi =54.7°)$ to within 0.5%, which suggests that even two measurements ($\psi =0°$ and 90°) are sufficient for accurate hydrostatic equation of state determination. The hydrostatic compression data yield a bulk modulus $K_0=270\pm 12\mathrm{GPa}$ and its pressure derivative $K_0^{\prime }=1.8\pm 0.3$. The ratio of differential stress to shear modulus ranges from 0.021 to 0.095 at pressures of $9.3–65.3\mathrm{GPa}$. Together with estimates of the high-pressure shear modulus, a lower bound to the yield strength is $26–30\mathrm{GPa}$ at the highest pressure. The yield strength of ${\mathrm{B}}_6\mathrm{O}$ is about a factor of 2 larger than for other strong solids such as ${\mathrm{Al}}_2{\mathrm{O}}_3$. The ratio of yield stress to shear modulus derived from lattice strain theory is also consistent with the result obtained by the analysis of x-ray peak width. This ratio might be a good qualitative indicator of hardness as it reflects the contributions of both plastic and elastic deformation.

• Received 28 May 2004

DOI:https://doi.org/10.1103/PhysRevB.70.184121