High-pressure radial X-ray diffraction study of osmium to 58 GPa

Nonhydrostatic compression behavior of osmium (Os) was investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell. The apparent bulk modulus of Os derived from RXRD data varies from 262 GPa to 413 GPa, depending on Ψ, the orientation of the diffraction planes with respect to the loading axis. Fitting to the third-order Birch-Murnaghan equation of state, the RXRD data obtained at Ψ = 54.7° yields a bulk modulus K0 = 390 ± 6 GPa with pressure derivative K′0 fixed at 4. The ratio of differential stress to shear modulus t/G ranges from 0.024 to 0.029 at the pressures of 15.7–58.2 GPa. The yield strength was observed to increase with compression and reach the value of 11.7 GPa at the highest pressure. This confirms that Os is the strongest known pure metallic material compared with the reported stiff elemental metals such as W, Mo and Re. It was found that the apparent c/a ratio changed with the nonhydrostatic compression, as well as the orientation Ψ in our experiments. Moreover, the aggregate moduli of Os at high pressure were determined from the RXRD measurements.