We have measured the isothermal equation of state (EOS) of osmium to 75 GPa under hydrostatic conditions at room temperature using angle-dispersive x-ray diffraction. A least-squares fit of this data using a third-order Birch-Murnaghan EOS yields an isothermal bulk modulus $K_0=411\pm 6\mathrm{G}\mathrm{P}\mathrm{a}$, showing osmium is more compressible than diamond. Most importantly, we have documented an anomaly in the compressibility around 25 GPa associated with a discontinuity in the first pressure derivative of the $c/a$ ratio. This discontinuity plausibly arises from the collapse of the small hole-ellipsoid in the Fermi surface near the $L$ point.

• Received 4 December 2003
• Corrected 26 August 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.095502