The longitudinal acoustic phonon dispersion of polycrystalline cobalt was determined by inelastic x-ray scattering up to 99 GPa, throughout the entire stability field of the hcp phase. The obtained aggregate compressional and shear sound velocities are compared with recent single crystal results, impulsive stimulated light scattering and ambient pressure ultrasonic measurements, as well as first principle calculations. We observe a linear evolution of the sound velocities with density up to 75 GPa. In this pressure range, the aggregate elastic properties of the polycrystalline sample are reproduced within 3% by a Voigt-Reuss-Hill average of the single crystal Cij. Above 75 GPa both aggregate velocities show a softening. Our comparative analysis of single-crystalline and polycrystalline results points towards a magnetic origin of the anomaly.
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