Measurement of elastic velocities of MgO under shock compression to 500 kilobars

The velocities of rarefaction waves in shock‐compressed MgO have been measured by observing the reduction of the shock front velocity near the sample edges due to the rarefaction waves propagating from the edges. The extent of this ‘edge effect’ is difficult to determine accurately because of its emergent nature. Arrangements sensitive to differences in shock front velocity yielded rarefaction wave velocities close to predicted longitudinal velocities in the high‐pressure shock state. Velocities closer to the hydrodynamic sound speed in the shock state were obtained from less sensitive arrangements. These results can be interpreted in terms of a two‐stage elastoplastic model of the decompression. The longitudinal velocities measured in shock states up to 528 kb imply second pressure derivatives of the elastic moduli c ij ″, given by K 0 c ij ″ = −1 ± 15, where K is the bulk modulus.