Hugoniot and release measurements in diamond shocked up to 26 Mbar

The equation of state (EOS) of carbon in its high-pressure solid and liquid phases is of interest to planetary astrophysics and inertial confinement fusion. Of particular interest are the high-pressure shock and release responses of diamond as these provide rigorous constraints on important paths through the EOS. This paper presents experimental Hugoniot and release data for both single-crystal diamond (SCD) and nanocrystalline diamond (NCD), which is comprised of nanometer-scale diamond grains and is ~5% less dense than SCD. We find that NCD has a stiffer Hugoniot than SCD that can be attributed to porosity. A Gr\"uneisen parameter of ~1 was derived from the data, which suggests increased coordination in the high-pressure fluid carbon compared to ambient diamond.