What the nanosecond shock measured
In the peer-reviewed Nature Physics experiment, microcrystalline diamond was driven by a nanosecond shock at the OMEGA Laser Facility. Optical velocimetry, pyrometry and X-ray diffraction were read together as the sample crossed the solid-to-liquid boundary. The melting temperature sits near 7,300 K and falls slightly as pressure rises, while the diamond structure itself persists up to 1 TPa.[1]
That temperature sits more than 700 degrees Celsius away from earlier experimental estimates of the same boundary. The new curve is offered as a benchmark for inertial confinement fusion capsules and for interior models of ice giant planets, where carbon is thought to freeze and fall.[1]
The phase that this path does not show
Density-functional theory has long placed the BC8 phase of carbon as the thermodynamically stable solid above pressures around 1 TPa. Along this Hugoniot the X-ray diffraction intensity falls as the sample melts, and the diamond reflections remain the ones that are seen. A previously reported BC8 transition is not recovered here.[1]
The observation licenses a narrower claim than a global ban on BC8: on a single nanosecond shock, carbon stays diamond until melt. A rival reading is that a thin BC8 layer formed and stayed below the diffraction threshold. Either way, the search for that phase has to move to a different compression path rather than rest on this one.[1]
The next compression path
If a later X-ray diffraction series on carbon, driven by more than one shock at or above 1 TPa, is published from NIF, the National Ignition Facility, a BC8 reflection before melt would reopen this path; an empty interval there would leave the single-shock constraint in place.[1]