Diamond’s melting behavior at Neptune-level pressures reconciles 20-year theory-experiment gap

Diamond’s response to pressures exceeding those inside Neptune and Uranus has been a persistent puzzle. Earlier experiments and simulations diverged by roughly 20% on melting temperatures, while indirect evidence hinted at an intermediate crystalline phase that no experiment could directly observe. The new laser-driven compression work closes both gaps by capturing X-ray diffraction data during the brief, extreme conditions. This alignment between measurement and theory removes a key uncertainty for materials under such pressures.

The practical stakes are highest in inertial-confinement fusion. Diamond capsules hold fusion fuel, and their behavior under compression dictates energy yield. The newly confirmed phase diagram suggests that capsule performance could be optimized by tuning pressure and temperature to exploit diamond’s denser liquid phase. This could triple energy gain in current designs, though the exact engineering path remains untested at scale.

For planetary science, the results sharpen models of ice giants. Diamond rain, long theorized to form in Neptune and Uranus, depends on how carbon behaves at high pressures. The experiments show diamond floating in liquid carbon, a detail that may explain the distribution of carbon in these planets’ interiors. However, the findings stop short of proving diamond rain exists; they only provide the material properties needed to model it.

The experimental technique itself is a constraint. Laser-driven compression lasts mere nanoseconds, limiting the time window for measurements. While X-ray diffraction captured atomic structure, other properties like electrical conductivity remain unmeasured. Future work will need to extend observation times or develop new diagnostics to fill these gaps, especially for applications requiring sustained high-pressure conditions.