Measuring temperature and observing graphitization of heavy-ion-heated diamond.

The graphitization of diamond is of significant interest for nanodiamond synthesis from laser-shocked hydrocarbons, for its use as a detector material, and for the application in diamond anvil cells. The transition can be triggered either locally through sufficiently rapid energy deposition, or thermally at temperatures above 1800 K. We report on an experiment in which a heavy-ion beam was used to volumetrically heat monocrystalline diamond, investigating the intermediate regime between these two competing mechanisms. Our sample is probed using x-ray radiation generated from a laser-driven titanium plasma. The ratio of elastic to inelastic scattering in backward direction allows for measuring the samples' bulk temperature. We observe good agreement between the x-ray based technique and stopping power simulations up to ∼ 2000 K. When increasing the heating further by fully stopping the ions in the target, we reach conditions where graphitization is predicted to occur. In this regime, elastic x-ray scattering is notably stronger than expected for pristine, heated diamond. Simultaneous x-ray diffraction measurements show a modification of the previously sharp diamond peaks in this range, which is also accompanied by changes in the optical properties of the sample. A thermally induced transition to graphite provides the most plausible explanation of the observations.