Melting temperature of diamond and cubic boron nitride at 15 gigapascals

The melting temperature of diamond has not yet been experimentally measured under static pressure and these data for cubic boron nitride (cBN) are scarce. This lack of information is understandable due to the difficulty of heating optically transparent diamond via absorption of intense laser radiation in a diamond anvil cell and obtaining a high temperature with traditional resistance heating methods in a large-volume, multiple-anvil apparatus. Here, we report the melting temperature of diamond (5968 +/- 457 K) and cBN (5689 +/- 411 K) at a pressure of 15 GPa based on a two-stage multiple-anvil apparatus coupled with a so-called flash-heating method and show that the previous theoretical predictions underestimate the melting points of these two superhard materials. Our results indicate that diamond has the highest melting point in nature, and strengthen our understanding of the melting behavior of those two superhard materials.