Lonsdaleite: The diamond with optimized bond lengths and enhanced hardness.

Diamond is known as the hardest substance due to its ultra-strong tetrahedral sp3 carbon bonding framework. The only weak link is its cubic cleavage planes between (111) buckled honeycomb layers. Compressing graphite single crystals and heating to moderate temperatures, we synthesized a bulk, pure, hexagonal diamond (lonsdaleite) with distorted carbon tetrahedrons that shorten the bond between its hexagonal (001) buckled honeycomb layers, thus strengthening their linkage. We observed direct transformation of graphite (100) to lonsdaleite (002) and graphite (002) to lonsdaleite (100). We find the bulk lonsdaleite has superior mechanical properties of Vicker hardnesses HV = 164+/-11 GPa and 124+/-13 GPa, measured on the surface corresponding to the original graphite (001) and (100) surfaces, respectively. Properties of lonsdaleite as the supreme material can be further enhanced by purifying the starting material graphite carbon and fine-tuning the high pressure-temperature synthesis conditions.