Lonsdaleite: The diamond with optimized bond lengths and enhanced hardness

TL;DR

This paper reports the synthesis of bulk lonsdaleite, a hexagonal diamond with optimized bond lengths, demonstrating superior hardness and mechanical properties compared to traditional cubic diamond.

Contribution

It introduces a novel synthesis method for bulk lonsdaleite with enhanced mechanical properties through controlled high pressure-temperature processes.

Findings

Lonsdaleite exhibits Vickers hardness of approximately 164 GPa.

Direct transformation pathways from graphite to lonsdaleite were observed.

Properties can be further improved by purifying starting materials and optimizing synthesis conditions.

Abstract

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.