Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

Abstract

Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies.

Body-Centered OrthorhombicC16: A Novel Topological Node-Line Semimetal

“…It has long been known [1] that at ambient conditions graphite is the thermodynamically most stable carbon allotrope and that many structural transformations and modifications of carbon structures in various sp 2 and sp 3 bonding networks can be produced under various pressure and temperature conditions. Under high static pressure and high temperature conditions, graphite can be converted to cubic diamond [2][3][4][5][6] or twinned cubic diamond with {111} hexagonal-diamond-like stacking faults [5,6]; meanwhile, under cold static compression graphite can transform to diamond-like sp 3 carbon forms [7][8][9][10][11][12][13][14]. Additional cubic modifications of carbon have been produced during the heating of carbon soot or shock compression of polycrystalline graphite [15][16][17][18], which led to a proposed simple cubic carbon phase termed SC24 in P a3 symmetry [19] and a body-centered cubic carbon phase termed BC12 in Ia3d symmetry [20].…”

Effects of stacking disorder on thermal conductivity of cubic ice

“…They 86 observed the {113} twins and/or {111} stacking faults over a 2 nm scale by using ultra-high resolution electron microscope images. 86 Their findings seems intriguing because the density of Lonsdaleite is 3.2-3.52 g/cm 3 , [http://www.handbookofmineralogy.com/ pdfs/lonsdaleite.pdf], which is comparable to the measurable density of 3.514 g/cm 3 of (cubic) diamond. (Theoretical density of Lonsdaleite is 3.51 g/cm 3 , which is the same as the measured density of 3.514 g/cm 3 of cubic diamond.…”

“…It remains to be investigated if the requirement that two crystal lattices have the same density and nearly the same energy to form stacking faults can be fulfilled by some structures formed by carbon atoms alone. We also note a recent finding that twinning and/or stacking faults may be present 86 in meteoritic-impact shock wave-produced (and also laboratory-produced) hexagonal diamond known as Lonsdaleite. They 86 observed the {113} twins and/or {111} stacking faults over a 2 nm scale by using ultra-high resolution electron microscope images.…”

Is graphane the most stable carbon monohydride?

“…The energy difference between lonsdaleite and diamond is less than 10 meV per bond (it is significantly less than the 20 meV per atom difference between diamond and graphite), so for practical purposes, it can be neglected. There is even a tendency to regard lonsdaleite as a diamond structure, but with a considerable number of stacking faults [23]. In a more formal way, this procedure can be described as a virtual symmetry breaking operation so the equivalence of lattice nodes in diamond (space group F d3m, single Wyckoff position 8a) and lonsdaleite (P 6 3 /mmc, 4f ) structures aren't retained and their occupancies may vary 1 .…”