title: Nanocrystalline hexagonal diamond formed from glassy carbon.
abstract: Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by "locking in" favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.
authors.affiliation: Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia.
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Shiell
authors.affiliation: School of Science, RMIT University, Melbourne, VIC 3001, Australia.
authors.forename: Dougal G
authors.initials: DG
authors.lastname: McCulloch
authors.affiliation: Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia.
authors.forename: Jodie E
authors.initials: JE
authors.lastname: Bradby
authors.affiliation: Chemical and Engineering Materials Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
authors.forename: Bianca
authors.initials: B
authors.lastname: Haberl
authors.affiliation: Geophysical Laboratory, Carnegie Institute of Washington, 5251 Branch Rd., NW Washington, DC 20015, USA.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Boehler
authors.affiliation: School of Physics, The University of Sydney, NSW 2006, Australia.
authors.forename: David R
authors.initials: DR
authors.lastname: McKenzie