Abstract
Carbon exhibits a large number of allotropes and its phase behaviour is still subject to signifcant
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-defned 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 100GPa and 400°C. The nanocrystalline material was recovered at ambient and analysed
using difraction and high resolution electron microscopy. We propose that the transformation is the
result of intense radial plastic fow 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 frst principles calculations
of transformation pathways and explains why the new phase is found in an annular region. Our
fndings establish that high purity lonsdaleite is readily formed under strain and hence does not require
meteoritic impacts.
