  We explore the structural evolution of highly oriented pyrolytic graphite
(HOPG) under detonation-induced shock conditions using in-situ synchrotron
X-ray diffraction in the ns time scale. We observe the formation of hexagonal
diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with
recent gas gun experiments. First-principles density functional calculations
reveal that under uniaxial compression the energy barrier for the transition
towards hexagonal diamond is lower than cubic diamond. Finally, no indication
of cubic diamond formation was observed up to >70 GPa.
