Detonation-induced transformation of graphite to hexagonal diamond.*

The structural evolution of graphite under elevated thermodynamic conditions has been the subject of intense research interest. Early studies, back in 90s, clearly indicate the shock-induced transformation of graphite to a phase with much higher density, presumably a sp³ allotrope, above 20 GPa. However, only recently the capability of in-situ X-ray diffraction under shock conditions allowed the structural characterization of the relevant phases. Here 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. Moreover, we observe an extended pressure stability of the initial HOPG crystal structure up to ~50 GPa, in contrast with previous shock and static compression results.