Shock-formed carbon materials with intergrown sp3- and sp2-bonded nanostructured units.

ABSTRACT

Studies of dense carbon materials formed by bolide impacts or produced by laboratory compression provide key information on the high-pressure behavior of carbon and for identifying and designing unique structures for technological applications. However, a major obstacle to studying and designing these materials is an incomplete understanding of their fundamental structures. Here, we report the remarkable structural diversity of cubic/hexagonally (c/h) stacked diamond and their association with diamond-graphite nanocomposites containing sp3-/sp2-bonding patterns, i.e., diaphites, from hard carbon materials formed by shock impact of graphite in the Canyon Diablo iron meteorite. We show evidence for a range of intergrowth types and nanostructures containing unusually short (0.31 nm) graphene spacings and demonstrate that previously neglected or misinterpreted Raman bands can be associated with diaphite structures. Our study provides a structural understanding of the material known as lonsdaleite, previously described as hexagonal diamond, and extends this understanding to other natural and synthetic ultrahard carbon phases. The unique three-dimensional carbon architectures encountered in shock-formed samples can place constraints on the pressure-temperature conditions experienced during an impact and provide exceptional opportunities to engineer the properties of carbon nanocomposite materials and phase assemblages.