DIAMOND AT HIGH RESOLUTION: COMPLEX STRUCTURES AND REPORTED POLYMORPHS

High-resolution, aberration-corrected, transmission electron microscopy has revealed interesting features regarding the nanostructure of diamond. Although diamond has been studied intensively, controversy exists over the types of polymorphs that exist, with several suggested as nanocrystals. Reported examples include h-, i-, m-, and n-polymorphs in addition to the ordinary cubic (Fd-3m) diamond. Collectively, these forms have received considerable attention in the materials and planetary science literature because they are thought to possess exceptional physical properties and in some cases indicate diagnostic formation conditions. Most attention has been devoted to lonsdaleite, also called hexagonal or h-diamond, which has been widely used as an indicator of ancient asteroidal impacts on Earth, including those linked to mass extinctions. However, pure crystals, even tiny ones, of lonsdaleite or the other purported polymorphs have neither been found nor synthesized. Rather than from direct observations, their identification has been based on diffraction features. Using aberration-corrected transmission microscopes, we find that images and diffraction features of both natural and synthetic examples of the various types contain abundant defects. A combination of {113} and <011> twinning produces HRTEM images and d-spacings that match those attributed to h-, i-, and m-diamond, and the diagnostic features of n-diamond in TEM images can arise from crystal-thickness effects. Our data and interpretations strongly suggest that the reported nanodiamond polymorphs are all actually Fd-3m diamond containing intimate twinning and nanometer-scale structural complexity. These results imply that twins are widespread in diamond nanocrystals, and the evidence for the reported polymorphs suggests they can be more readily explained as “common” Fd-3m diamond.