title: Twinning of cubic diamond explains reported nanodiamond polymorphs.
abstract: The unusual physical properties and formation conditions attributed to h-, i-, m-, and n-nanodiamond polymorphs has resulted in their receiving much attention in the materials and planetary science literature. Their identification is based on diffraction features that are absent in ordinary cubic (c-) diamond (space group: Fd-3m). We show, using ultra-high-resolution transmission electron microscope (HRTEM) images of natural and synthetic nanodiamonds, that the diffraction features attributed to the reported polymorphs are consistent with c-diamond containing abundant defects. Combinations of {113} reflection and <011> rotation twins produce HRTEM images and d-spacings that match those attributed to h-, i-, and m-diamond. The diagnostic features of n-diamond in TEM images can arise from thickness effects of c-diamonds. Our data and interpretations strongly suggest that the reported nanodiamond polymorphs are in fact twinned c-diamond. We also report a new type of twin (<121> rotational), which can give rise to grains with dodecagonal symmetry. Our results show that twins are widespread in diamond nanocrystals. A high density of twins could strongly influence their applications.
authors.affiliation: Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, H-1117 Budapest, Magyar Tudósok Körútja 2, Hungary.
authors.forename: Péter
authors.initials: P
authors.lastname: Németh
authors.affiliation: Center for Meteorite Studies, Arizona State University, Tempe, Arizona 85287-6004, USA.|School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, USA.
authors.forename: Laurence A J
authors.initials: LA
authors.lastname: Garvie
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, USA.|School of Molecular Sciences, Arizona State University, Tempe, AZ 85287-1604, USA.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Buseck