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 (<11> 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.
Highlights
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
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
Summary
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). Of particular interest are reports of a world-wide nanodiamond-rich layer associated with the onset of the Younger Dryas (YD) cooling episode at ~12,800 before present[29,30,31,32,33] These nanodiamonds are postulated to have formed from terrestrial carbon during an extraterrestrial impact, producing c-, h-, i-, and n-diamonds[33]. YD reports commonly use the presence of nanodiamond polymorphs as the impact signature
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