Twinning of cubic diamond explains reported nanodiamond polymorphs

Area of Science:

• Materials Science
• Mineral Physics
• Crystallography

Background:

• Unusual physical properties and formation conditions of nanodiamond polymorphs (h-, i-, m-, n-) have garnered significant interest.
• Identification of these polymorphs relies on diffraction features distinct from ordinary cubic (c-) diamond.

Purpose of the Study:

• To re-evaluate the nature of reported nanodiamond polymorphs using advanced microscopy.
• To determine if these polymorphs represent distinct structures or variations within cubic diamond.

Main Methods:

• Ultra-high-resolution transmission electron microscopy (HRTEM) of natural and synthetic nanodiamonds.
• Analysis of diffraction features and d-spacings.
• Investigation of crystallographic defects and twinning mechanisms.

Main Results:

• Diffraction features attributed to h-, i-, m-, and n-nanodiamonds are consistent with defects and specific twinning in cubic diamond.
• {113} reflection and <011> rotation twins accurately reproduce observed HRTEM images and d-spacings.
• Thickness effects in cubic diamond can explain features attributed to n-diamond.
• A new <121> rotational twin, producing dodecagonal symmetry, was identified.

Conclusions:

• The reported nanodiamond polymorphs are likely twinned cubic diamonds, not distinct phases.
• Twinning is widespread in diamond nanocrystals, influencing their properties.
• A high density of twins may significantly impact the applications of nanodiamonds.