(color online) (a) High resolution transmission electron microscope (HRTEM) image of NtD synthesized from carbon onions (2000 °C, 20 GPa), the insert pattern is a selected area electron diffraction (SAED) result. Twin boundaries (TB) are each marked by a long white line with two short lines either side; the two short lines indicate the direction of the lattice fringes. Twins are terminated at grain boundaries (GB). Stacking faults are found in the lattice, which are marked as SF and SFs. (b) The twin thickness distribution, as obtained by measuring the distance of two long white lines. A total of 823 twins were measured to acquire this twin thickness distribution. (c) Vickers hardness of NtD and NPD.^[20]