Fig. 1

HRTEM imaging of the order–disorder phase transition in Cu_2−xSe HNPs. a Selected movie frames from continuous HRTEM imaging of an HNP undergoing a transition from the vacancy-ordered (VO) phase to the superionic (SI) phase. The time relative to the start of the movie is indicated for each frame. Before t = 30 s, there is nucleation of a cation-disordered region at the location labeled by the red arrow. The SI phase (marked by green outlines) grows across the HNP over time. b FFT of the VO region (top) and SI region (bottom) of the HNP, obtained from the t = 35 s frame shown in Fig. 1a. The VO phase shows characteristic vacancy super-lattice spots (d ~ 0.68 nm, marked by blue circles) in the FFT, which are absent for the SI phase. c HRTEM snapshots of an HNP before, around, and after the point of nucleation of the order–disorder transition. Corresponding FFTs are shown in Supplementary Fig. 3. This evolution is shown for three different HNPs (one per row). The nucleation site, where vacancy-ordering is first observed to be lost, is marked by a red arrow in each case. This site was identified on the basis of a difference in the local lattice fringe pattern in the before and after frames. It is found that nucleation occurs at a vertex site at the intersection of (\(\bar 1\)11)_c and (100)_c surface facets. These surface facets (labeled in the leftmost image of each row) were assigned on the basis of FFT analysis of the HRTEM images of individual HNPs, as described in Supplementary Fig. 4. Scale bars in images are 5 nm in length