Direct imaging of layered O3- and P2-Na x Fe 1/2 Mn 1/2 O ₂ structures at the atomic scale

Using aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques, the atomic-scale structures of the O3 and P2 phases of NaxFe1/2Mn1/2O2 are investigated systematically. The Na, transition metal M (Fe and Mn) and O columns are well revealed and precisely assigned to the O3 and P2 phase layered structures. The O3 phase sample demonstrates larger atomic site fluctuations along [001] direction but with less structural imperfections (e.g. interlayer structure and stacking defaults) than the P2 phase sample. Furthermore, a clear surface with a regular structure is observed for the O3-NaFe1/2Mn1/2O2 sample, while a surface with a large amount of Na-M antisites is observed for the P2-Na2/3Fe1/2Mn1/2O2 sample.