Column-like La arrangement in (Li, La)NbO3 lithium ion conductor unveiled by x-ray fluorescence holography

We applied x-ray fluorescence holography (XFH) to a Li0.15La0.85/3NbO3 Li ion conductor to investigate the La3+ ionic arrangement. Thus far, this compound has been believed to have the A-site-deficient perovskite structure with the alternately arranged La-/Li-rich and La-/Li-empty layers (A1 and A2 layers, respectively) along the c axis. However, our XFH results revealed that A sites in the A2 layers are partially occupied by La3+ ions. From the detailed analysis of the obtained atomic images, it was indicated that the La3+ ions are not randomly distributed but form a column-like arrangement, where the La3+ ions in the A2 layers (La2 ions) are aligned in the c-axis direction and those in the A1 layers are aggregated near these La2 ions. We discuss the reason why the La3+ ions are located in the A2 layers in terms of crystal growth processes, and we also examine the structural origin of the enhanced ionic conductivity of the present sample.