 BA-7Nb for Mu-20 is a high ion conductor with a unique structure consisting of two layers of cations and anions. The inner layer consists of oxygen vacancies, while the outer layer contains lithium and sodium ions. The researchers used a combination of techniques including resonant x-ray diffraction, solid state nuclear magnetic resonance, NMR, and first principles calculations to determine the chemical order and disorder of the solids. They found that the Mo-atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. This finding provides a basis for developing ion conductors with improved performance. This article was authored by Yuta Yatui, Masotaka Tansho, Kotaro Fuji, and others.