Interstitial oxide-ion conduction with unconventional polyhedral units in Bi_2-x Te_1+x O_5+x/2

Interstitial oxide-ion conductors are promising candidates for intermediate-temperature solid oxide fuel cells because of low migration barriers. However, such conductors with cation-centered polyhedral frameworks are relatively rigid and afford insufficient lattice free volume for the migration of interstitial oxygen. Herein, we report an interstitial oxide-ion conductor in layered Bi_1.9Te_1.1O_5.05, which exhibits a conductivity of 1.41 × 10^-3 S cm^-1 at 650 °C, comparable to those of advanced oxide-ion conductors. The average structure shows that stereochemically active lone-pair electrons of Bi³⁺ and Te⁴⁺ induce highly distorted, non-cation-centered coordination units, thereby generating sufficient lattice free volume. Neutron diffraction coupled with maximum entropy method analysis determines the location and concentration of interstitial oxygen atoms, which are stabilized through coordination with adjacent Te atoms. The incorporation of interstitial oxygen fundamentally alters the local Te environment, driving a transition from exclusively three-coordinated units to a diverse coexistence of three-, four-, and five-coordinated units. Concurrently, neighboring lattice oxygen atoms are displaced by interstitial oxygen, giving rise to pronounced local structural relaxation and enhanced lattice flexibility that are conducive to interstitial oxide-ion transport. This work provides new insights into the design of interstitial oxide-ion conductors beyond conventional polyhedral framework systems.