Fast Oxide Ion Conduction in Ba₃MoNbO_8.5 Enhanced by Acceptor Substitution and Dehydration

Ba₃MoNbO_8.5 (BMN) exhibits high O^2- conductivity by disordered partially occupied O2 + O3 sites. Hydration decreases, and certain dopants increase the conductivity, and we present a novel model for defects and transport over two sites that rationalizes these behaviors. A broad experimental and computational study shows that hydration increases the activation energy of migration by immobile protons and fills the O2 + O3 plane. Acceptor substitution reverses this by emptying the O2 + O3 plane and spreading protons, and 5 at % Y substitution yields a bulk oxide ion conductivity exceeding 10 mS·cm^-1 at 650 °C and effectively suppresses the effect of humidity. The effects of hydration and acceptor substitution on the grain boundary (GB) resistance suggest a positively charged GB core by vacancy and proton accumulation with adjacent space charge regions with higher filling and activation energy and that doping and GB engineering may pave the way for operando utilization of Ba₃MoNbO_8.5-related oxide ion conductors.