Acceptor Doping and Local Structure in Ba7nb4moo20 Investigated by 93nb Nmr Spectroscopy

Ba7Nb4MoO20 (BNM) with a layered hexagonal perovskite-type structure is a new type of ionic conductor and a candidate electrolyte for intermediate temperature SOFC due to its high oxide-ion and proton conductivity at 300-600°C. This conductivity originates from disordered structures in the ionic conduction layer containing structurally intrinsic oxygen vacancies. Recently, cation doping acting as the doner has been reported to improve its oxide-ion conductivity. However, acceptor doping and its influence on the local structure of the ionic conduction layer remains unclear. This study aimed to clarify the local structure and the effect of Ti doping on the conductivity of BNM. In the 93Nb NMR spectrum of Ti-doped BNM, a new peak appears on the right-hand side of the 93Nb peak assigned to 4-coordinated Nb, indicating Ti substitutes the Nb sites in the ionic conduction layer. While electrical conductivity decreases with Ti substitution, the contribution of proton conduction increases with the substitution, which is consistent with an increase in proton concentration in the Ti-doped sample determined by TGA. These results indicate that Ti substitution positively affects proton incorporation and conduction; however, it negatively impacts the stability of the BNM phase and the total conductivity.