Protonic conduction, crystal and electronic structures of La0.9Ba1.1Ga0.95Mg0.05O4 − δ

•La0.9Ba1.1Ga0.95Mg0.05O4 − δ was successfully synthesized.•Simultaneous Ba and Mg substitutions in LaBaGaO4 could improve the conductivity.•Proton sites in La0.9Ba1.1Ga0.95Mg0.05O4 − δ could be determined experimentally.•Electron density distributions in LaBaGaO4-based materials were investigated.

We synthesize La0.9Ba1.1Ga0.95Mg0.05O4 − δ and then investigate electrical conduction properties, crystal structure, and nuclear and electron density distributions. From conductivity measurements, it is demonstrated that this sample exhibits higher conductivity than La0.9Ba1.1GaO4 − δ which is well-known as a protonic conductor. In addition, protonic conduction in La0.9Ba1.1Ga0.95Mg0.05O4 − δ is confirmed by an H/D isotope effect on the conductivity. In order to clarify proton site in the specimen, we measure neutron diffraction and then perform the Rietveld and maximum entropy method analyses. As a result, it is indicated that protons coordinate oxygens at O3 site and form hydrogen bonds. It is also found that electronic states around oxygens are affected by the Ba and Mg substitutions.