BaTiWO: An Oxygen- and B-Site-Deficient 9R Hexagonal Perovskite Exhibiting Triple Conduction.

The hexagonal perovskite derivatives BaM'M″O featuring a hybrid structure composed of 9R hexagonal perovskite and palmierite structure motifs exhibit significant oxide ionic conductivity due to the highly disordered oxide-ion and M-cation sublattices. Herein, we report the structure and electrical properties of the perovskite BaTiWO. Three-dimensional (3D) electron diffraction (ED), neutron powder diffraction (NPD), and neutron pair distribution functions (nPDF) revealed a 9R hexagonal perovskite structure for BaTiWO with fully occupied central M2 sites, partially occupied outer M1 sites, and oxygen-deficient cubic c-BaO sublayers. These cation and oxygen arrangements differ significantly from those in BaM'M″O and enable BaTiWO to capture atmospheric water and O, resulting in triple conduction (oxide ion, proton, and hole) under wet air conditions. Proton and oxide ion conductions predominate at temperatures <400 and >650 °C in wet Ar and dry air, respectively. Bond-valence site energy calculations, together with structure analysis, deciphered that the two-dimensional oxide-ion diffusion pathways along the c-BaO layers are disrupted by the M1 vacancies, thereby resulting in relatively low oxide ionic conductivity. Our findings open up a new strategy of utilizing the cation's propensity of coordination geometry to design new oxygen- and B-site-deficient perovskites and thus achieve desired conductivity.