Ba₃Ti_0.4W_1.6O_8.6: An Oxygen- and B-Site-Deficient 9R Hexagonal Perovskite Exhibiting Triple Conduction

The hexagonal perovskite derivatives Ba₃M'M″O_8.5 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 Ba₃Ti_0.4W_1.6O_8.6. Three-dimensional (3D) electron diffraction (ED), neutron powder diffraction (NPD), and neutron pair distribution functions (nPDF) revealed a 9R hexagonal perovskite structure for Ba₃Ti_0.4W_1.6O_8.6 with fully occupied central M2 sites, partially occupied outer M1 sites, and oxygen-deficient cubic c-BaO_2.6 sublayers. These cation and oxygen arrangements differ significantly from those in Ba₃M'M″O_8.5 and enable Ba₃Ti_0.4W_1.6O_8.6 to capture atmospheric water and O₂, 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_2.6 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.