Syntheses, structures, and ionic conductivities of perovskite-structured lithium–strontium–aluminum/gallium–tantalum-oxides

The ionic conductivities of new perovskite-structured lithium–strontium–aluminum/gallium–tantalum oxides were investigated. Solid solutions of the new perovskite oxides, (Li_xSr_1−x)(Al_(1−x)/2Ta_(1+x)/2)O₃ and (Li_xSr_1−x)(Ga_(1−x)/2Ta_(1+x)/2)O₃, were synthesized using a ball-milled-assisted solid-state method. The partial substitution of the smaller Ga⁺³ for Ta⁺⁵ resulted in new compositions, the structures of which were determined by neutron diffraction measurements using a cubic perovskite structural model with the Pm−3m space group. Vacancies were introduced into the Sr(Li) sites by the formation of solid solutions with compositions (Li_xSr_1−x−y☐_y)(Ga_{[(1−x)/2]−y}Ta_[(1+x)/2]+y)O₃, where the composition range of 0≤y≤0.20 was examined for x=0.2 and 0.25. The highest conductivity, 1.85×10⁻³ S cm⁻¹ at 250 °C, was obtained for (Li_0.25Sr_0.625☐_0.125)(Ga_0.25Ta_0.75)O₃ (x=0.25, y=0.125). Enhanced ionic conductivities were achieved by the introduction of vacancies at the A-sites. - Graphical abstract: Novel lithium-conducting oxides with the cubic perovskite structure (Li_xSr_1−x−y☐_y)(Ga_{[(1−x)/2]−y}Ta_[(1+x)/2]+y)O₃ provide a specific solid-solution region with various x and y values, exhibiting the highest ionic conductivity (1.85 S cm⁻¹ at 250 °C) for (Li_0.25Sr_0.625☐_0.125)(Ga_0.25Ta_0.75)O₃ (x=0.25, y=0.125 in (Li_xSr_1−x−y☐_y)(Ga_{[(1−x)/2]−y}Ta_[(1+x)/2]+y)O₃). The vacancies (☐) introduced into the A-sites contribute to the enhancement of lithium diffusion in the perovskite structure because of the enlargement of the bottleneck size and suppression of the interaction between lithium and oxygen. - Highlights: • The perovskite-structured novel Li–Sr–Al/Ga–Ta oxides were investigated. • The Ga cation offers a larger bottleneck by increasing the B−O bond length. • The greater conductivity was observed upon Ga-containing perovskite. • The ionic conductivity was improved by the introduction of vacancies into A-site