Wide range lattice parameter control by aliovalent substitution to the rare-earth site in cubic garnet Li_6.75(La_1−xSm_x)₃Zr_1.75Ta_0.25O₁₂

Abstract

Garnet-type Li-ion conductor Li_6.75(La_1−xSm_x)₃Zr_1.75Ta_0.25O₁₂ powders were synthesized with 0 ≤ x ≤ 1 using solid-state reaction method. Almost single-phased garnet phase was obtained by synthesis at 900 °C for all Sm contents. The lattice parameters decrease linearly, concomitantly with increasing Sm content, following Vegard’s law. Results of X-ray diffraction and Raman spectroscopy show that Sm is substituted to the rare-earth element site and that the cubic structure of garnet is stable as a main phase at room temperature. Therefore, this study demonstrates that Sm substitution to La site in the garnet-type Li-ion conductor provides continuous lattice parameter control in a wide range up to a = 1.27170(8) nm. It is noteworthy that the wavenumber shifts of Raman mode related to the four-coordinated Li–O bond are observed only slightly in spite of the large lattice constriction and clear observation of peak-top shifts of other cation-oxygen modes, implying that Li-ion conductivity would be less affected by aliovalent substitution to the rare-earth site.