New cation conducting solid electrolytes with the Sc2(WO4)3 type structure

Abstract

The Sc ₂ (WO ₄ ) ₃ type structure has been used in order to create new trivalent cation conducting solid electrolytes by forming solid solutions of the type (Sc ₂ (WO ₄ ) ₃ ) _{1– x} (M ₂ (MoO ₄ ) ₃ ) _x (M=Nd, Sm, Gd, Lu). In these compounds, the anionic part of the structure is partly replaced by molybdate (WO ₄ →MoO ₄ ) whereas the Sc ³⁺ cations are substituted by different larger lanthanide cations M (Sc→M). Increasing the size of M gradually leads to a restriction of the M ₂ (WO ₄ ) ₃ solubility in the scandium tungstate phase (e.g., x=0-1 for M=Lu but x=0-0.1 for M=Nd). The electrical properties of the resulting materials have been characterized in detail. All solid solutions exhibit neither electronic nor anionic O ^2– conduction but rather a mixed trivalent Sc ³⁺ /M ³⁺ cationic conduction with the Sc ³⁺ cations as the main charge carrying species for low M ₂ (MoO ₄ ) ₃ concentrations (x<0.5) and the ionic transference number is >99%. For a given substitution rate of M for Sc, the electrical conductivity increases with increasing size of M. The highest conduction data were observed for the solid solution (Sc ₂ (WO ₄ ) ₃ ) _0.75 (Sm ₂ (MoO ₄ ) ₃ ) _0.25 exhibiting a conductivity of 2.4×10 ^–4 S cm ^–1 at 600 °C with an activation energy of 45.8 kJ mol ^–1 . Furthermore, in all different systems the maximum conductivity appears for the same average trivalent cationic radius indicating an optimized spatial spacing for the mobile cationic species.