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

The Sc-2(WO4)(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(WO4)(3))(1-x)(M-2( MoO4)(3))(x) (M = Nd, Sm, Gd, Lu). In these compounds, the anionic part of the structure is partly replaced by molybdate (WO4 --> MoO4) whereas the SC3+ cations are substituted by different larger lanthanide cations M (S --> M). Increasing the size of M gradually leads to a restriction of the M-2(WO4), 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 O2- conduction but rather a mixed trivalent Sc3+/M3+ cationic conduction with the Sc3+ cations as the main charge carrying species for low M-2(MoO4)(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(WO4)3)(0.75) (Sm-2(MoO4)(3))(0.25) exhibiting a conductivity of 2.4 x 10(-4) S cm(-1) at 600 degrees 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.