Anomalies in Bulk Ion Transport in the Solid Solutions of Li(7)La(3)M(2)O(12) (M = Hf, Sn) and Li(5)La(3)Ta(2)O(12)

[Image: see text] Cubic Li(7)La(3)Zr(2)O(12)(LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ′(bulk). Here, we synthesized solid solutions of Li(7–x)La(3)M(2–x)Ta(x)O(12) with M = Hf, Sn over the full compositional range (x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li(7–x)La(3)M(2–x)Ta(x)O(12). The maximum in total conductivity for LLHTO (2 × 10(–4) S cm(–1)) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ′(bulk) turned out to be 7.94 × 10(–5) S cm(–1) (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer–Neldel’s compensation behavior showing that a decrease in E(a) is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li(7)La(3)Zr(2)O(12) as LLHTO is also anticipated to be highly stable against Li metal.