Unveiling the presence of mixed oxidation states of Europium in Li _7+δ Eu _x La _3−δ Zr _2−δ O _12−δ garnet and its impact on the Li‐ion conductivity

Abstract

Abstract Recently, lanthanides have been employed by researchers to examine their impact on the structure and properties of Li 7 La 3 Zr 2 O 12 (LLZO) garnets. In this regard, we developed Europium oxide (Eu 2 O 3 ) doped LLZO (Li 7+δ Eu x La 3−δ Zr 2−δ O 12−δ ) solid electrolyte which demonstrates a cubic phase with the symmetry of Ia d (No.230) at room temperature. In this investigation, different concentrations of Eu ranging from 0.1 to 0.6 atoms per formula unit (pfu) were doped into Li 7 La 3 Zr 2 O 12 to evaluate the impact of Eu on the stability of the cubic phase and thereby the ionic conductivity. The results unveiled that upon doping Eu 3+ ions, the Eu 2+ state is also formed and is then self‐doped into the structure in which Rietveld refinement coupled with XPS, EPR, and solid‐state NMR suggests that Eu 3+ ions most probably partially occupy Zr 4+ (16a) site, the Eu 2+ ions occupy La 3+ (24d) site, and the Li + ions occupy two different sites (24d and 96h). It was further found that such a site preference induces distortion at LaO 8 polyhedrons opening up the neck for Li‐ions diffusion, thereby enhancing the ionic conductivity. Moreover, it was revealed that Li‐ions probably hop from 96h to 24d and then to 96h site to generate the Li‐ion movement. Overall, by introducing Eu ions into the LLZO structure, an enhanced bulk ionic conductivity of 0.30 × 10 −3 S/cm at 298 K with a minimum electronic conductivity of 2.547 × 10 −9 S/cm at 298 K was achieved.