Phase Stability and Electrochemical Performance of La-Site-Doped Li ₆ La ₃ Zr _0.5 Nb _0.5 Ta _0.5 Hf _0.5 O ₁₂ High-Entropy Garnets

We investigate La-site substitution in the high-entropy garnet Li₆La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ (LLZNTH) using Ba²⁺, Sr²⁺, and Sm³⁺ to elucidate how dopant governs phase stability, Li-site distribution, and electrochemical behavior. X-ray diffraction shows that Sr²⁺ is incorporated homogeneously into the garnet lattice, whereas the larger Ba²⁺ and smaller Sm³⁺ ions partially exceed the structural tolerance, generating secondary phases. Nevertheless, the Sm-doped composition (<i>x</i> = 0.05) exhibits the highest room-temperature ionic conductivity (2.7 × 10^-4 S cm^-1). Neutron powder diffraction reveals that Sm substitution drives a redistribution of Li⁺ from the tetrahedral 24 d sites into the higher-mobility 96 h positions, enhancing the connectivity of the three-dimensional Li-ion migration network. A Sm-doping series (<i>x</i> = 0.01-0.05) further shows that only sufficiently high Sm levels induce this redistribution, whereas lower concentrations retain Li arrangements similar to the undoped garnet. Critical current density measurements demonstrate that La-site dopants also influence interfacial stability against Li metal, underscoring a trade-off between bulk transport enhancement and mechanical robustness. Collectively, these findings reveal that in high-entropy garnets improved ionic conductivity can originate not only from phase-pure structures but also from targeted modification of the Li sublattice, even when accompanied by secondary phases, offering a compositional design principle for garnet electrolytes.