Abstract
We investigate La-site substitution in the high-entropy garnet Li(6)La(3)Zr(0.5)Nb(0.5)Ta(0.5)Hf(0.5)O(12) (LLZNTH) using Ba(2+), Sr(2+), and Sm(3+) to elucidate how dopant governs phase stability, Li-site distribution, and electrochemical behavior. X-ray diffraction shows that Sr(2+) is incorporated homogeneously into the garnet lattice, whereas the larger Ba(2+) and smaller Sm(3+) ions partially exceed the structural tolerance, generating secondary phases. Nevertheless, the Sm-doped composition (x = 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 (x = 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.