Solids with Two Mobile Ions: Proton H⁺ Self-Diffusion in Li-H Exchanged Garnet-Type Li₆La₃ZrTaO₇ as Seen by Solid-State ¹H NMR Relaxation

The development of ceramic proton conductors is currently attracting great attention, as they might be useful to construct new energy storage systems. Li₆La₃ZrTaO₁₂ (LLZTO) is known for its rapid Li⁺ diffusivity as has been directly revealed by ⁷Li NMR measurements. Exchanging parts of the highly mobile Li⁺ ions by protons through treatment of a single crystal in water or glacial acetic acid yields a mixed proton-lithium ionic conductor. Here, H⁺ proton diffusivity and Li⁺ diffusivity have separately been studied with element-specific ¹H and ⁷Li NMR spectroscopy. While long-range ⁷Li diffusion is noticeably slowed in Li-H exchanged LLZTO, we directly observe rather high H⁺ diffusivity, which is, however, significantly slower than Li⁺ dynamics. With the help of spin-lattice relaxation measurements we were able to measure local (and long-range) energy barriers (0.20(1) eV vs 0.45(3) eV) as well as the self-diffusion coefficient D_H of H⁺ dynamics (1.2 × 10^-15 m² s^-1 at 125 °C). These encouraging results are assumed to open new directories in designing ceramics offering fast transport pathways for protons.