Crystal chemistry of "Li7La3Zr2O12" garnet doped with Al, Ga, and Fe: a short review on local structures as revealed by NMR and Mossbauer spectroscopy studies

Cubic Li7La3Zr2O12 (LLZO) garnets stabilized by substitution of Li by supervalent cations (Al3+, Ga3+ and Fe3+) are exceptionally well suited to be used as protecting layer to enable Li-metal based battery concepts. On the one hand this dopants are needed to provide the outstanding properties of LLZO at room temperature (RT), but on the other hand dopants occupying Li sites are suspected to hinder the long-range Li-ion transport properties within the structure. This depends on the type of dopant species and their amount in the LLZO garnet. In particular, the way these dopants can be distributed in the garnet structure is thought to play a critical role in the Li-diffusion behaviour. This short review addresses the difficulty to obtain structural information on minor amounts of cations in a large complicated structure such as LLZO by diffraction methods and the advantages of the application of complementary spectroscopic methods, such as Mossbauer and NMR spectroscopy, which provide information on the valence state and the distribution of the dopants Al, Ga, and Fe over the possible cation positions of the garnet structure. Finally, (i) NMR spectroscopy at very high magnetic fields (21.1 T) shows that Al and Ga are similarly distributed over the 24d and 96h sites in the garnet structure and (ii) Mossbauer spectroscopy proves that Fe occurs in the trivalent state, also at the 24d and 96h sites of the cubic garnet framework. The solubility limit of Fe, Al, and Ga is up to 0.25 pfu, 0.39 pfu, and 0.72 pfu, respectively.