Site Occupation of Ga and Al in Stabilized Cubic Li7–3(x+y)GaxAlyLa3Zr2O12 Garnets As Deduced from 27Al and 71Ga MAS NMR at Ultrahigh Magnetic Fields

Li-containing garnets, which are stabilized in their cubic modification by doping with Al or Ga, show very high Li-ion conductivities. This property qualifies them to be used as solid electrolytes in advanced all-solid-state batteries. The relation between local structures and dynamic properties, however, is still not fully understood. Here, cubic mixed-doped Li7–3(x+y)GaxAlyLa3Zr2O12 garnet solid solutions with different portions of Al and Ga were synthesized. It turned out that the solubility of Ga is higher than that of Al; the evaluation of 42 different doping compositions indicated an increase of the lattice parameter a0 with increasing Ga content. 71Ga MAS NMR spectra recorded at 21.1 T revealed two 71Ga NMR resonances, corresponding to Ga occupying both the 24d (243 ppm) and 96h sites (193 ppm). This behavior, which has been observed for the first time in this study, is very similar to that of Al. The 71Ga NMR line at 193 ppm observed here remained invisible in previous NMR studies that were carried out at lower magnetic fields. The invisibility at lower field is because of large second-order quadrupolar broadening that has a lower effect on the 71Ga NMR spectra at higher magnetic field. Most importantly, the similarity in site preference of Al and Ga found here inevitably raises a question about the significance of a blocking effect on long-range Li-ion transport. It weakens the assumption that the site preference of dopants is responsible for the higher Li diffusivity of Ga-doped samples compared to the Al-doped analogues. Concerning Li-ion dynamics, our 7Li NMR line shape measurements indicate that the change in lattice constant a0 with increasing doping level seems to have a larger influence on Li-ion dynamics than the Al:Ga ratio.