Site Occupation of Ga and Al in Stabilized Cubic Li_7–3(x+y)Ga_xAl_yLa₃Zr₂O₁₂ 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 carrie...