DFT Study of the Role of Al3+ in the Fast Ion-Conductor Li7-3xAlx3+La3Zr2O12 Garnet

We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7-3xAlx3+La3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured Al-27 NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad Al-27 NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.