DFT Study of the Role of Al3+ in the FastIon-Conductor Li7–3xAl3+xLa3Zr2O12 Garnet

摘要

We investigate theoretically the site occupancy of Al³⁺ in the fast-ion-conducting cubic-garnet Li7–3xAl³⁺xLa3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured ²⁷Al 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 ²⁷Al 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.