x 1 garnet unit cell using FASTOSH 23 and FEFF8 to a limit of 5 Å. To support this analysis, structural models were generated from a 1 x 1 x 1 Gd3Al5O12 unit cell (160 atoms; P1) by substituting 0, 1, 2 or 3 Gd 3+ ions for Al 3+ at the B sublattice (corresponding to x = 0, 0.125, 0.25 or 0.375), and 0, 1 or 2 Tm (Yb) 3+ ions at the A and/or B sublattices (corresponding to dopant concentrations of 0, 4 or 8 %). For each composition, all symmetrically-inequivalent structural configurations were generated using Supercell. Due to the large number of inequivalent configurations for highly nonstoichiometric Tm(Yb)-doped compositions (e.g. 453 unique configurations for x = 0.25 with 8% Tm doping), a small number of representative configurations from each composition were selected for relaxation via plane-wave based DFT calculations using VASP. These used the PBEsol functional with the projector augmented wave method for the treatment of core electrons. Structures were optimized with a 600 eV plane-wave cutoff and a 5×5×5 k-point grid until forces fell below 10 -3 eV Å -1 .