Three series Gd3+xAl5-xO12, (Gd0.9Tm0.1)3+xAl5-xO12 and (Gd0.9Yb0.1)3+xAl5-xO12 were prepared with x = 0, 0. corresponding EXAFS fits and DFT-relaxed models are shown in Supporting Information S19-23. For the Tm 3+ and Yb 3+ doped series, two opposing scenarios are considered: Scenario 1, where Tm 3+ or Yb 3+ populate only the AO8 sites (displacing Gd 3+ to the BO6 sublattice when x > 0), and Scenario 2 which maximises the population of Tm 3+ or Yb 3+ at the BO6 sites (Figure ). These two scenarios give very different expected trends in (Tm, Yb)-O distance (Figure ), either negligible change with x (Scenario 1) or a strong decrease followed by a plateau at higher x values when the number of available B sites becomes greater than the number of (Tm, Yb) ions (Scenario 2). The corresponding experimental data show decreasing trends for both Tm-O and Yb-O distance with x, with a similar magnitude to that predicted for Scenario 2 (Figure ). This indicates that Tm and Yb both tend to occupy the BO6 sites in (Gd0.9RE0.1)3+xAl5-xO12 when x > 0. The weak variation of Gd-O distance with x in both scenarios is consistent with the undoped Gd3+xAl5-xO12 series, where the prevailing lattice parameter increase is countered by an increasing population of octahedral Gd 3+ (see Supporting Information S23).