Based on this analysis, the compositional limit of the garnet phase in Gd3+xAl5-xO12 lies close to x = 0.60. Structural evolution and detailed structural information were analyzed by Rietveld refinement against high-resolution SPD data for the nominal compositions x = 0, 0.20, 0.40 and 0.60. The starting models were based on the Gd3Al5O12 garnet structure for nonstoichiometric compositions (x = 0.20, 0.40 and 0.60). The refined models were based upon the Y3+xAl5-xO12 family , with excess Gd 3+ included by replacing a nominal amount of Al 3+ at the B sublattice. The Gd 3+ /Al 3+ occupancy ratio at this site was then allowed to refine freely. The lattice parameter was refined, in addition to the oxygen atomic coordinates and isotropic atomic displacement parameters for each site. Figure shows the refinement profile of Gd3.4Al4.6O12 (see Supporting Information S13-18 for x = 0, 0.2 and 0.6 and refined parameters for all samples). The occupancy by Gd 3+ of the B sublattice was found to increase linearly with x, consistent with the theoretical solid solution formula (Figure , upper panel). The Gd 3+ and Al 3+ respectively. These trends are consistent with those observed in the nonstoichiometric YAG system (Supporting Information Figure ).