The laboratory PXRD patterns of Gd3+xAl5-xO12 with nominal compositions 0 ≤ x ≤ 0.6, after crystallization, are shown in Figure . The patterns from 0 ≤ x ≤ 0.5 are phase-pure with a cubic garnet as the sole crystalline phase. The patterns of x = 0.55 and 0.60 are dominated by the same cubic garnet, but contain a set of weak Bragg peaks corresponding to a perovskite impurity phase (GdAlO3), with the x = 0.6 member containing 16.3(9) wt.% of this phase. An abrupt change was observed at x = 0.7, which yielded a complex multiphase mixture, indicating the end of the solid solution (see Supporting Information S3). Across the series, the lattice parameter of the garnet phase shows a clear evolution, with a systematic peak shift towards lower 2θ angle (see trend in the (420) peak position, expanded in Figure ) corresponding to a linear expansion of the unit cell in the range 0 ≤ x ≤ 0.60 (Figure ).