The thermal stability of Gd3+xAl5-xO12 was evaluated by in situ PXRD, for compositions x = 0, 0.2, 0.4 and 0.6. The nonstoichiometric members (x > 0) showed similar decomposition behaviors, exemplified by the x = 0.4 member in Figure (x = 0.2 and 0.6 members are shown in Supporting Information S10-11).