Here, it can be seen that the Bragg peaks shift systematically on heating from 30 -1150°C due to thermal expansion, with garnet retained as the only crystalline phase. Between 1150 and 1200°C, two new sets of new Bragg peaks appear, indicating the onset of decomposition into perovskite GdAlO3 and stoichiometric garnet Gd3Al5O12, as shown in Figure and the upper panel of 4b. The lattice parameter evolution of the nonstoichiometric garnet phase from RT to 1600°C is shown in Figure (lower), demonstrating first a linear increase due to thermal expansion from room temperature up to ~1200°C, followed by an abrupt change in gradient associated with the onset of decomposition into Gd3Al5O12 and GdAlO3, which are the only observed crystalline phases at 1400°C. The decomposition onset temperature decreases slightly with increasing x, from 1200 -1250°C for x = 0.2, to 1050 -1100°C for x = 0.6. In contrast, the stoichiometric member (x = 0) is stable until 1350°C, before decomposing into GdAlO3 and Al2O3, consistent with previous ex situ studies.