Leucite (ideally K[AlSi₂O₆]) exhibits a complex sequence of structural transformations upon heating, the nature of which—particularly the existence of intermediate phases and the behaviour of the extra-framework potassium cation—has long been debated. In this study, highly accurate in-situ high-temperature powder X-ray diffraction (HT-PXRD) data were collected over the 303–1173 K range and analysed using the Rietveld method. The results support a continuous transition sequence from the low-temperature tetragonal I41/a phase to an intermediate tetragonal I41/acd modification, followed by the high-temperature cubic Ia \(\:\stackrel{-}{3}\) d phase. In the 873–898 K interval, the data are best described by the coexistence of two phases, namely I41/a + I41/acd at 873 K and I41/acd + Ia \(\:\stackrel{-}{3}\) d at 898 K. Given the long acquisition times and adequate thermal equilibration, the observed coexistence is best attributed to minor thermal gradients along the capillary rather than sluggish transition kinetics, providing a consistent explanation for previously reported lattice volume discontinuities. Furthermore, spontaneous strain analysis confirms that the ferroelastic transition is fully consistent, within experimental uncertainty, with a second-order Landau free energy expansion. While the intrinsic limitations of conventional PXRD preclude the refinement of Al/Si ordering, structural data indicate that the framework evolution is closely coupled to the extra-framework K cation. Bond-valence analysis indicates an apparent underbonding of K throughout the investigated range; however, this is consistent with pronounced dynamic disorder (rattling) within the framework cavities, prior to attaining a more regular 12-fold coordination in the cubic phase.
