Phase transitions and structural evolution of leucite (K[AlSi₂O₆]) from in-situ high-temperature powder X-ray diffraction

Abstract 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 I 4 1 / a phase to an intermediate tetragonal I 4 1 / 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 I 4 1 / a + I 4 1 / acd at 873 K and I 4 1 / 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.