Sol–Gel‐Prepared Pure and Lithium‐Doped Hexacelsian Polymorphs: An Infrared, Raman, and Thermal Expansion Study of the β‐Phase Stabilization by Frozen Short‐Range Disorder

Abstract

Monoclinic and hexagonal forms of celsians (BaAl2Si2O8) were synthesized by hydrolysis–polycondensation of Si/Al alkoxide mixtures. Dilatometry, Raman scattering, IR absorption, and X‐ray diffraction techniques were used to identify various hexacelsian phases and their conversion into the monoclinic phase as a function of thermal treatments and doping. Emphasis is given to the relationship between the shrinkage behavior and the static and dynamic short‐range disorders in the XO4 tetrahedra (X = Si/Al) arrangement. The α hexagonal phase exhibits a well‐defined α–βHT transition at about 300°C but a “statically disordered”βSD form, which does not vary very much with temperature and is very similar to the βHT form obtained below 300°C for some materials. This statically disordered phase is preferentially observed for fast‐sintered lithium‐free compositions and transforms gradually in the ordered form by annealing above 1100°C. Such stabilization of the high‐temperature phase by the static disorder arising from the peculiar synthesis through alkoxide hydrolysis and the gel route, which freeze the dynamic disorder of gel‐forming entities in a static disorder at the gelation step, is discussed in the light of previously observed cases. The thermal expansion behavior is very sensitive to the synthesis conditions, doping, and thermal treatments. The ordered α phase is more easily achieved with lithium‐doped compositions, but lithium addition shifts the hexagonal‐to‐monoclinic phase transformation onset toward lower temperatures and promotes complete transformation with thermal annealing.