Sol-gel-prepared pure and lithium-doped hexacelsian polymorphs: An infrared, Raman, and thermal expansion study of the beta-phase stabilization by frozen short-range disorder

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 i dentify 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 c w hexagonal phase exhibits a well-defined alpha-beta (HT) transition at abo ut 300 degreesC but a "statically disordered" beta (SD) form, which does no t vary very much with temperature and is very similar to the beta (HT) form obtained below 300 degreesC for some materials. This statically disordered phase is preferentially observed for fast-sintered lithium-free compositio ns and transforms gradually in the ordered form by annealing above 1100 deg reesC, Such stabilization of the high-temperature phase by the static disor der 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 previous ly observed cases. The thermal expansion behavior is very sensitive to the synthesis conditions, doping, and thermal treatments. The ordered a phase i s more easily achieved with lithium-doped compositions, but lithium additio n shifts the hexagonal-to-monoclinic phase transformation onset toward lowe r temperatures and promotes complete transformation with thermal annealing.