Crystallization and Seeding Effect in BaAl₂Si₂O₈ Gels

Two sol‐gel routes have been used to prepare celsian (BaAl₂Si₂O₈) precursors. The crystallization behaviors of unseeded gels and the gels seeded with 5 wt% feldspar, rutile, LiAISi₂O₂, and ZrO₂ crystals as well as SrAl₂Si₂O₈ gel have been studied using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric and differential thermal analyses (TGA and DTA). It was found that metastable hexacelsian, rather than the preferred monoclinic celsian, was the major crystallization product in the two unseeded gels. Seeding largely enhanced the crystallization of monoclinic celsian, but the mechanisms involved were different for seeding with different crystals. SrAl₂Si₂O₈ gel and ZrO₂ crystals acted as normal heterogeneous centers and had little catalytic effect on monoclinic celsian crystallization. Feldspar and rutile crystals worked as epitaxial substrates for monoclinic celsian and produced pure monoclinic celsian in some temperature regions. LiAISi₃O₈ seeds lowered the glass transition temperature of the immediately adjacent gel matrix and led to the crystallization of pure monoclinic celsian.