Crystallization and Seeding Effect in BaAl ₂ Si ₂ O ₈ Gels

Two sol‐gel routes have been used to prepare celsian (BaAl 2 Si 2 O 8 ) precursors. The crystallization behaviors of unseeded gels and the gels seeded with 5 wt% feldspar, rutile, LiAISi 2 O 2 , and ZrO 2 crystals as well as SrAl 2 Si 2 O 8 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. SrAl 2 Si 2 O 8 gel and ZrO 2 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. LiAISi 3 O 8 seeds lowered the glass transition temperature of the immediately adjacent gel matrix and led to the crystallization of pure monoclinic celsian.