Phase Stability of Hexacelsian Initially Formed as Low-Crystalline Spherical Particles

Hexacelsian, a layered polymorph of BaAl2Si2O8, is attractive for high-temperature applications but eventually transforms into celsian, the stable polymorph without a layered structure. In this study, spherical BaAl2Si2O8 precursors were prepared by spray pyrolysis. The as-prepared precursor was identified as low-crystalline hexacelsian with a spherical morphology. Upon heat treatment, crystalline hexacelsian formed exclusively over a wide temperature range. Notably, hexacelsian persisted even after heat treatment at 1300 °C for 136 h and 1500 °C for 181 h, conditions under which celsian formation has been reported for clay-derived and conventionally mixed precursors. X-ray diffraction, solid-state nuclear magnetic resonance, electron diffraction, and Rietveld refinement showed that the specimens calcined at 1000 °C consisted of mixed α- and β-hexacelsian phases, whereas the calcined specimen at 1500 °C was assigned to the β-phase. Mechanistic investigations indicate that the overall composition, together with the mixing state of Ba, Al, and Si achieved by spray pyrolysis, contributes to the stabilization of hexacelsian. Furthermore, compact specimens prepared from the spherical precursor exhibited reduced preferred orientation of platy hexacelsian grains compared with those derived from platy clay-based precursors. Thus, spray-pyrolyzed spherical precursors offer a useful route for improving the phase stability and processing characteristics of hexacelsian.