Mutual Phase Stability between Kaolinite-Derived Hexacelsian and Zirconia in a Mixed System

This study demonstrates the mutual phase stability and chemical inertness of hexacelsian─the metastable phase of BaAl₂Si₂O₈ with a layered structure─and yttria-stabilized zirconia (YSZ) across a wide temperature range up to 1600 °C, through solid-state reactions between kaolinite and BaCO₃, with or without YSZ. Hexacelsian remained without transformation to celsian─the stable BaAl₂Si₂O₈ phase without a layered structure─under heating when copresent with ZrO₂. Additionally, while YSZ alone shows both tetragonal and monoclinic ZrO₂ phases upon calcination, only the tetragonal phase is retained when calcined in the presence of hexacelsian. This mutual stability is supported by complementary analyses─including X-ray diffraction, scanning electron microscopy, thermal analysis, and solid-state magnetic resonance spectroscopy, which revealed the rapid formation of low-crystalline hexacelsian at 800 °C even in the presence of YSZ. These results offer insights into the design and functionality of thermally stable two-dimensional aluminosilicates in multiphase systems.