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.