Phase transformation of BaAl2Si2O8 through metastable phase-growth in a kaolinite system containing minor impurities

Abstract

Hexacelsian, the metastable phase of BaAl₂Si₂O₈, is composed of stacked tetrahedral aluminosilicate layers; however, practical utilization of this layered structure is hindered by impurity-driven transformation to celsian, the stable phase of BaAl₂Si₂O₈ with a three-dimensional framework crystal structure. In this study, the hexacelsian-to-celsian transformation prepared from kaolinite containing relatively small amounts of by-products was examined by varying a pre-heating step to grow the hexacelsian phase prior to calcination at 1600 °C as the main heating step. X-ray diffraction patterns and solid-state ²⁷Al nuclear magnetic resonance spectra revealed the formation of poorly crystalline and then crystalline hexacelsian, accompanied by small amounts of BaAl₂O₄ and Ba₂SiO₄ originating from the by-products. Although defect signals were not detected by electron spin resonance, the presence of BaAl₂O₄ and Ba₂SiO₄, together with the sharpening and intensity changes of the ²⁷Al signals, were used as qualitative indicators of defect-related factors, which correlate with an accelerated transformation to celsian. Comparisons among specimens with nearly constant crystallinity and surface area confirm this correlation. Comparisons at nearly constant surface area show that increased crystallinity retards the transformation, whereas comparisons at similar crystallinity indicate that surface area still influences the transformation. Therefore, the results indicate that defect-related factors, crystallinity, and surface area dominate the transformation from hexacelsian to celsian, and their individual influences can be qualitatively distinguished through the present comparisons. Thus, this study provides practical guidelines for stabilizing hexacelsian in impurity-bearing raw materials and for exploiting metastable layered aluminosilicates in high-temperature ceramic applications.