BaAl2Si2O8 polymorphs and a novel reversible transition of BaAlF5 in supercooled oxyfluoride aluminosilicate liquids

Crystallization plays a critical role in realizing the functionalization of glass materials. However, it is imperative to understand the microscopic mechanism of both the formation and the phase transitions of nanocrystals during heat processes. Here, in the as-prepared SiO₂-Al₂O₃-BaF₂-Na₂O glass, we studied the polymorphic phase transition of BaAl₂Si₂O₈ during heating-cooling circles, and a novel reversible transformation is found by serendipity. Using differential scanning calorimetry (DSC), we observed an endothermic event around 1047 K during heating at 20 K/min to 1173 K, and an exotherm event at 1024 K during the subsequent cooling. Based on the X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), the reversible process was attributed to the transition between two morphs of BaAlF₅ nano-crystal. Na₂O introduced in the studied glass system promotes the transformation between hexacelsian (Hexagonal) and celsian (Monoclinic). The results also show the formation of different types of crystals, such as, BaF₂, Na₅Al₃F₁₄ and BaAlF₅ etc.