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 SiO 2 -Al 2 O 3 -BaF 2 -Na 2 O glass, we studied the polymorphic phase transition of BaAl 2 Si 2 O 8 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 BaAlF 5 nano-crystal. Na 2 O 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, BaF 2 , Na 5 Al 3 F 14 and BaAlF 5 etc.