Hot-stage transmission electron microscopy study of phase transformations in hexacelsian (BaAl₂Si₂O₈)

Phase transformations in synthetic hexacelsian were investigated by hot-stage transmission electron microscopy. A second phase transformation from an orthorhombic to hexagonal structure was identified in the synthetic hexacelsian at approximately 700°C. The hexacelsian was found to exhibit a sequence of phase transformations on heating of hexagonal (P6₃/mcm)-orthorhombic (Immm)-hexagonal (P6/mmm). Antiphase domain boundaries, which were observed in P6₃/mcm and Immm phases, were absent in the P6/mmm phase. Crystal symmetries of the three phases were determined by convergent beam electron diffraction, and space group symmetries were derived by comparison of experimental selected area electron diffraction patterns with computer-simulated patterns.