Synthesis of BaAl₂Si₂O₈from solid Ba–Al–Al₂O₃–SiO₂precursors: Part III. The structure of BaAl₂Si₂O₈formed by annealing at ≤650 °C and at 1650 °C

Analytical TEM and HREM have been used to examine the structure of BaAl₂Si₂O₈crystals produced within oxidized Ba–Al–Al₂O₃–SiO₂precursors upon annealing: (i) at ≤650 °C and (ii) up to 1650 °C. A BaAl₂Si₂O₈polymorph with ac-axis parameter of 15.6 Å was detected after annealing at ≤650 °C. Stacking faults and antiphase boundaries were detected within this polymorph after the 650 °C treatment. After a 15 h heat treatment at 1650 °C, convergent beam diffraction patterns and HREM confirmed that the predominant phase wasβ–hexacelsian. Although antiphase boundaries were absent in theβ–hexacelsian crystals, dislocations and stacking faults were detected after the 1650 °C anneal. The generation of defects in BaAl₂Si₂O₈crystals within specimens annealed at ≤650 °C and at 1650 °C is discussed in light of prior structural analyses.