Synthesis of BaAl2Si2O8 from solid Ba–Al–Al2O3–SiO2 precursors: Part III. The structure of BaAl2Si2O8 formed by annealing at ≤650 °C and at 1650 °C

Analytical TEM and HREM have been used to examine the structure of BaAl2Si2O8 crystals produced within oxidized Ba–Al–Al2O3–SiO2 precursors upon annealing: (i) at ≤650 °C and (ii) up to 1650 °C. A BaAl2Si2O8 polymorph with a c-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 was β–hexacelsian. Although antiphase boundaries were absent in the β–hexacelsian crystals, dislocations and stacking faults were detected after the 1650 °C anneal. The generation of defects in BaAl2Si2O8 crystals within specimens annealed at ≤650 °C and at 1650 °C is discussed in light of prior structural analyses.