Phase transitions and associated domains in hexacelsian (BaAl2Si2O8)

Abstract

Phase transitions and associated domains of BaAl2Si2O8-hexacelsian were studied by means of transmission electron microscopy combined with energy-dispersive X-ray microanalysis. Two polymorphs were found: a hexagonal with space group P63/mcm and a pseudohexagonal orthorhombic one with space group Immm. P63/mcm-hexacelsian displays antiphase domains with the displacement vector 1/2 c and Immm-hexacelsian contains twin domains with the three-fold twin axis parallel z and antiphase domains with the displacement vector 1/2 c. When P63/mcm-hexacelsian is heated above ∼ 300°C it inverts to Immm-hexacelsian. The reversible and rapid transition and the associated formation of domains was studied by in situ experiments using a heating holder. P63/mcm and Immm are not connected among each other by a group-subgroup relationship, but both are subgroups of P6/mmm. The occurrence of twin and/or antiphase domains respectively in the polymorphs agrees with the symmetry reductions.