TWINNING IN AN ORTHORHOMBIC ALUMINATE SODALITE, CA-8[AL12O24](CRO4)(2)

Transmission electron microscopy (TEM) was used to study the aluminate sodalite, Ca-8[Al12O24](CrO4)(2). This aluminate sodalite has four po lymorphs: one cubic, one modulated, one tetragonal, and an orthorhombi c cell that arise from ordering of [Ca-4 . CrO4](6+) clusters that occ ur in the cages of the structure. The transition temperatures are 432, 453, and 610 K. The cell parameters for the orthorhombic phase, with the cubic cell parameter of a = b = c = 9.222 Angstrom, are 3a x 2b x c. Transformation twins are formed at 432 K during the transition from the tetragonal to the orthorhombic phase. The twin plane is {110} in the cubic cell, and {320} in the orthorhombic cell.