Twinning in an orthorhombic aluminate sodalite, Ca8[A112024] (CI'O4)2

Transmission electron microscopy (TEM) was used to study the aluminate sodalite, Cas[Al12024](CrO4)2. This aluminate sodalite has four polymorphs: one cubic, one modulated, one tetragonal, and an orthorhombic cell that arise from ordering of [Ca,,.CrO4] 6§clusters that occur 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 A, 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. KE'CWORDS: aluminate sodalite, twinning, sodalite group, nosean, haiiyne, lazurite, TEM, superstructures