DIRECT OBSERVATION OF PHASE-TRANSITIONS IN ALUMINATE SODALITE, CA-8[AL12O24](CRO4)(2)

Transmission electron microscopy (TEM) was used to study the structura l changes that occur in aluminate sodalite, Ca-8[Al12O24](CrO4)(2), wh en it is heated with the electron beam. Initially, a twinned orthorhom bic crystal was heated, and it transformed to tetragonal and then to c ubic symmetry. The cubic phase contains two-dimensional periodic antip hase boundaries (APBs) along the {110} planes. The APBs are about 54 A ngstrom apart. Structural modulations also occurred during the transfo rmations. The transition from tetragonal to orthorhombic symmetry is a lso associated with random transformation twins on a {110} plane; this plane is both the composition and twin plane.