2-DIMENSIONAL PERIODIC ANTIPHASE BOUNDARIES IN A SYNTHETIC ALUMINATE SODALITE, CA-8[AL12O24](CRO4)(2)

Transmission electron microscopy (TEM) has provided direct evidence fo r ordering of [Ca-4 . CrO4](6+) clusters in a synthetic aluminate soda lite, Ca-8[Al12O24](CrO4)(2). The ordering of the above clusters cause s antiphase domains that are separated by two-dimensional periodic ant iphase boundaries (APBs). Such APBs are rare in minerals and inorganic solids, but they do occur in alloys. The APBs are at 90 degrees to ea ch other, parallel to {110} planes, and separated by about 54 Angstrom . At high temperature, the [Ca-4 . CrO4](6+) clusters in the aluminate sodalite (space group I (4) over bar 3m) are disordered, together wit h the framework oxygen atoms. As the temperature is lowered, the [Ca-4 . CrO4](6+) clusters are ordered and cause two-dimensional periodic A PBs that are probably accompanied by positional modulations of the fra mework oxygen atoms.