Structural modulations in a pseudo-cubic aluminate sodalite, Ca8[Al12O24](CrO4)2

Abstract Complex satellite reflections in aluminate sodalite, Ca8[Al12O24](CrO4)2, arise from incommensurate modulations of the framework oxygen-atom positions in several directions, e.g. 〈001〉, 〈110〉, and 〈112〉; their periodicities differ with direction. The orientational ordering of the [Ca4 · CrO4]6+ clusters influences the positional ordering of the framework oxygen atoms. This cluster ordering accounts for the two well-defined framework oxygen-atom positions (O1 and O2) in nosean and lazurite, and a similar type of framework oxygen-atom ordering probably occurs in the cubic aluminate sodalite and produces the modulated structure. The clusters are disordered together with the framework oxygen atoms at higher temperature.