A COMPUTER-SIMULATION STUDY OF AL SI ORDERING IN GEHLENITE AND THE PARADOX OF THE LOW TRANSITION-TEMPERATURE/

The ordering of Al and Si in gehlenite is considered using computer si mulation. The enthalpy of ordering DELTAH per 2Al + 2Si atoms is found to be 0.52 eV. It is dominated by the nearest neighbour interaction, but the analysis is carried out to fifth neighbours. The nearest neigh bour interaction differs significantly from that for other materials. The structure does not have a connected network of ordering sites, whi ch mainly accounts for the unobservably low transition temperature for Al/Si ordering. Two alternatives are given for the likely ordering pa ttern.