ON THE ROLE OF AL-SI ORDERING IN THE CUBIC-TETRAGONAL PHASE-TRANSITION OF LEUCITE

The cubic-tetragonal phase transition in leucite has been examined usi ng static lattice energy calculations with realistic, tested interatom ic potentials. The calculations have demonstrated that the transition is not triggered by Al-Si ordering and can occur without any ordering. Any Al-Si order will have only a minor effect on the observed spontan eous strain and structure distortion. Calculations show that any long- range Al-Si order is associated with the energetics of next-nearest Al -O-Al linkages rather than nearest neighbor interactions, leading to a low ordering temperature.