THE DETERMINATION OF RIGID-UNIT MODES AS POTENTIAL SOFT MODES FOR DISPLACIVE PHASE-TRANSITIONS IN FRAMEWORK CRYSTAL-STRUCTURES

This paper describes a computational method for the determination of a ll possible phonon modes in framework crystal structures that leave th e fundamental structural units (tetrahedra and octahedra) undistorted. Such rigid-unit modes (RUMs) are prime candidates as soft modes for d isplacive phase transitions, such as in the perovskite structure, and this computational method can be used to rationalize the phase transit ions in any framework structure. The method has been programmed for ge neral use. The RUM approach is illustrated by consideration of the per ovskite, quartz and cristobalite structures.