MOLECULAR DYNAMIC SIMULATIONS OF THE ALPHA-BETA PHASE-TRANSITION IN SILICA CRISTOBALITE

The phase transformation between the alpha- and beta-cristobalite modi fications of SiO2 was studied using molecular dynamic simulations. The transformation was induced in two ways: (a) thermally using constant pressure simulations, and (b) isothermally at room temperature by cont rolling the pressure within the structure through an externally applie d constant stress or by changing the simulation box volume. The atomic scale mechanisms of the transformation have been observed by means of a time-correlation function describing the spatial orientation of the planes that contain the Si-O-Si bonds. These planes undergo cooperati ve rotations by 90 degrees in the course of the transition. One can di stinguish two groups of bonds for which rotations occur independent of one another. Bonds belonging to both groups are aligned in the [110] directions. Both, the bulk modulus, calculated for static structures, as well as the vibrational spectra of the Si-O-Si planes reflect the s oftening of phonon modes in the midst of the transition, which is char acteristic of displacive phase transformations. Although the aperiodic shift of atomic positions upon passage of the transformation front co uld be held responsible for a momentary softening of the structure, th is is not the only reason, since the behavior persists when maintainin g the structure at intermediate densities, which correspond neither to alpha-, nor to beta-cristobalite. (C) 1998 Elsevier Science Ltd. All rights reserved.