Molecular dynamic simulations of the α-β phase transition in silica cristobalite

• Published in: The Journal of physics and chemistry of solids 1998-06, Vol.59 (6), p.1025-1037
• Main Authors: Duffréne, Lucas, Kieffer, John
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; elastic properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Lattice dynamics; oxides; phase transitions; phonons; Physics; Solid-solid transitions; Specific phase transitions; Statistical mechanics of lattice vibrations and displacive phase transitions; Structure of solids and liquids; crystallography
• ISSN: 0022-3697; 1879-2553
• DOI: 10.1016/S0022-3697(98)00016-X

The phase transformation between the α- and β-cristobalite modifications of SiO 2 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 controlling the pressure within the structure through an externally applied 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 cooperative rotations by 90 ° in the course of the transition. One can distinguish 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 softening of phonon modes in the midst of the transition, which is characteristic of displacive phase transformations. Although the aperiodic shift of atomic positions upon passage of the transformation front could be held responsible for a momentary softening of the structure, this is not the only reason, since the behavior persists when maintaining the structure at intermediate densities, which correspond neither to α-, nor to β-cristobalite.