Molecular dynamics study of the alpha-beta transition in quartz: elastic properties, finite size effects, and hysteresis in the local structure

The alpha-beta transition in quartz is investigated by molecular dynamics s imulations in the constant stress ensemble. Based on a frequently used two- body interaction potential for silica. it is found that anomalies in the el astic constants are at least in semiquantitative agreement with experiment despite the fact that no anomaly in the c/a ratio is observed in the simula tions. A finite-size scaling analysis shows that first-order Landau theory is applicable to the employed model potential surface. This statement also applies to the susceptibility below the transition temperature T-tr which h as not yet been measured experimentally. Examination of the local order nea r T-tr reveals that the deformation Of SiO4 tetrahedral units is equally la rge in the beta phase as in the alpha phase. However, large hysteresis effe cts can be observed in the local structure for distances r > 4 Angstrom. Th e results are in agreement with the picture of a first-order displacive pha se transformation which is driven by the motion of deformed tetrahedral SiO 4 units. Yet, the fast oscillations of oxygen atoms are around (time-depend ent) positions that do not correspond to the ideal oxygen positions in beta -quartz. The averaged configurations resemble the ideal structure only if averaged over at least a few nanoseconds.