Simulation of modulated structures in quartz

We report the results of molecular-dynamics calculation of equilibrium modu lated phases of quartz in the vicinity of pressure induced alpha - beta tra nsition. A numerical method suitable for simulation of modulated phases in the vicinity of transition point was developed. The phase diagram with resp ect to the unit cell deformations was obtained from the pairwise interatomi c potentials by [Tsuneyuki et al.: Phys. Rev. Lett. 61 (1988) 869]. The res ults show that the modulated structures with various wavevectors appear bet ween alpha- and beta -phases suggesting that the softening may occur at any point on the Sigma line in the Brillouin zone and that a structure with la rge wavevector seems to be more probable than one with a small wavevector.