The mechanism of self-diffusion of ions in silicate systems

The special features of diffusion processes in noncrystalline models of SiO2, CaO · SiO2, and 2CaO · SiO2 with small admixtures of sodium oxide were studied by the method of molecular dynamics at 2000 and 3000 K. A purely ionic model with Born-Mayer potentials and parameters taken from the literature was used. In addition to the usual analysis of the mean-square displacements of particles, the maximum square displacements in each separate run 10000 time steps long were analyzed. Fairly long tunnels in the structure of silica were detected in the SiO2-Na2O system containing two sodium atoms only. Sodium ions can traverse these tunnels and execute a reciprocating motion in them at an approximately thermal velocity. The addition of calcium oxide gradually makes the silicate structure more compact, and long tunnels cease to exist in calcium meta-and orthosilicate models.