Molecular dynamics study of tridymite.

Structural changes in tridymite have been investigated by molecular dynamics simulation. Two thermal processes were carried out, one cooling from the high-temperature hexagonal structure of tridymite (HP-tridymite) and the other heating from the low-temperature monoclinic structure of tridymite (MX1-tridymite). The former process showed that HP, LHP (low-temperature hexagonal structure), OC (orthorhombic structure with 222 symmetry) and OP (orthorhombic structure with 222 symmetry)-like structures appeared in sequence. In contrast, the latter process showed that MX1, OP, OC, LHP and HP-like structures appeared in sequence. Detailed analysis of the calculated structures showed that the configuration underwent stepwise changes associated with several characteristic modes. First, the structure of HP-tridymite determined from diffraction experiments was identified as a time-averaged structure in a similar manner to β-cristobalite, thus indicating the important role of floppy modes of oxygen atoms at high temperature - one of the common features observed in silica crystals and glass. Secondly, the main structural changes were ascribed to a combination of distortion of the six-membered rings in the layers and misalignment between layers. We suggest that the slowing down of floppy oxygen movement invokes the multistage emergence of structures with lower symmetry on cooling. This study therefore not only reproduces the sequence of the main polymorphic transitions in tridymite, except for the appearance of the monoclinic phase, but also explains the microscopic dynamic structural changes in detail.