Disorder models of oxygen positions in P63/ mmc, C222~ and P212121 tridymites were given in applying geometrical and lattice dynamical calculations. Sixmembered rings of rigid SiO 4 units are all collapsed in these forms; with silicon atoms fixed, SiO 4 units can take six different orientations in forming tridymite frameworks in both the P63/mmc and C222~ forms, and three orientations in the P212121 form. Atomic distances and angles obtained from the distance least-squares method are about equal for the three forms: (Si-O) (mean Si-O)-l.611 A, (O-O)=2.629 A, and (Si-O-Si) = 147 ~ Domain formation models are given for the three forms.
The tridymite framework structures may possibly undergo lattice vibrations with low frequencies in two kinds of pair-wise rotational modes of SiO 4 units joined by the apical oxygen atoms, at the F-point: one is around (100) (or (210) for the hexagonal case), and the other is around (010). As temperature approaches the hexagonal-orthorhombic transition from below, the rotational mode around (100) remarkably softens at the F-point. The behavior of the atoms at the hexagonal-orthorhombic transition is explained in terms of a coupled softening of the two rotational modes of neighboring local domains in different orientations.