Crystal structures, Raman spectra and atomic displacements in monoclinic tridymite and its higher temperature orthorhombic phase

The crystal structures of monoclinic tridymite and its higher temperature orthorhombic modification were studied in single-crystal X-ray structure-refinements with newly measured intensity data at five points of temperature. The temperature dependences of unit-cell dimensions, DTA curves and Raman spectra were also studied. Structural parameters involving unit-cell dimensions, atomic coordinates and temperature factors change only slightly and almost linearly in the monoclinic phase up to the monoclinic-orthorhombic transition point at about 110°C, and no intermediate phase was observed between these two phases, whose existence has been believed in some previous studies. With the newly measured intensity data for the orthorhombic phase, atomic parameters involving anisotropic temperature factors of atoms were fully determined. The atomic mean square displacements are highly anisotropic and large particularly for oxygen atoms, and interpreted in terms of coupling of librational motions of SiO4 and disordering motions.Frequencies and atomic displacement patterns in optic phonons at the zone center of monoclinic tridymite and its higher temperature orthorhombic modification were successfully calculated for the ordered structures re-refined or re-determined in a recent study. Atomic displacements in selected modes are described in details for the two phases. Disorder in the orthorhombic structure, favored in previous NMR and X-ray studies, is not of much effective in Raman scattering, and an interpretation is presented for the orthorhombic phase in terms of coupling between disordering motions and phonon modes.