Thermal parameters for α-quartz: a lattice-dynamical calculation

Using empirical atomic charges and valence force fields, the atomic thermal parameters have been calculated for α-quartz (SiO 2 ) at room temperature. These force fields have been derived from the best fit to the IR- and Raman-active vibrational frequencies, and phonon dispersion curves: in this process, together with quartz, some members of the olivine group, such as forsterite Mg 2 SiO 4 , monticellite CaMgSiO 4 , tephroite Mn 2 SiO 4 and fayalite Fe 2 SiO 4 have been considered. Agreement with the experimental values is good, and good agreement is also obtained for the estimates of thermodynamic functions such as entropy and molar heat. Similarly, as for other silicates and oxides, the zero-point contribution to the vibrational energy amounts to more than 80% at room temperature; the corresponding contribution to thermal parameters is about 30%