THERMAL PARAMETERS FOR ALPHA-QUARTZ - A LATTICE-DYNAMICAL CALCULATION

Using empirical atomic charges and valence force fields, the atomic th ermal parameters have been calculated for alpha-quartz (SiO2) at room temperature. These force fields have been derived from the best fit to the IR- and Raman-active vibrational frequencies, and phonon dispersi on curves: in this process, together with quartz, some members of the olivine group, such as forsterite Mg2SiO4, monticellite CaMgSiO4, teph roite Mn2SiO4 and fayalite Fe2SiO4 have been considered. Agreement wit h the experimental values is good, and good agreement is also obtained for the estimates of thermodynamic functions such as entropy and mola r heat. Similarly, as for other silicates and oxides, the zero-point c ontribution to the vibrational energy amounts to more than 80% at room temperature; the corresponding contribution to thermal parameters is about 30%.