Free-energy calculations of thermodynamic, vibrational, elastic, and structural properties of alpha-quartz at variable pressures and temperatures

The reliability of two shell model potentials and a rigid-ion model potential has been investigated by comparing available experimental data with predicted values for a large number of cu-quartz properties at variable temperatures and pressures. Calculations were done with a recently developed free-energy minimization code. Predictions for the structure, Raman modes, and elastic constants of cr-quartz at elevated temperatures and pressures are discussed. Calculations of the constant pressure entropy and heat capacity as a function of temperature are presented. Calculated low-frequency phonon dispersion curves in the [xi xi 0], [xi 0 0], and [0 0 xi] direction are also discussed. Analysis of these predictions shows which atomic interactions are most accurately described by each potential. Furthermore, the conditions are discussed under which the predictions of each potential are the most reliable.