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 po tential has been investigated by comparing available experimental data with predicted values for a large number of cu-quartz properties at v ariable temperatures and pressures. Calculations were done with a rece ntly developed free-energy minimization code. Predictions for the stru cture, Raman modes, and elastic constants of cr-quartz at elevated tem peratures and pressures are discussed. Calculations of the constant pr essure entropy and heat capacity as a function of temperature are pres ented. 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.