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 was studied by comparing available exptl. data with predicted values for a large no. of a-quartz properties at variable temps. and pressures. Calcns. were done with a recently developed free-energy minimization code. Predictions for the structure, Raman modes, and elastic consts. of a-quartz at elevated temps. and pressures are discussed. Calcns. of the const. pressure entropy and heat capacity as a function of temp. are presented. Calcd. low-frequency phonon dispersion curves in the [x x 0], [x 0 0], and [0 0 x] direction are also discussed. Anal. of these predictions show which at. interactions are most accurately described by each potential. Also, the conditions are discussed under which the predictions of each potential are the most reliable