Lattice dynamics of andalusite: Prediction and experiment

Abstract

Static energy minimization calculations have been used to obtain a relaxed structure of andalusite using a core-shell model with a set of transferable potential parameters. Subsequent lattice dynamics calculations have been used to calculate the eigenvalues and the eigenvectors of the dynamical matrix. The results of these calculations are compared with experimental data obtained by inelastic neutron scattering experiments and powder FTIR spectroscopy and previously published single crystal infrared and Raman data. The agreement of the calculated with observed phonon frequencies at the Γ point is satisfactory. The TO-LO splitting is modelled reasonably well. Coherent inelastic neutron scattering measurements have been made along one high symmetry direction up to energy transfers of 230 cm^-1. The model predicts phonon dispersion curves which are in good qualitative agreement with experimental data, but the calculated frequencies are consistently too low by about 10–15%. Macroscopic thermodynamic properties were calculated from the phonon density of states. The calculated specific heat is in excellent agreement with previously published data. The transferable potential parameters in the predictive model used in the present study give, within certain limitations, a realistic description of the static and dynamic aspects of the andalusite structure.