A new interatomic potential model for calcite; applications to lattice dynamics studies, phase transition, and isotope fractionation

Abstract

A microscopic model for calcite and aragonite has been developed that includes short range repulsive atom-atom interactions; two-, three-, and four-body potentials; and longrange Coulomb interactions. The structures and elastic constants of both phases are reproduced satisfactorily, as are the phonon frequencies for calcite. The observed softening of a transverse acoustic phonon in the [1,0,4] direction in calcite is reproduced by our model. Applications of the model include the calculation of reduced fractionation coefficients for 160/180 of calcite and isotope exchange coefficients of calcite and H20 that are in reasonably good agreement with experimental data. The lattice parameters of the hightemperature disordered phase of calcite have been calculated and are in good agreement with experimental data. It is inferred that the temperature dependence of the spontaneous strain accompanying the phase transition in calcite is due to short range order of the C03 groups, which also explains the anomalous negative thermal expansion along [100].