INTERATOMIC POTENTIALS FOR CACO3 POLYMORPHS (CALCITE AND ARAGONITE), FITTED TO ELASTIC AND VIBRATIONAL DATA

Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO3 groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges: z(O) = -0.995 e, z(C) = 0.985 e, z(Ca) = 2.0 e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1 % for calcite and of 0.2, 19.4, 2.5, 11.8 % for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO3 system, and is discussed and compared to other potentials.