A COMPUTER-SIMULATION STUDY OF POINT-DEFECTS IN DIOPSIDE AND THE SELF-DIFFUSION OF MG AND CA BY A VACANCY MECHANISM

Computer simulation techniques have been used to investigate defect fo rmation and the diffusion of Ca and Mg in diopside. It was found that isolated, non-interacting CaO and MgO Schottky defects had the lowest formation energies (3.66 and 3.97 eV respectively); oxygen Frenkel def ects are the most favourable oxygen defects (formation energies 3.93 e V). Magnesium and calcium self-diffusion in the c-direction of diopsid e is easiest by a vacancy mechanism involving either direct jumps alon g the c-direction, or double jumps in the b-c plane. In the extrinsic regime, diffusion activation energies for Mg are predicted to be 9.82 eV (direct route) and 1.97 eV (double jump route); for Ca diffusion, a ctivation energies are predicted to be 6.62 eV (direct route) and 5.63 eV (double jump route). If additional vacancies (oxygen or magnesium) are present in the vicinity of the diffusion path, Ca migration energ ies fall to 1.97-2.59 eV. At elevated temperatures in the intrinsic re gime, diffusion activation energies of greater than or equal to 5.95 e V are predicted for Mg self-diffusion and 9.29-10.28 eV for Ca self-di ffusion. The values for Ca diffusion are comparable with published exp erimental data. It is inferred that a divacancy mechanism may operate in diopside crystals.