ABSOLUTE IONIC-DIFFUSION IN MGO - COMPUTER CALCULATIONS VIA LATTICE-DYNAMICS

Computer calculations based upon lattice dynamics have enabled us to c alculate self-diffusion coefficients (D-sd) in MgO, a significant comp onent of the lower mantle. We have employed the supercell method to st udy the mechanisms governing the diffusion process thereby enabling us to calculate values for activation enthalpies and entropies for migra tion and formation. In the intrinsic regime we calculate Magnesium: D- sd = 2.20 x 10(-5) exp(-5.70/kT) Oxygen: D-sd = 1.24 x 10(-5) exp(-5.7 2/kT) and in the extrinsic regime we calculate Magnesium: D-sd = N(V)2 .84 x 10(-6) exp(-1.985/kT) Oxygen: D-sd = N(V)1.60 x 10(-6) exp(-2.00 3/kT) where N-v is the extrinsic defect concentration. We have also co nfirmed that for this system the diffusion path is not simple, but the re is a predicted bifurcation of the saddle surface. Our results are c omparable with previous embedded defect calculations, and suggest that current experimental data are unable to probe the intrinsic regime.