Static lattice calculation of the structure and energy of calcium-doped magnesium oxide indicated that the surface should be heavily segregated with calcium in accord with experimental measurement. At moderate temperatures the enthalpy of segregation was sufficient to produce a monolayer coverage of impurity at equilibrium. The phonons for the present segregated surface exhibited an instability that was not found in the static calculation. A phonon of imaginary frequency over much of the Brillouin zone indicated that a lower energy structure of larger periodicity should exist. A self-consistent static-dynamic procedure was proposed that used the phonon results to suggest a restructuring that was calculated in a static calculation. The phonon calculation was then repeated. When a lowest energy static calculation gave surface phonons with no softening, it was probable that this was the predicted structure. In this case, a segregation-induced surface restructuring was predicted. The restructured cell was c(√2 x √2)R45° with half the oxygen ions pushed high out of the surface. The small long wavelength phonon anomaly which remained suggested that there might also be a longer range rumpling of surface cations.

Surface Phonons and Surface Reconstruction in Calcium Doped Magnesium Oxide. P.Masri, P.W.Tasker: Surface Science, 1985, 149[1], 209-25