Surface structure and lattice vibrations of MnO(0 0 1) analyzed by high-resolution medium energy ion scattering spectroscopy

Clean MnO(0 0 1)-1 × 1 surfaces were successfully prepared by cleavage in N2-ambience followed by annealing at 400 °C for 20 min in O2-pressure of 1 × 10−6 Torr. The rumpled relaxation and lattice dynamics of the MnO(0 0 1) surface was analyzed in situ by high-resolution medium energy ion scattering (MEIS) using 120 keV He+ ions. We determine the vertical displacements from the bulk lattice sites for the top- and 2nd-layer Mn and O atoms on the basis of the 3rd-layer Mn. It is found that the top-layer Mn atoms with a smaller polarizability are displaced toward the vacuum side relative to the top-layer O. This is consistent with the recent ab initio calculation but in conflict with the prediction based on the Coulombic interactions combined with a pair potential. Correlated thermal vibrations are also analyzed quantitatively by MEIS and the result obtained deviates pronouncedly from molecular dynamics simulations using the pair potential. The present result shows that the semi-classical treatment using pair potentials is no longer applicable to transition metal oxides with covalent bond and a fully quantum mechanical approach is required to calculate exact electron charge density distributions, which contribute to the force constant (corresponding to spring constant).