Segregation of Isovalent Impurity Cations at the Surfaces of MgO and CaO

Enthalpies of segregation for isovalent impurities in magnesium and calcium oxide as a function of surface concentration were calculated by using an atomistic computer simulation method. We have considered Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Ni²⁺, segregating to both (001) and (110) faces. The results obtained can be extrapolated to predict the behavior of other impurities including Mn²⁺, Fe²⁺, and Co²⁺, We find, for example, that Fe²⁺, Mn²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ will concentrate at the (001) surface of MgO, while Ni²⁺ will be depleted. The enthalpy of segregation is found to vary substantially with coverage particularly for the larger impurities. The enthalpy becomes less negative with increasing impurity concentration due to the increasing lattice strain until the surface is nearly saturated. Then additional stabilization is obtained by restructuring of the surface layer. We predict reconstructed surfaces for both the (001) and (110) faces, which contain a high concentration of a larger impurity ion. The enthalpy of segregation shows a maximum at around 50% surface coverage implying a bimodal surface distribution of segregant. The influence of segregation on surface energy suggests two unusual effects. The (001) surface energy of the impure crystal becomes negative for surface concentrations of impurity greater than 10% Ba²⁺ or 75% Sr²⁺ in MgO. This implies a thermodynamic barrier to sintering. At high coverages of Ba²⁺ in MgO the (110) surface becomes more stable than the (001) face suggesting that facetting may occur.