Segregation isotherms at the surfaces of oxide

A non-Arrhenius segregation isotherm is derived which includes the change in the heat of segregation with surface coverage due to impurity-impurity interactions. It is shown that a linear dependence of log(xs) on the reciprocal temperature, where xs is the surface atomic ratio, can derive either from a constant heat of segregation, i.e., Arrhenius behavior, or from a heat of segregation that varies as xs-1. This isotherm is then used to calculate the equilibrium surface coverages of Ca at the {001} surface of MgO, Mg at the {0001} surface of α-Al2O3, Y at the {1012} and {1 bar 120} surfaces of α-Al2O3, and Na at the {111} and {110} surfaces of Li2O from the calculated heats of segregation. Where possible, comparisons are made with experiment. The more useful operational definition of the heat of segregation, namely, that derived from the measured coverage or that defined atomistically and obtained by calculation, is discussed