High temperature point defect equilibria in iron‐doped MgO: An in situ Fe‐K XAFS study on the valence and site distribution of iron in (Mg1‐xFex)O

Abstract

AbstractThe point defect structure of a doped (oxide) system was quantitatively studied by applying in situ X‐ray absorption spectroscopy in the vicinity of the absorption edge of the dopant at high temperature (T>270 K) and under defined oxygen activity. Based on refined phase diagrams of second kind (oxygen activity vs. composition x, at constant T), the nearest neighbour dopant‐oxygen bond distances, the corresponding coordination numbers and intensity changes in the near K‐edge structure of iron in (Mg1‐xFex)1‐δO were analysed in terms of the iron oxidation degree at various compositions, x = 0.011–0.106, varying the oxygen activity within the stability field of the mixed oxide (T = 1273–1468 K). At low oxygen activities, near the metal 1 mixed oxide phase boundary (a10−12), iron was found to be divalent and octahedrally coordinated by oxygen. Over a wide range of higher oxygen activities, however, the EXAFS is predominated by two iron species. Identified via the bond length and confirmed by XANES analysis of the pre‐edge region, one species is attributed to Fe2+ (FeO bond distance R = 215.8 pm), the other to Fe3+ (R = 205.0 pm) (averaged over T and x), both occupying magnesium lattice sites. The obtained dependencies of the oxidation degree of iron can be described consistently within a point defect model in which divalent iron ions are randomly distributed within the cation sublattice and trivalent iron ions form defect associates involving two Fe3+ and a cation vacancy. Near the phase boundary to MgFe2O4, indications for trivalent iron ions dissolved in the interstitial sublattice of the NaCl type structure were found.