Thermal Expansion of MgFe2O4, FeO, and MgO·2FeO

The thermal expansion of magnesium ferrite, wüstite, and magnesio-wüstite has been measured up to 1000°C. Good agreement has been found with published data obtained at lower temperatures, but the reported extrapolations have been shown to be low. The thermal expansion of MgFe₂O₄ has been interpreted to support Gilleo's hypothesis that the super-exchange energy for Fe³⁺-O^2--Fe³⁺ linkages is independent of the Fe³⁺-O^2- distance. This conclusion is supported by the fact that the energy per linkage is the same as that for Fe³⁺-O^2-Fe³⁺ linkages in a variety of crystal structures.