Thermal Expansion of MgFe 2 O 4 , 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 MgFe 2 O 4 has been interpreted to support Gilleo's hypothesis that the super‐exchange energy for Fe 3+ ‐O 2‐ ‐Fe 3+ linkages is independent of the Fe 3+ ‐O 2‐ distance. This conclusion is supported by the fact that the energy per linkage is the same as that for Fe 3+ ‐O 2‐ Fe 3+ linkages in a variety of crystal structures.