Electronic Structure and Anomalous Thermal Expansion in FeF2 and VO2

Abstract
The negative thermal expansion coefficient (TEC) in FeF2 and the large positive one in VO2, along the c axis, are explained by a model which emphasizes the changes in force constants, resulting from the spatial rearrangement of the electronic charge surrounding the cation, when low‐lying electronic states are thermally populated. The TEC data are compared for 14 rutile‐type crystals. It is significant that all the compounds studied which involve unequally occupied t 2g electronic orbitals display TEC anomalies.