Theoretical explanation of the octahedral distortion in FeF2 and MgF2

Abstract

Abstract The equilibrium geometries of rutile type FeF2 and MgF2 are determined using precise density functional theory calculations that lead to good agreement with experimental data. The strong deviation from the “ideal” octahedral geometry in FeF2 comes from the Fe2+ ion with the d 6 configuration in the high-spin state, which would require a singly occupied spin-down d orbital. The stability can only be understood by going beyond the first coordination shell considering direct Fe–Fe interactions mainly along the c-axis. The orbital population of the various Fe-d orbitals is analyzed using constarint distortions between the ideal and the equilibrium structure. Above a critical Fe–Fe distance of about 3.15 Å the population numbers change drastically, a single Fe-d spin down orbital becomes occupied and the system can relax to an insulating state. In MgF2 no such d-orbitals are present and thus the distortions are much smaller.