Correlation of Magnetic Susceptibility and Infrared Spectra of FeF2

Abstract

Precise measurements of the magnetic susceptibility of single crystal FeF₂ parallel and perpendicular to the tetragonal axis are reported. Measurements of the polarized infrared spectra of crystalline FeF₂ show transition to electronic levels arising from the splitting of the ⁵D state of the Fe²⁺ ion by the ligand field of D_2h symmetry. A magnetic dipole transition observed at 1115 cm⁻¹ when the magnetic field of the light is polarized parallel to the tetragonal axis is tentatively identified with a transition from the ground orbital state of symmetry A_1g to an excited level of symmetry B_1g arising from the octahedral ⁵T_2g levels. In the helium temperature range the magnetic susceptibility parallel to the tetragonal axis is (5.6±0.1) × 10⁻⁴ cm³ mole⁻¹. This value is correlated with that calculated from the matrix element of the orbital angular momentum parallel to the tetragonal axis connecting the ground state with the excited orbital state.