Far-Infrared Spectra of Iron-Doped MgO

The far-infrared absorption spectra of two iron-doped MgO samples were measured at 4.2, 20.3, and 77.4^∘{}K, using a large grating spectrometer. A line at 105.0 cm^-1 was observed with a peak absorption coefficient of 1.5 cm^-1 and a width of {}9.0 cm^-1 at 20.3^∘{}K. This line is attributed, with the aid of oxidation and reduction data, to MgO: Fe²⁺ at cubic sites. The transition Γ5g→Γ3g, Γ4g of MgO: Fe²⁺ would be expected from crystal-field theory to be at around 200 cm^-1. However, Ham has postulated that vibronic effects could displace these levels to a position near 100 cm^-1. The location of the levels Γ3g, Γ4g at {}100 cm^-1 would account for both anomalous quadrupole splitting in the M\"ossbauer spectrum of MgO: Fe²⁺ at T<14^∘K and the rates of spin-lattice relaxation in the ground state. This prediction is now confirmed. Other lines were also observed in one or both of the samples investigated, but they are not related to the 105-cm^-1 line. The origin of these lines is also briefly discussed.