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 ${\mathrm{cm}}^{-1\mathrm{}}$ was observed with a peak absorption coefficient of 1.5 ${\mathrm{cm}}^{-1\mathrm{}}$ and a width of ≃9.0 ${\mathrm{cm}}^{-1\mathrm{}}$ at 20.3°K. This line is attributed, with the aid of oxidation and reduction data, to MgO: ${\mathrm{Fe}}^{2+}$ at cubic sites. The transition ${\Gamma }_{5g}\to {\Gamma }_{3g}$, ${\Gamma }_{4g}$ of MgO: ${\mathrm{Fe}}^{2+}$ would be expected from crystal-field theory to be at around 200 ${\mathrm{cm}}^{-1\mathrm{}}$. However, Ham has postulated that vibronic effects could displace these levels to a position near 100 ${\mathrm{cm}}^{-1\mathrm{}}$. The location of the levels ${\Gamma }_{3g}$, ${\Gamma }_{4g}$ at ≃100 ${\mathrm{cm}}^{-1\mathrm{}}$ would account for both anomalous quadrupole splitting in the Mössbauer spectrum of MgO: ${\mathrm{Fe}}^{2+}$ at $T<\mathrm{}14\mathrm{}°$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-${\mathrm{cm}}^{-1\mathrm{}}$ line. The origin of these lines is also briefly discussed.