Jahn-Teller Effects in the Far-Infrared, EPR, and Mössbauer Spectra of MgO:Fe2+

Measurements by Wong of the far-infrared absorption spectrum of the ${\mathrm{Fe}}^{2+}$ ion at undistorted cubic sites in MgO have revealed that the first excited spin-orbit level (evidently comprising both the ${\Gamma }_{3g}$ and ${\Gamma }_{4g}$ states) lies 105 ${\mathrm{cm}}^{-1\mathrm{}}$ above the ${\Gamma }_{5g}$ ground state, in contrast to the value ∼200 ${\mathrm{cm}}^{-1\mathrm{}}$ predicted by crystal-field theory. A calculation has been made to determine if this reduced spin-orbit splitting can reasonably be attributed to a dynamic Jahn-Teller effect without leading to inconsistencies with other EPR and Mössbauer data for MgO: ${\mathrm{Fe}}^{2+}$. It is concluded that this reduction is indeed predominantly the result of the Jahn-Teller coupling, that this reduction is consistent with the strength of Jahn-Teller coupling indicated by strain data for the ${\Gamma }_{5g}$ level, and that significant Jahn-Teller corrections occur also in other parameters which characterize the EPR and Mössbauer spectra, such as the $g$ factor, hyperfine field, and nuclear quadrupole coupling coefficients of the ${\Gamma }_{5g}$ level. These Jahn-Teller corrections are calculated using the strength of the Jahn-Teller coupling to both $E_g$ and $T_{2g}$ vibrational modes as parameters, which are adjusted to give the observed spin-orbit reduction, and the resulting values are compared with estimates of corrections resulting from covalency. The calculations are carried out by treating the Jahn-Teller interaction by perturbation methods, and the calculation is formulated both for coupling with discrete vibrational modes and also for coupling with the continuum of lattice phonons.