Mössbauer Spectrum ofFe2+in MgO

Abstract

The appearance of a quadrupole doublet in the Mössbauer spectrum of ${\mathrm{Fe}}^{2+}$ in MgO at $T<\mathrm{}14\mathrm{}°$K, as observed originally by Pipkorn and Leider, is explained on the basis of crystal-field theory. The doublet is the result of the electric field gradient produced at the nucleus by the valence electrons of ${\mathrm{Fe}}^{2+}$, combined with long electronic relaxation times and the effect of random strains in lifting the electronic degeneracy of the ground-state triplet by ∼${10}^{-2\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Although the Jahn-Teller effect plays no essential role in the appearance of the doublet, it may change its splitting slightly and cause broadening. An Orbach-type process is proposed for the electronic relaxation that is responsible for motional narrowing of the doublet for $T>\mathrm{}14\mathrm{}°$K. The form predicted for the Mössbauer spectrum is also given for zero strain and for an applied magnetic field; in the latter case, in addition to a hyperfine splitting, there is a quadrupole splitting which changes sign as H rotates from [100] to [111].