Electron Paramagnetic Resonance ofFe3+in NaF

The EPR absorption of x-irradiated NaF containing iron as an impurity has been investigated at controlled temperatures below 180°K. Three spectra have been distinguished: (i) the $F$-center spectrum, (ii) a spectrum isotropically centered near $g=2.002\mathrm{}$ and with a partially resolved anisotropic structure, and (iii) the ${\mathrm{Fe}}^{+}$ spectrum centered near $g=4.344\mathrm{}$. The spectrum near $g=2.002\mathrm{}$ is satisfactorily described in terms of a spin Hamiltonian pertaining to an ${}_{}{}^6{}_{}{}^{}S_{\frac{5}{2}}^{}{}_{}{}^{}$ ion located at a lattice site with cubic symmetry and interacting with six equivalent nuclei of spin ½ each. This spectrum is attributed to ${\mathrm{Fe}}^{3+}$ at a cation site. Computer analyses of the observed temperature dependence of the ${\mathrm{Fe}}^{3+}$ spectrum have indicated that the fine-structure constant decreases as the first power in $T$, and the line width increases as the square root of $T$, when $T$ changes from 77 to 180°K. The results are consistent qualitatively with the existing theories of the $S$-state ion. Quantitatively, however, the computed $T$ dependence of the fine-structure constant disagrees substantially with current theories.