No abstract

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 and with a partially resolved anisotropic structure, and (iii) the Fe + spectrum centered near g=4.344. The spectrum near g = 2.002 is satisfactorily described in terms of a spin Hamiltonian pertaining to an Q S&/2 ion located at a lattice site with cubic symmetry and interacting with six equivalent nuclei of spin | each. This spectrum is attributed to Fe 3+ at a cation site. Computer analyses of the observed temperature dependence of the 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 5-state ion. Quantitatively, however, the computed T dependence of the fine-structure constant disagrees substantially with current theories.

Iron-group impurities in x-irradiated NaF have been studied using electron paramagnetic resonance. Special attention was given to the measurement and understanding of the temperature dependence of a 8&~2 spectrum, which previously had been attributed to Fe'+ located substitutionally at a cation site with octahedral symmetry. Concentration measurements and comparisons with the work of others indicate that the spectrum is also consistent with NaF:Cr+. The effective-spin-Hamiltonian parameters were measured for temperatures in the range of 21 to 198 K, and indicate 65% decrease in the fine-structure constant from 60 to 198 K, and a similar but less-well-defined increase in the transition width. The superhyperfine constants associated with the surrounding F ligands show little temperature dependence, and all parameters appear to have constant values below 60 K. The temperature dependence of the fine-structure constant has been interpreted according to the splitting of ionic states by the electron-phonon interaction mechanism, and calculations indicate possible agreement with data for a reasonable choice of atomic parameters.