The paramagnetic resonance of the ${}_{}{}^6{}_{}{}^{}S$ state of ${\mathrm{Fe}}^{3+}$ has been studied in the monoclinic $C_s$ symmetry which arises from ${\mathrm{Fe}}^{3+}$ associated with a monovalent metal impurity (${\mathrm{Cu}}^{+}$, ${\mathrm{Ag}}^{+}$, or ${\mathrm{Li}}^{+}$) in ZnS, ZnSe, ZnTe, CdTe, and ZnO. The ${\mathrm{Fe}}^{3+}$ and ${\mathrm{}\{\mathrm{C}\mathrm{u},\mathrm{A}\mathrm{g},\mathrm{o}\mathrm{r}\mathrm{L}\mathrm{i}\}\mathrm{}}^{+}$ impurities are substitutional for the metal ions at one of the nearest possible sites. The zero-field splitting due to the crystalline electric fields is frequently large compared to the Zeeman interaction. It is observed that no specific ratio of the two quadratic fine-structure terms in the spin Hamiltonian occurs. This suggests that in many cases the observation of nearly isotropic lines near $g=4.3\mathrm{}$ results from a fortuitous set of values for these fine-structure terms, supporting the view that a pure "rhombic" term need not follow from the symmetry of the environment.

• Received 13 December 1967

DOI:https://doi.org/10.1103/PhysRev.169.359