Extra EPR Spectra of Iron-Doped Rutile

We have discovered that besides the well-known EPR spectrum of ${\mathrm{Fe}}^{3+}$ substitutional ions in iron-doped rutile there are several other EPR spectra related to ${\mathrm{Fe}}^{3+}$ ions. The interpretation of these extra spectra was made considering the intensity variation with heat treatment and the orientation of the magnetic axes of the extra spectra. Isofrequency diagrams were recorded at 9.2 GHz and isogon plots were measured up to 62 GHz. One extra spectrum, spectrum I, was found to have zero-field splittings (ZFS) of 32.1 and 50.7 GHz. Spectrum I could be described by the spin-Hamiltonian parameters $D=13.1\mathrm{}$ GHz, $E=2.3\mathrm{}$ GHz, $a=-0.7\mathrm{}$ GHz, $F=1.9\mathrm{}$ GHz, $g_x=1.993\mathrm{}$, $g_y=2.040\mathrm{}$, and $g_z=1.998\mathrm{}$. We interpret spectrum I as due to a substitutional ${\mathrm{Fe}}^{3+}$ ion which is perturbed by a proton nearby. The position of this proton in the lattice is determined. Another spectrum, spectrum II, has one ZFS of 61.4 GHz. The other ZFS is expected around 81.6 GHz but could not be observed because of lack of a signal source. Spectrum II could be described by the following approximate spin-Hamiltonian parameters: $D=22.9\mathrm{}$ GHz, $E=2.7\mathrm{}$ GHz, $a=-0.1\mathrm{}$ GHz, $F=6.3\mathrm{}$ GHz, $g_x=2.001\mathrm{}$, $g_y=1.993\mathrm{}$, and $g_z=2.001\mathrm{}$. We interpret spectrum II as due to a substitutional ${\mathrm{Fe}}^{3+}$ ion that is perturbed by a nearest-neighbor oxygen vacancy. Several lines of a third spectrum were also identified and tentatively interpreted as due to a substitutional ${\mathrm{Fe}}^{3+}$ ion with a ${\mathrm{Ti}}^{4+}$ interstitial ion nearby. Further weak spectra were seen but could not be given any definite interpretation.