Strong Axial Electron Paramagnetic Resonance Spectrum of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Fe</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>in SrTi<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Due to Nearest-Neighbor Charge Compensation

An axial electron paramagnetic resonance (EPR) spectrum in iron-doped (cubic) strontium titanate has been observed at 3.3 and 1.85 cm wavelength with effective $g$ values ${{g}_{\ensuremath{\perp}}}^{e}=5.993\ifmmode\pm\else\textpm\fi{}0.001$ and ${{g}_{\ensuremath{\perp}}}^{e}=5.961\ifmmode\pm\else\textpm\fi{}0.001$, respectively, and ${{g}_{\mathrm{II}}}^{e}={g}_{\ensuremath{\parallel}}=2.0054\ifmmode\pm\else\textpm\fi{}0.0007$. The same spectrum was found after charge displacement due to heat treatment in the dark or by reduction in crystals which contained, in addition to iron, other transition metal ions. The spectrum is attributed to $\ensuremath{\Delta}M=1$ transitions of the ${S}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ level of ${\mathrm{Fe}}^{3+}(3{d}^{5})$ in a strongly tetragonal electric crystalline field produced by local charge compensation at a nearest-neighbor oxygen site. The theory is developed for ions of half-integral spin in an axial field much greater than the Zeeman splitting. By applying the theory to the special case of $S=\frac{5}{2}$ and using the measured effective ${{g}_{\ensuremath{\perp}}}^{e}$ values and resonance magnetic fields ${g}_{\ensuremath{\perp}}=2.0101\ifmmode\pm\else\textpm\fi{}0.0008$ and a zero-field splitting parameter $|2D|$ of 2.85\ifmmode\pm\else\textpm\fi{}0.15 ${\mathrm{cm}}^{\ensuremath{-}1}$ is obtained. This is the largest splitting which has so far been observed for ${\mathrm{Fe}}^{3+}$ in any inorganic crystal and could be useful for submillimeter maser applications.