Effect of Temperature and Pressure on the Electron Paramagnetic Resonance Spectra of Substitutional Impurities in Cubic SrTiO3

The electron paramagnetic resonance spectra of ${\mathrm{Fe}}^{3+}$, ${\mathrm{Gd}}^{3+}$, and ${\mathrm{Cr}}^{3+}$ in cubic SrTi${\mathrm{O}}_3$ have been studied as a function of hydrostatic pressure to 10 000 bars and as a function of temperature between 300 and 110°K. The optical spectrum ${\mathrm{Cr}}^{3+}$:SrTi${\mathrm{O}}_3$ has been examined at 4.2, 77, and 300°K. From a comparison of the pressure data for ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ with those for these ions in other hosts, we deduce that the local compressibility at the site occupied by these impurities is enhanced by a factor of two over the bulk compressibility. The isoelectronic ions ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Eu}}^{2+}$ show a striking difference in the temperature dependence of the ground-state cubic field parameter, $b_{40}$. This difference is due to the polarization of the lattice by the uncompensated charge of ${\mathrm{Gd}}^{3+}$ and manifests itself as a Curie-Weiss term in the temperature dependence of $b_{40}$. This polarization effect is also seen in the temperature dependence of $b_{40}$ for ${\mathrm{Fe}}^{3+}$. Theoretical justification for the temperature dependence of this term is given.