Electron paramagnetic resonance of three manganese centers in reduced SrTiO3

Three paramagnetic centers associated with manganese substituting for ${\mathrm{Ti}}^{4+}$ have been observed at 300 K by EPR in reduced crystals of SrTi${\mathrm{O}}_3$. The first is a ${\mathrm{Mn}}^{2+}$ ion having cubic symmetry, with isotropic $g$ and $A$ values given by $g=2.0036\mathrm{}\pm 0.0005$ and $A=(82.6\mathrm{}\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The second center is characterized by an axially symmetric EPR spectrum with effective $g$ values of $g_{\parallel }^e\cong 2.00$ and $g_{\perp }^e\cong 5.9$ at $X$ band. The spectrum is attributed to $\Delta M=\pm 1$ transitions between the $S_z=\pm \frac{1}{2}$ levels of a ${\mathrm{Mn}}^{2+}$ ion located in a strong tetragonal crystalline field. The tetragonal field presumably arises from a nearest-neighbor oxygen vacancy, hence we designate it as ${\mathrm{Mn}}^{2+}-V_{\mathrm{o}}$, in analogy with the previously investigated ${\mathrm{Fe}}^{3+}-V_{\mathrm{o}}$ center. An exact computer diagonalization of the $S=\frac{5}{2}$ matrix, together with data on a second fine-structure transition, yielded a zero-field splitting parameter $\left|2\mathrm{}{D}^{\prime \prime }\right|=1.088\mathrm{}\pm 0.0010$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $g$ values of $g_{\perp }=2.0030\mathrm{}\pm 0.0005$ and $g_{\perp }=2.0082\mathrm{}\pm 0.0060$. The hyperfine-structure splitting parameters were determined to be $A=(76\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $B=(65\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The normally forbidden $\Delta m=\pm 1$ transitions were also observed for this center, and their separations are consistent with an $S=\frac{5}{2}$ formalism. The third paramagnetic center also gives rise to a highly-anisotropic spectrum with effective $g$ values of $g_{\parallel }^e=7.945\mathrm{}\pm 0.001$ and $g_{\perp }^e<0.4\mathrm{}$, and a hyperfine constant $A=(37.3\mathrm{}\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The center is assigned to a ${\mathrm{Mn}}^{3+}$ ion associated with a nearest-neighbor oxygen vacancy, hence we designate it as ${\mathrm{Mn}}^{3+}-V_{\mathrm{o}}$. The one fine-structure transition observed is assigned to occur between the $M=\pm 2$ levels of ${\mathrm{Mn}}^{3+}(3\mathrm{}{d}^4,S=2)$. By careful measurements at $X$- and $K$-band frequencies, the cubic zero-field splitting between these levels was determined to be $a=0.054\mathrm{}\pm 0.001$ ${\mathrm{cm}}^{-1\mathrm{}}$.