Electron-Paramagnetic-Resonance Spectrum ofMo5+in SrTiO3-An Example of the Dynamic Jahn-Teller Effect

The EPR spectrum of ${\mathrm{Mo}}^{5+}$ in SrTi${\mathrm{O}}_3$ at $T=1\mathrm{}$. 65 °K is reported. The spectrum is stable in suitably prepared samples or light-induced (3900-4300-Å light) in photochromic samples, and can only be observed at liquid-helium temperatures. The $g$ tensor is $g_{\parallel }=1.72\mathrm{}$, $g_{\perp }\sim 0$. The hyperfine (hf) constant is $A_{\parallel }=39\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. $A_{\perp }$ was not measured but the hf splitting is not very anisotropic. The linewidth varied from 3 to 8 G for $\overrightarrow{\mathrm{H}}\parallel 〈100〉$ direction but broadened to 60 G at $\theta =60\mathrm{}°$. The spectrum cannot be explained by crystal-field theory alone but does fit the dynamic Jahn-Teller effect as calculated by Macfarlane, Wong, and Sturge for the $d^1$ ions ${\mathrm{Ti}}^{3+}$ and ${\mathrm{V}}^{4+}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$. We find a Jahn-Teller energy $E_{\mathrm{J}\mathrm{}\mathrm{T}}=480\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for SrTi${\mathrm{O}}_3$:${\mathrm{Mo}}^{5+}$. To obtain a reasonable first excited state (∼10 ${\mathrm{cm}}^{-1\mathrm{}}$), a small tetragonal distortion (∼20 ${\mathrm{cm}}^{-1\mathrm{}}$) must be assumed, as expected for SrTi${\mathrm{O}}_3$ below the 105 °K phase transition.