Dynamic Jahn-Teller Effect of an Impurity in a Spontaneously Distorted Crystal

The local symmetry of ${\mathrm{Ti}}^{4+}$ sites in SrTi${\mathrm{O}}_3$ is known to be cubic above 106°K, and tetragonal below. We have investigated the electron-spin-resonance spectrum of substitutional ${\mathrm{Ni}}^{3+}$ in the tetragonal phase of this compound. The components of the $g$ tensor are observed to vary monotonically from $g_{\perp }=2.215\mathrm{}$ and $g_{\mathrm{II}}=2.103\mathrm{}$ at 4°K, to $g_{\mathrm{II}}=g_{\perp }=2.180\mathrm{}$ at 106°K. Three models of the dynamical Jahn-Teller effect in the presence of coupling to strain are examined. The magnitude and temperature dependence of $g_{\perp }-g_{\mathrm{II}}$ are best explained by a model in which the $g$ tensor is given by a Boltzmann average over the two lowest vibronic states. The tunneling splitting appears to exceed by far the maximum spontaneous splitting of these states (23 ${\mathrm{cm}}^{-1\mathrm{}}$). Our results indicate that the elongation of the nearest-neighbor octahedron is too small to be the principal cause of the $g$ splitting.