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

The local symmetry of Ti⁴⁺ sites in SrTiO₃ is known to be cubic above 106^∘{}K, and tetragonal below. We have investigated the electron-spin-resonance spectrum of substitutional Ni³⁺ in the tetragonal phase of this compound. The components of the g tensor are observed to vary monotonically from g_⊥=2.215 and gII=2.103 at 4^∘{}K, to gII=g_⊥=2.180 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_⊥-gII 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 cm^-1). Our results indicate that the elongation of the nearest-neighbor octahedron is too small to be the principal cause of the g splitting.