ESR of Cu2+(H2O)6. II. A quantitative study of the dynamic Jahn-Teller effect in copper-doped zinc Tutton's salt

Abstract

The X-band ESR spectra of 63Cu2+ doped zinc Tutton's salt enriched to 16 at.% in 17O isotope was studied in a single crystal at 20, 77, and 290°K. The powder spectrum of the same material was studied over the temperature range 10–350°K at X band and 120–290°K at Q band. The observed values of the g tensor and 63Cu and 17O hyperfine tensors were found to be strongly temperature dependent except when the magnetic field was parallel to g3, one of the principal axes of the g tensor. The directions of the principal axes of the g tensor and 63Cu hyperfine tensor were shown to be temperature independent, in opposition to the findings of other workers. In the single crystal, marked mI dependent linewidth variations in the 63Cu hyperfine components were observed in the temperature range 14–80°K, except when the external field was parallel to g3. The experimental data are explicable in terms of a dynamic Jahn-Teller effect, in contrast to the previously held assumption that dilute copper Tutton's salt is a static Jahn-Teller system. The energy splittings between the three inequivalent valleys were determined directly from the experimental results and found to be δ1,2 = 75 cm−1, δ1,3 ≃ 450 cm−1. The splittings are discussed in terms of published crystallographic data on Tutton's salts, and a crystal field calculation gave a value of δ1,2/δ1,3 = 1/5 in good agreement with experiment. The rate of intervalley jumping between the lowest two valleys was determined from linewidth measurements and found to be characteristic of an Orbach process, following the law τ = τ0 exp(120/k T). The value of 120 cm−1 is identified with the height of the ``warping'' barrier between valleys 1 and 2. Phonon-assisted tunneling makes no significant contribution to the jumping rate.