We have measured the spin-orbit splitting, Zeeman effect, and splitting under $〈001〉$ and $〈111〉$ stress of the lowest vibronic levels of the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ term of ${\mathrm{Co}}^{2+}$ in KMg${\mathrm{F}}_3$. The results are in good agreement with the Ham theory of the dynamic Jahn-Teller effect, in which interaction is only with a single $\epsilon$ (tetragonal) distortion mode. The convergence of this theory is checked and found to be good, and the influence of $\tau$ vibrations is shown to be small. We conclude that the cluster model, on which the Ham theory is based, gives an adequate description of the Jahn-Teller effect in perovskite fluorides.

• Received 7 June 1971

DOI:https://doi.org/10.1103/PhysRevB.4.2092