Dynamical Jahn-Teller coupling of Fe2+ in the cluster approach applied to the Mössbauer quadrupole data of 57Fe doped K2ZnF4 and Ba2ZnF6

We have analysed the Mössbauer quadrupole data of high-spin Fe2+ located in the tetragonally distorted octahedra of K2ZnF 4 and Ba2ZnF6. The dynamical linear Jahn-Teller coupling between the orbital excited states Eg, in D4h symmetry, involves the vibrational modes A1g,B1g, B 2g of the cluster FeF6 ; spin-orbit coupling is introduced, and a full mathematical treatment is performed in a basis of dimension 5 (orbit) x 5 (spin) x 10 (vibrations). This leads to a spectacular improvement of the calculated thermal variations of the quadrupole splitting in both systems, when compared to the results of the static crystal field model.