Unified description of the cooperative Jahn-Teller effect in FeCr2S4and the impurity Jahn-Teller effect in CoCr2S4:Fe2+

With a molecular-field theory that includes local vibronic dynamics, the phase transition at about 10K in the ferrimagnetic spinel FeCr2S4 is described as a cooperative Jahn-Teller effect caused by antiferrodistortive coupling between (Fe2+S42-) Jahn-Teller complexes, using the same parameters that describe the impurity Jahn-Teller effect in CoCr2S4:Fe2+ within the E(X) epsilon cluster model. The estimate for the frequency of the local vibration of the sulphur tetrahedron is omega approximately=200 cm-1, and for the Jahn-Teller coupling k=(2EJT/h(cross) omega )12/=1.00+or-0.25. The spin-orbit splitting of the electronic E ground doublet of Fe2+ is delta =17+or-2 cm-1, the covalency of that state is alpha 2=0.79+or-0.07 corresponding to a spin-orbit coupling constant lambda =81+or-7 cm-1. The connection between the cooperative Jahn-Teller effect and the lattice dynamics of the spinel structure is discussed.