Dynamical Jahn-Teller effect in polymorphs: Model for the optical vibronic states ofMn2+in polymorphic ZnS

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Abstract
The structures of the E4, T24, and T14 states of Mn++ in the axial centers of ZnS polytypes are compared to the structures observed in cubic sites, and a unified model for all vibronic states considered is elaborated which provides a coherent description of a complex spectral distribution in a polymorph. First, from a detailed analysis of recent experimental results obtained from site selection spectroscopy and uniaxial stress experiments, it is shown that the structures of the E4 states are identical for the cubic and axial sites and that all the observed structures for the orbital triplet states associated with the cubic and axial sites can be analyzed in terms of a Jahn-Teller coupling to degenerate or almost degenerate E vibrational modes. The strength of the Jahn-Teller coupling and the Jahn-Teller shift with respect to the electronic states is determined for each level of Mn++ in the cubic and axial centers. Then, the shifts of the electronic states E4, T24, and E4 for Mn++ in axial symmetry with respect to the electronic states in cubic symmetry are analyzed in terms of totally symmetric electronic interactions with local strains and with the ligand field of distant neighbors. The E4 states of a d5 configuration being most likely sensitive to the ligand field of the first neighbors only, the shifts of the E4 electronic states for the axial centers are interpreted in terms of local strains of A1 symmetry acting on the MnS4 clusters. By interpreting the stress-induced shifts of the E4 states and the shifts in axial centers in a molecular cluster model it is shown that the local A1 strains most likely correspond to dilatations of the MnS4 clusters for axial centers. Finally, the shifts of the T24 and T14 electronic states are interpreted in part in terms of the local A1 strains and in part in terms of the variation of the ligand field of A1 symmetry in stacking faults. The energy levels of the Mn++ in wurtzite are also tentatively analyzed in the framework of the proposed model.