Jahn-Teller interaction at Cr²⁺(d⁴) centres in tetrahedrally coordinated II-VI lattices studied by optical spectroscopy

Optical spectra of the d4 configuration have been studied with II-VI compounds. Failure of static crystal-field description led various authors to introduce static or dynamic Jahn-Teller models for the Cr2+ ground and excited states. With ZnS, the main optical transition 5E(D)↔5T2(D) now appears as a doublet at 5218 and 5212cm-1 coinciding for emission and absorption spectra. This no-phonon structure indicates a splitting of the gound- and excited-state multiplets under combined Jahn-Teller and spin-orbit interactions. In the cubic host ZnSe and in the hexagonal compounds CdS and CdSe, the optical properties exhibit the same general behaviour but with a decreasing shaping of the structural details. In addition to the NIR measurements, middle-infrared spectra have been recorded by Fourier spectroscopy in the transmission mode. For the first time a broad band could be detected for ZnS and interpreted as the 5Ê(T2)←5B̂2(T2) absorption between the sheets of the respective adiabatic potential surfaces in D2d symmetry. Its maximum near 1300 cm-1 indicates v¯JT≈430cm-1 following the common interpretation of this transition