Infrared Absorption and Luminescence Spectra of ${\mathrm{Fe}}^{2+}$ in Cubic ZnS: Role of the Jahn-Teller Coupling

A revised interpretation of the ${}_{}{}^5{}_{}{}^{}E\to {}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ optical absorption spectrum of ${\mathrm{Fe}}^{2+}$ in cubic ZnS is proposed. It is shown that certain absorption peaks, lacking a counterpart in the luminescence, cannot be interpreted, as previously done, as two- and three-phonon sidebands of the zero-phonon transitions, and also cannot be attributed to crystal-field electronic transitions or to simple unshifted one-phonon sidebands. A Jahn-Teller coupling within the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ level, weaker than the spin-orbit coupling, is shown to be capable not only of partially quenching the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ spin-orbit splitting but also of shifting the associated one-phonon levels in the manner required to account for the observed spectra. Such a dynamic Jahn-Teller effect, involving simultaneous coupling to a low-frequency mode (∼ 100 ${\mathrm{cm}}^{-1\mathrm{}}$) and a higher-frequency mode (∼ 300 ${\mathrm{cm}}^{-1\mathrm{}}$), is proposed to account for the data for ${\mathrm{Fe}}^{2+}$ in ZnS in terms of coupling to the TA and TO and/or LO phonons of the ZnS lattice. This interpretation of the phonon coupling is shown to be consistent with a moment analysis of the broadening of the optical absorption and luminescence spectra using the methods of Henry, Schnatterly, and Slichter. The relation of this model to similar features in the spectrum of ${\mathrm{Fe}}^{2+}$ in CdTe, Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$, ZnSe, ZnTe, CdS, GaP, and GaAs is discussed. It is also shown that this model provides insights into the relationship between lattice phonons and the localized modes that dominate the Jahn-Teller coupling in strongly coupled systems, and that it shows why the cluster model for the Jahn-Teller ion and its nearest neighbors often works well for such systems.