Mössbauer absorption and emission study of diluteFe2+impurities in cubic ZnS. Observation of metastable electronic levels. II. Dynamical Jahn-Teller effect study

In the Mössbauer source ZnS:${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ the two excited spin-orbit triplets ${\Gamma }_4$ and ${\Gamma }_5$ of ${\mathrm{Fe}}^{2+}$ behave as metastable levels after being populated by the preceding nuclear decay. They contribute two slow relaxation quadrupole doublets to the emission spectra near zero kelvin. Both quadrupole splittings are reduced by the dynamical Jahn-Teller effect, but in very different ways: the reduction factors are, respectively, $q\left({\Gamma }_4\right)\simeq 0.92$ and $q\left({\Gamma }_5\right)\simeq 0.40$. These factors are interpreted by means of a coherent vibronic model where the low-energy $\mathrm{TA}\mathrm{}\left(L\right)\mathrm{}$ phonon of ZnS ($\hslash \omega \sim 70$ ${\mathrm{cm}}^{-1\mathrm{}}$), which contributes a ${\Gamma }_3$ vibrational mode, is coupled to the ${}_{}{}^5{}_{}{}^{}E$ state of ${\mathrm{Fe}}^{2+}$ with a Jahn-Teller energy $E_{\mathrm{JT}}\simeq 14$ ${\mathrm{cm}}^{-1\mathrm{}}$. The covalency reduction of the spin-orbit parameter $\lambda$ is found to be less important for ${\mathrm{Fe}}^{2+}$ in ZnS ($\lambda =-88.4\mathrm{}\pm 1.6$ ${\mathrm{cm}}^{-1\mathrm{}}$) than for ${\mathrm{Fe}}^{2+}$ in CdTe ($\lambda \simeq -79\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$). The isomer-shift values of the ${\Gamma }_1$, ${\Gamma }_4$, and ${\Gamma }_5$ contributions are not identical because of different second-order Doppler shift values in these vibronic levels.