Optical Absorption of Tetrahedral ${\mathrm{Fe}}^{2+}$ ($3d^6$) in Cubic ZnS, CdTe, and Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$

The optical absorption spectrum of substitutional ${\mathrm{Fe}}^{2+}$ ions at concentrations from 2×${10}^{18}$ to 4×${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ has been studied for cubic ZnS, CdTe, and Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$ single crystals from 3 to 300°K. The ${\mathrm{Fe}}^{2+}$ ions show a single broad absorption band at 300°K in the infrared region between 1500 and 7500 ${\mathrm{cm}}^{-1\mathrm{}}$ (1.3 to 6.7 μ) that arises from the ${}_{}{}^5{}_{}{}^{}E\to {}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ transition. At low temperatures this band shows many distinct lines which are identified as resulting from zero-phonon and phonon-assisted transitions between the spin-orbit levels of ${}_{}{}^5{}_{}{}^{}E$ and ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ in tetrahedral symmetry. The levels of ${}_{}{}^5{}_{}{}^{}E$ are found to be described well by crystal-field theory: There are five uniformly spaced levels split in second order by spin-orbit interactions, with an interval given by 15, 10, and 13 ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Fe}}^{2+}$ in ZnS, CdTe, and Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$, respectively. The levels of ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ do not fit the predictions of crystal-field theory; they can, however, be understood if a moderately strong Jahn-Teller effect occurs in the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ state, so that the first-order spin-orbit splitting of ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ is quenched to a small fraction of its crystal-field value. Values for this Jahn-Teller energy of 535, 255, and 945 ${\mathrm{cm}}^{-1\mathrm{}}$ are derived from the data for ZnS, CdTe, and Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$, respectively. A phenomenological Hamiltonian is found which describes the dynamical Jahn-Teller effects in ZnS very well, and which may also be appropriate for Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$ but does not suffice for CdTe. An alternative interpretation of the spectrum for ${\mathrm{Fe}}^{2+}$ in ZnS, not requiring so strong a Jahn-Teller effect, more nearly accords with the predictions of crystal-field theory, but at the expense of assuming that some of the observed zero-phonon lines arise from ${\mathrm{Fe}}^{2+}$ associated with some other defect common to all samples, or from a mixture of cubic and hexagonal regions within the crystals. Values of the cubic-field parameter $\mathrm{Dq}$ for ${\mathrm{Fe}}^{2+}$ in these crystals are -340, -248, and -447 ${\mathrm{cm}}^{-1\mathrm{}}$ for ZnS, CdTe, and Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$, respectively. The phonon-assisted transitions yield values for the transverse acoustic, longitudinal acoustic, transverse optic, and longitudinal optic phonons in ZnS and CdTe which are 115, 184, 296, 331 ${\mathrm{cm}}^{-1\mathrm{}}$ and 65, 105, 140, 180 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively.