DYNAMIC JAHN-TELLER EFFECT ON THE FAR-INFRARED SPECTRUM OF FE2+ IN CD1-XFEXTE COMPOUNDS

The optical absorption of substitutional Fe2+ ions in Cd1-xFexTe compounds has been measured in the far infrared (10-80 cm-1) at temperatures from 5 to 300 K. Electronic transitions inside the 5E multiplet of Fe2+ are observed at low temperature, but the crystal-field model is not sufficient to explain the experimental features. Calculations of the 5E electron states are performed, taking into account the Jahn-Teller interaction with the vibrations of the host lattice. The electron-phonon coupling is approximated by a single pair of E modes corresponding to TA(L) phonons. Energies and oscillator strengths for electric- and magnetic-dipole transitions from the ground state to excited levels are calculated within the Jahn-Teller model. An excellent quantitative agreement is obtained with the low-temperature absorption lines for a Jahn-Teller energy of 2.5 cm-1, an effective spin-orbit splitting lambda = -101.9 cm-1, a spin-spin interaction coefficient-rho = 0.18 cm-1, and a phonon energy of the TA(L) mode (28 +/- 0.5 cm-1) quite consistent with the neutron-scattering results. The high-temperature far-infrared absorption spectra give also evidence for a local mode of Fe in CdTe and for a two-phonon process involving the TA(X) mode of CdTe (35 +/- 0.5 cm-1).