Electron-phonon coupling and Jahn-Teller effect in KMgF₃:Cr ³⁺

Energy levels of the Cr³⁺ ion in the KMgF₃ crystal are analyzed by means of the crystal field theory. Electron-phonon coupling constants (the Stokes shift, the force constants and the constants of the linear vibronic Hamiltonian) are evaluated using experimental spectroscopic data. The T ⊗ e Jahn-Teller effect in the lowest excited ⁴T_2g state of the Cr³⁺ ion is considered using both Ham effect and analysis of the displacements of ligands due to the combined effect of the a_1g and e_g normal modes of the [CrF₆] ^3- octahedral complex. The net result of these vibrations for this cluster is an equatorial expansion by 0.070 Å and an axial compression by 0.008 Å. The value of the Jahn-Teller stabilization energy for the considered complex is about 357 cm^-1.