Effect of Exchange Coupling on the Spectra of Transition Metal Ions. The Ligand Field Spectrum and Crystal Structure of CsCrCl3

Abstract

The single crystal polarized absorption spectra of CsCrCl₃ and of CsCrCl₃ doped into CsMgCl₃ have been measured and interpreted in terms of an octahedral ligand field. The intensities of the spin forbidden transitions are found to be very dependent on the chromium ion concentration. In addition, an absorption band which is strongly polarized along the crystallographic c axis is observed at approximately twice the energy of the 10 Dq band. Both of these effects are attributed to magnetic coupling between the chromium ions; the anomalous band is assigned to a ``double excitation'' of the ${}^5{\mathrm{E\to }}^5{T}_2$ transition on coupled pairs of ions. The crystal structure of CsCrCl₃ has also been determined. The lattice is hexagonal with P6₃mc the most probable space group. The lattice constants are a=7.256(3) and c=6.224(3) Å, with ρ_obs=3.24(4) g/cm³ and ρ_calc=3.40 g/cm³. Least‐squares refinement, based on F, of counter data has led to a final R index of 0.042. The main structural feature is a linear array of [CrCl₆]⁻⁴ octahedra sharing faces. The structure is similar, but not identical, to that of CsMgCl₃ and CsNiCl₃. A small static Jahn‐Teller distortion of the chloride ions from trigonal symmetry about the Cr(II) ions or a Jahn‐Teller active E_1g vibrational mode is probably responsible for the structural differences.