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

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 E→55T2 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 ρobs=3.24(4) g/cm3 and ρcalc=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 E1g vibrational mode is probably responsible for the structural differences.