Symmetry of Transition Metal Impurity Sites in Crystals as Inferred from Optical Spectra

Abstract

The polarized optical spectra of several transition metal ion impurities in crystals with the wurtzite structure; Cs2ZnX4 (where X−=Cl− or Br−); and Al2O3 have been measured at temperatures ranging from that of liquid helium to room temperature. In all these cases the transition metal ion substitutes for a cation of pure compound. The gross features of the spectra are explained by crystal field theory for the impurity ions in a potential field arising from the nearest-neighbor ligands in their undistorted configuration. The finer details of the spectra and the polarization effects which are observed are used to infer the actual site symmetry of the impurity ion. The trigonal field splitting compared to the calculated value indicates that certain ions in Al2O3 are displaced from the aluminum site along the threefold axis. The breadth of certain bands, and in some cases the lattice vibration progressions which are observed, have been interpreted as being due to a reduction in the symmetry of the electronically exicted ion and its ligand environment from that of the ground-state configuration. In the case of Mn3+ in corundum there is very good evidence for a highly distorted site, presumably due to the Jahn-Teller effect.