A study of the rare earth ion sites in Gd3+ and Eu3+ doped CaSO4

Abstract

The principal Gd³⁺ centers in CaSO₄:Gd³⁺ single crystals have been studied using ESR and compared with the main Eu³⁺ centers evident in the optical spectra of CaSO₄:Eu³⁺. The results indicate that the rare earth ion occupies the same four structural centers in each case. Structural models are proposed for these centers based on the symmetry of the measured crystal field at the principal Gd³⁺ sites, all of which were found to exhibit either orthorhombic or monoclinic behavior. ESR spin‐Hamiltonian parameters have been calculated for these Gd³⁺ centers and accurate measurements made of the principal optical transition wavelengths for the corresponding Eu³⁺ centers. Analysis of the crystal field symmetry at the dominant Gd³⁺ site shows that it is not consistent with either an electrostatic model or a covalent ligand model, but that it can be well described by a combination of these two interactions in which each contributes approximately equally to the measured second order parameters. A pseudosymmetry analysis of this site’s measured fourth order parameters shows that the ligand contribution to the total crystal field possesses approximte D_2d symmetry and that its major contribution is along the crystallographic c axis, in agreement with the predictions of the ligand superposition model of Newman and Urban.