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 spinhyphen;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 sitersquo;s measured fourth order parameters shows that the ligand contribution to the total crystal field possesses approximteD2dsymmetry and that its major contribution is along the crystallographiccaxis, in agreement with the predictions of the ligand superposition model of Newman and Urban.
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