A triply-ionized rare earth ion in an axial crystal field: Praseodymium(+3) in beta'' alumina.

摘要

The superionic conductor {dollar}betasp{lcub}primeprime{rcub}{dollar} alumina offers an unusual host environment for the triply-ionized rare earth ions. Previous x-ray structure analyses, combined with optical and electron paramagnetic resonance spectroscopy, have shown that these ions occupy two sites. One of these sites, called the Beevers-Ross site, is extraordinary in that it has only four nearest neighbors. In this dissertation, Pr{dollar}sp{lcub}+3{rcub}{dollar} has been used as a probe of the crystal field in order to understand this unusual rare earth site.; The fluorescence spectrum of the {dollar}sp3{dollar}H{dollar}sb4{dollar} ground state of Pr{dollar}sp{lcub}+3{rcub}{dollar} can be interpreted on the basis of an axial crystal field with a dominant {dollar}Asb{lcub}20{rcub}{dollar} term, together with the selection rules for an electric-dipole interaction under C{dollar}sb{lcub}rm 3v{rcub}{dollar} symmetry, which is the site symmetry expected for the {dollar}betasp{lcub}primeprime{rcub}{dollar} alumina Beevers-Ross site. One of the values of {dollar}Asb{lcub}20{rcub}{dollar} obtained from the {dollar}sp3{dollar}H{dollar}sb4{dollar} fluorescence data is approximately the same as that obtained from the splitting of the Nd{dollar}sp{lcub}+3{rcub}{dollar} {dollar}sp4{dollar}F{dollar}sb{lcub}3/2{rcub}{dollar} level.; Polarized absorption spectra of single crystals of approximately 55% exchanged (Na, Pr) {dollar}betasp{lcub}primeprime{rcub}{dollar} alumina were obtained at several temperatures. Laser-induced fluorescence spectra were also recorded, as well as the polarized fluorescence spectra obtained from a broader-band blue excitation. These experimental spectra were compared with the predictions of the proposed axial crystal field model. The agreement between the calculated and experimental spectra is remarkably good for such a simple model, and indicates that this model might be useful in predicting the general features of other rare earth ions in this host.