| Solution growth of materials now offers a viable alternative to traditional high temperature melt growth processes. The sol-gel method, one such solution technique, chemically produces inorganic materials from aqueous solutions. The sol-gel process can be used to produce alumina, Al2O3 , in a variety of forms from porous, transparent monoliths to ceramics. Aluminum oxide, a large bandgap material (9 eV), has played important roles in the fields of impurity ion spectroscopy and catalysis. High quality single crystals, both natural and synthetic, can serve as a host for a variety of transition metal impurities, which are responsible for crystal coloration. Early studies of the spectroscopy of these crystals provided a proving ground for the quantum theory of solids. Meanwhile, the high surface area polycrystalline forms, referred to as transition aluminas, were finding wide application in the areas of catalysis. Despite the technological importance of solution grown materials, we know little about the role of impurities in these materials. The sol-gel growth technique enables the doping of transition aluminas with precise amounts of optically active impurity ions. These ions probe the structure and dynamics of the materials through interactions with the crystal field produced by the host ions. Recent work has begun to address this important class of materials. In this work, we report on the luminescent properties of trivalent ytterbium doped into alumina. Combining the results of optical measurements performed on single crystal and ceramic samples, we show that the dopants occupy identical sites in the two materials. We identify a set of crystal field parameters for ytterbium ions on the substitutional aluminum cation site. Comparison of the crystal field parameters with those of trivalent ytterbium in Bi4Ge3O12, a host with a larger cation site, shows that the ytterbium, the smallest of the lanthanides, sits near the center of the oxygen octahedron. Finally, studies of ytterbium in nanocrystalline gamma alumina demonstrate the role surfaces play in the relaxation of impurity ion excited states in the bulk. This work demonstrates a new method for studying metal oxide surfaces using optical probe ions. |
