A cosolvent-free solgel method to synthesize monolithic silica glasses containing both of rare-earth and aluminum ions has been studied. Fracture during drying and bloating during sintering are significantly suppressed by forming macroporous wet gels via macroscopic phase separation in parallel with gelation. The resultant rare-earth and aluminum codoped silica glasses exhibit good visible transparency, suggesting an improvement of dispersion of rare-earth ions in these glasses. The aluminum codoping increases the photoluminescence efficiency in glasses doped with Nd 3+ ions.

10917free path imposed by the structure, and thereby the rough physical characteristics of the free volume. All the results (both chemical shifts and relaxation rates) indicate there is a discontinuity in the surface properties at around [All, -2. We believe this expresses a nonrandom distribution of aluminum atoms in the lattice, in agreement with other observation of similar materials.Acknowledgment. Springuel-Huet, M. A.; Fraissard, J. Zeocat 90: Catalysis and Adsorption by Zeolites; Elsevier: Amsterdam; 1991; p 129, and refmnccs therein.( 5 ) Alexander, S.; Coddington, J. M.; Howe, R. F.(1 1) The vinal coefficients were converted to a common basis using a value for the amagat, the density of an ideal gas at STP, of 2.69 X IOl9 atoms an-'.Ultrafine Ti(1V)-doped a-FeOOH particles were dispersed on the high-surface-area carbon fibers of uniform micropores in order to elucidate the role of the surface electronic factor in the micropore filling of supercritical NO. EXAFS and XANES showed that dispersed Ti-doped a-FeOOH particles are ultrafine and have the same local structure as that of bulk a-FeOOH particles. The microporosity of the samples did not change significantly by the doping of Ti(1V) in dispersed a-FeOOH on the basis of Nz adsorption. Also, the surface acidic site concentration measured by the irreversible NH3 adsorption was almost constant regardless of the Ti doping. However, doping of Ti(1V) in the dispersed a-FeOOH enhanced the micropore fdling of supercritical NO by 40%, at the maximum. The enhancement of the NO micropore filling by Ti doping was presumed to come from the increase of quasi-free electrons in the dispersed a-FeOOH fine particles due to mixed valence formation, which is enhanced by the surface segregation of Ti(1V) from the XPS examinations.

When the Mg2+/Fetotal molar ratio in the initial aqueous suspension is below 0.1 at pH 9.0 and 65 °C, almost all Mg2+ is incorporated into the spinel type ferrite by the air oxidation of Fe(OH)2. The chemical composition of the Mg-bearing ferrite thus obtained is not stoichiometric, and is expressed as the MgFe2O4–Fe3O4–γ-Fe2O3system. When the Mg2+/Fetotal ratio exceeds 0.1, the excess Mg2+ produces a basic magnesium sulfate which has a composition of [Mg1.002+, Na0.64+] [OH1.36−,(SO42−)0.64]. At pH 8.0, a small amount of the Mg2+ is incorporated into the ferrite, between pH 9.0 and 10.0 only the Mg-bearing ferrite is formed, and at pH 11.0, the α-FeOOH type compound is formed together with the Mg-bearing ferrite.

Al codoping can improve the poor solubility of rare-earth ions in silica glasses. However, the mechanism is not well understood. The coordination structure around Nd ions in sol-gel-derived Nd-Al-codoped silica glasses with different Al content was investigated by optical and pulsed electron paramagnetic resonance spectroscopies. Both tetrahedral AlO4 and octahedral AlO6 units were observed around Nd ions as ligands. The average total number of these two types of ligands for each Nd(3+) ion was ∼ 2 irrespective of Al content and was larger by 1-2 orders of magnitude than that calculated for a uniform distribution of codopant ions (∼ 0.08-0.25). With increasing Al content, AlO4 units disappeared and AlO6 units became dominant. The preferential coordination of AlOx (x = 4, 6) units to Nd ions enabled the amount of Al necessary to dissolve Nd ions uniformly in silica glass at a relatively low temperature of 1150-1200 °C to be minimized, and the conversion of AlO4 units to AlO6 units around Nd ions caused the asymmetry of the crystal field at the Nd sites to increase and the site-to-site distribution to decrease.

A cosolvent-free sol–gel method yields monolithic silica glasses containing (Tb,Ce)PO4 nanocrystals, free from Rayleigh scattering, and exhibiting ultraviolet-induced bright green photoluminescence.