B. Dunn

Luminescence of Cu+-β“-alumina

Abstract A series of monovalent copper containing β -alumina single crystals have been prepared by standard ion exchange techniques. The crystals exhibit strong luminescence which we attribute to the 3d 9 4s → 3d 10 interconfigurational transition. The peak wavelength of this transition can be varied by 135 nm by the presence of other cations in the β “-alumina conduction plane, allowing the luminescence to be varied throughout the visible spectrum. Single-pass gain measurements of several of these crystals show gains of the order of 0.1/cm, suggesting the potential use of these materials for solid-state tunable lasers.

Analysis of the absorption spectrum of neodymium:sodium beta double prime alumina

The Judd–Ofelt analysis has been applied to the sigma‐polarized absorption spectrum of Nd3+:NaB‘alumina to determine the modified Judd–Ofelt parameters Ωσ,2, Ωσ,4, and Ωσ,6. A model for the near‐neighbor environment of Nd3+ has been proposed and tested using a simplified point charge model for the crystal field. The model indicates that the observed spectrum may be explained by a large linear field term in the crystal field induced by the shift in position of conduction plane oxygen and neodymium ions away from sites of inversion symmetry.

Structural and Electrochemical Properties of amorphous and crystalline molybdenum oxide aerogels

Abstract The structure and chemical differences among amorphous, crystalline and nanocrystalline molybdenum oxide aerogels were determined using extended X-ray absorption fine structure (EXAFS), Fourier transform infrared (FTIR) analysis and powder X-ray diffraction (XRD). These different forms of the same nominal material are produced by heat treatment. The influence of the structural differences on electrochemical properties was examined using stepped cyclic voltammetry. The most interesting material was the MoO 3 aerogel heated to 300 °C. The material was found to be nanocrystalline; there are no XRD peaks but the EXAFS were virtually identical to orthorhombic MoO 3 . The electrochemcial response of the nanocrystalline material contains characteristics of both the amorphous and crystalline forms, but with better lithium capacity than either one.

Interactions between organic dyes and sol-gel matrices

Abstract The sol-gel process is a chemical route that allows one to prepare amorphous and transparent oxide matrices doped with organic dyes. Luminescence of pyranine-doped aluminosilicate gels was studied during gel formation. The pyranine molecules adsorb on the surface of the oxide particles formed early in the gelation process. The changes in luminescence indicate a strong decrease in the hydrophilic character of the particles during gelation and aging. Absorption studies of rhodamine B- and 6G-doped silica gels show that the initial pH of the gels has a significant effect on the chemistry of the dye. Results suggest that both rhodamine B and 6G are influenced by the electrical charge developed on the silica gel surface.

Preparation and structure of Li-Stabilized Na+ β″-Alumina single crystals

Single crystals of Li-stabilized Na + β″-alumina were grown. LiVO 3 fluxes were used to provide a homogeneous melt and crystals were grown by slow cooling of the melt from 1600 to 1300°C, Single crystal X-ray diffraction indicates that Li substitutes for Al(2) and Al(4) in an ordered fashion.

ESR and optical spectroscopy of Ce3+: β″-alumina

Abstract ESR and optical absorption spectroscopy are reported for Ce 3+ doped β″-alumina and codoped Ce 3+ Nd 3+ -β″-alumina. The doping was achieved by ion exchange techniques. The spectra indicate that the Ce 3+ ions are found at the mid-oxygen sites in the β″-alumina conduction plane. The g -tensor values were found to be g x = 2.58, g y = 2.15, and g z = 0.85. A small second site occupation (less than 1%) is observed from Ce 3+ ions located in the Beevers-Ross sites. In the codoped crystals, it is shown that the site occupations of Ce 3+ and Nd 3+ ions are not changed when both of these ions are present in the same crystal, an important consideration in energy transfer studies.

Spectroscopic studies of Nd3+-exchanged β″ alumina

Abstract Optical absorption spectroscopy and electron spin resonance (ESR) techniques were used to study the site occupations for Nd3+ in β″ alumina. In partially exchanged compositions (3.2 × 1020 Nd/cm3) the Nd3+ occupy midoxygen sites primarily. There is some population of the tetrahedral site; however, the concentration is too low to be clearly observed by ESR. Crystals which were completely exchanged (∼1.8 × 1021 Nd/cm3) exhibited rather different behavior. The spectroscopy results for these materials indicate that there is a combination of both isolated Nd3+ ions and discrete pairs.

Optical properties of β-Ga2O3:Cr3+ single crystals for tunable laser applications

Abstract Single crystals of β-Ga2O3:Cr3+ with various doping levels have been grown by the floating zone technique. In this material, the room temperature fluorescence is dominated by the broad band 4T2→4A2 emission extending between 650 and 950 nm, with a 210 μs lifetime. At 77 K, the 4T2 level is depopulated so that fluorescence originates only from the 2E level with a 2.3 ms lifetime. The temperature dependence of the lifetime indicates that the 4T2 level lies approximately 600 cm-1 above the 2E level. These characteristics are similar to those of alexandrite: Cr3+. Therefore, β-Ga2O3:Cr3+ may be an interesting new material for tunable laser applications.

Preparation and properties of transition metal β″ aluminas

Abstract Ion exchange techniques were used to produce a family of 3d transition metal β″-aluminas by substitution of the sodium ion content in Na + -β″-alumina single crystals. Complete or nearly complete replacement was obtained for Cr 3+ , Mn 2+ , Co 2+ , and Ni 2+ compositions. Optical properties of the crystals were studied and both broad absorption and broad fluorescence bands were observed. Environmental exposure produced detrimental effects over long time periods, suggesting hydration of the conduction planes.

The transport properties and defect chemistry of Na+, K+ and Ag+β″-aluminogallates

Abstract A series of Na+β″-aluminogallate crystals with gallium substitution for aluminum ranging from 30 to 52 at% was grown by flux evaporation. X-ray diffraction analysis confirms that the structure is analogous to the rhombohedral Na+β″-alumina, and the a- and c-axis lattice parameters increase with increasing gallium content. Two-point ac impedance measurements indicate that the Na+β″-aluminogallates are fast ion conductors with conductivities equal to and slightly lower than that of Na+β″-alumina, 0.36 to 1.18 S/cm at 300°C, with comparable activation energies. Despite the increase in c-axis lattice parameter, the conductivities of the Na+ aluminogallates are actually somewhat lower as the c-parameter becomes larger. Conductivities of K+ and Ag+ ion exchanged crystals are also presented.