Determination of the Fe2+/Fe3+ Ratio in Nuclear Waste Glasses

The Fe²⁺/Fe³⁺ ratios of 47 simulated nuclear waste glass samples with ratios varying from 0.01 (oxidized) to 1.6 (reduced) were determined by wet-chemical and Mössbauer spectral analyses. The wet-chemical method involved the spectrophotometric determination of Fe²⁺ and total iron using remote spectroscopy with fiber optic chemical sensing. Interferences from other species present in these glasses were examined and alternative analytical techniques were investigated. Results of wet-chemical and Mössbauer spectral analysis were comparable; however, the wet-chemical method is probably preferable for the analysis of highly radioactive glasses until such glasses have been shown to have satisfactory Mössbauer spectra.     

The Iron-Oxygen Equilibrium in Glass: Effect of Platinum on the Fe2+/Fe3+ Equilibrium

A number of experiments were performed on iron-containing sodium disilicate melts in air. It was found that it was not possible to obtain an equilibrium between Fe²⁺ and Fe³⁺ in platinum crucibles owing to the reaction between platinum and iron, whereas in alumina crucibles the equilibrium was rapidly established. Thermodynamic calculations of the reaction 2FeO (in Na²O-2SiO²) +½O² (g) = Fe²O³ (in Na²O-2SiO²) showed that the equilibrium went more and more to the right with increasing temperature. The standard free energy, enthalpy, and entropy for the reaction were calculated.     

Occurrence of Ti3+ and Fe2+ in Mullite

Electron paramagnetic studies showed that Ti³⁺ and Fe²⁺ occur in mullites taken from a refractory material which was fused-cast under a reducing atmosphere. Exposure of the mullite samples to temperatures >1600°C caused oxidation of Ti³⁺ and Fe²⁺ to Ti⁴⁺ and Fe³⁺, respectively.     

Influence of Fe3+/Fe2+ Ratio on the Crystallization of Iron-Rich Glasses Made with Industrial Wastes

The influence of the Fe³⁺/Fe²⁺ ratio on the crystallization of iron-rich glasses was investigated in this study. The glass batches were made from two hazardous industrial wastes: mud (goethite and jarosite) originating from the zinc hydrometallurgical process and electric arc furnace dust (EAFD). Glass compositions were prepared by adding different percentages of carbon powder. The crystallization process was investigated by a combined thermogravimetry/differential thermal analysis technique, in air or nitrogen atmospheres, using powder and bulk glass samples. The crystalline phases formed, i.e., pyroxene and spinels, and their relative ratio were determined by X-ray diffractometry. The experimental results indicated that melting temperature and crystallization behavior were influenced by the initial Fe³⁺/Fe²⁺ ratio and by the amount of carbon added to the glass batch. For goethite and jarosite glass compositions, decreasing the Fe³⁺/Fe²⁺ ratio increased the crystallization rate by favoring magnetite formation. For EAFD glass compositions, the addition of carbon to the batch inhibited chromite–magnetite spinel formation and favored the attainment of an amorphous glassy phase.     

Polymorphism of BaTiO3 Acceptor Doped with Mn3+, Fe3+, and Ti3+

The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO₃ has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi_{1− x} Mn _x O_3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO₃ with Fe³⁺ and observations made concerning acceptor doping with Ti³⁺.     

Effect of Fe4+ in the System SrFeO3-SrTiO3

Perovskites of the system SrFeO₃-SrTiO₃ were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe⁴⁺ varied from 72.5% for SrFeO_2.86 to about zero for Sr(Fe_0.1Ti_0.9)O_2.95; the remainder of the iron was in the Fe³⁺ state and electrical balance was achieved by oxygen loss. Sr(Fe_{1- x} Ti _x )O₃ was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10⁻⁸ ohm⁻¹ cm⁻¹ for x = 1.0 to 10⁻² for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe⁴⁺. The Fe⁴⁺ content depended on the heat treatment and atmosphere during formation.     

The Effect of Atmosphere on the Sintering of Fe3O4 and Fe2O3 Powders at Low Temperatures

The sintering kinetics of submicrometer Fe₃O₄ and Fe₂O₃ powders were investigated at 300° to 500°C. Using measurements of the rate of reduction of surface area, the coefficients of surface diffusion on the oxides are estimated for a range of oxygen partial pressures. The surface-diffusion coefficients appear to be independent of P _O2 for magnetite and only slightly dependent on P _O2 for hematite.     

Kinetic Reactions in Quenched MgO:Fe3+ Crystals

Quenched single crystals of trivalent iron-doped MgO were studied using etching and EPR spectroscopy. Quench-rate measurements indicated that a liquid nitrogen quench from 1100°C is not rapid enough to prevent localized association and clustering. The distribution, size, and concentrations of precipitates, clusters, and associates in quenched crystals are critically dependent on both solute concentration and quench rate. At T<600°C, the mobility of iron is reduced as a result of cluster formation. Above 600°C, clusters dissociate, liberating mobile associates and vacancies.     

Nucleation and Growth of Magnesioferrite in MgO Containing 0.9% Fe3+

The initial stage of precipitation of magnesioferrite from a supersaturated solid solution of 0.9 cation % Fe³⁺ in MgO at 500 °C was studied by fitting the magnetization curves to 7000 Oe at low temperatures with the Brillouin function. The averase umber of Fe³⁺ ions in a precipitate particle, increased monotonically with aging time from 9 in the as-quenched condition to 88 after 16 h. The average spin quantum number per Fe³⁺ ion decreased to near its final value at an aging time corresponding to = 50. Thus the volume fraction of precipitate is near the final value when the average particle size is only about three unit cells of magnesioferrite.     

Structural and Optical Characterization of Flower-Like Rutile Nanostructures Doped with Fe3+

Flower-like agglomerates with sizes of 200–400 nm of pure and Fe³⁺-doped TiO₂ with rutile crystalline structure were synthesized by the coprecipitation method. The morphology of the agglomerates was determined by electron microscopy (TEM and HRTEM). TiO₂ agglomerates consist of nanorods with clearly visible crystalline faces, parallel to the axis of elongation whose direction was along the [101] direction of pure TiO₂ and the [111] direction of doped TiO₂. Furthermore, nanorods consist of "chains" of spherical particles, most likely interconnected through the so-called oriented attachment or grain-rotation-induced grain coalescence (GRIGC) process. UV/Vis reflection measurements revealed that the absorption of pure TiO₂ was significantly shifted from UV toward the visible spectral region upon the incorporation of Fe³⁺ into the TiO₂ host.     

On the Formation of Fe2+ in the System MgO-Fe2O3-MgFe2O4 at High Temperatures

The formation of ferrous iron in the system MgO-Fe₂O₃MgFe₂O₄ is of interest in connection with its deleterious effect on the microwave performance of magnesium ferrite, MgFe₂O₄. The partial reduction of ferric iron in the system at relatively low temperatures is discussed in terms of the preferential diffusion of iron, and its partial stabilization as ferrous iron from lattice energy considerations.     

Hyperfine Interactions of cis and trans Octahedral Fe2+ Sites in the Layered Silicate Annite

Spin-polarized first-principles calculations based on density functional theory were performed for 23- and 101-atom embedded clusters representing annite. Hyperfine interactions and magnetic moments were obtained and compared with experimental Mössbauer spectra and SQUID magnetometry measurements. It was found that the electric field gradients and the magnetic fields are profoundly affected by the position of the hydroxyls. The calculated magnetic moments are close to 4 μ_B in both octahedral sites.     

Upconversion Luminescence in Er3+ Doped and Yb3+/Er3+ Codoped Yttria Nanocrystalline Powders

Y_1.9Er_0.1O₃ and Y_1.7Yb_0.2Er_0.1O₃ nanocrystalline powders were prepared via a reverse-strike coprecipitation method using nitrates and ammonia as raw materials. The obtained powders were of cubic-phase structure of Y₂O₃ and the particle size was in the range of ∼60–80 nm. Strong red (⁴F_9/2–⁴I_15/2) and green (²H_11/2/⁴S_3/2–⁴I_15/2) upconversion luminescence were observed in all the samples when excited with a 980-nm continuous wave diode laser. The possible upconversion mechanisms in Y_1.9Er_0.1O₃ and Y_1.7Yb_0.2Er_0.1O₃ were discussed. Power studies indicated that two-photon processes are responsible for the green and red upconversion luminescence in these systems. The codoping of Yb³⁺ greatly enhanced the red (⁴F_9/2–⁴I_15/2) upconversion emission.     

Diffusion of Co2+ and Ni2+ in Magnesium Oxide

Rates of diffusion of Co²⁺ and Ni²⁺ ions in the MgO lattice were determined by a micrographic method. The energies of activation for the transport process were 65 and 36 kcal per mole, respectively. The penetration curve in these systems exhibited a plateau followed by a sudden decrease in concentratior.     

Gamma-Induced Absorption Bands Associated with Tl+, Pb2+, or Bi3+ in Potassium Borate Glasses

Gamma irradiation of potassium borate glasses containing about 1.8 mole % Tl⁺, Pb²⁺, or Bi³⁺ resulted in two induced absorption bands which the authors call T and L bands. The T band was observed at about 1.1, 1.55, or 1.8 ev, and the L band was observed at about 1.8, 2.6, or 2.7 ev in alkali borate glasses containing Tl⁺, Pb²⁺, or Bi³⁺, respectively. The shifting of the band positions to higher energies was attributed to the increasing polarizing power of these ions as in the case of the F band induced in alkali halides. The effect of the addition of cerium on the intensity of the T and L bands induced in glasses containing lead or thallium and prepared under different melting conditions suggested strongly that these two bands are associated with electron trap centers near these ions.     

Lattice Parameters, Ionic Conductivities, and Solubility Limits in Fluorite-Structure MO2 Oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] Solid Solutions

Changes in the lattice parameters of fluorite type MO₂ oxides (M = Hf⁴⁺, Zr⁴⁺, Ce⁴⁺, Th⁴⁺, U⁴⁺) due to the formation of solid solutions can be predicted by proposed empirical equations. The equations show the generalized relationship between dopant size and ionic conductivity in the binary systems of these oxides, illustrating that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. The solubility limit of the same periodic group elements in fluorite-structure MO₂ oxides decreaes linearly with the square of Vegard's slope for each solute as determined from the proposed equations.     

Optical Spectra of Cu2+ and Ni2+ in Intermediate Aluminate Spinels

Optical absorption spectra were determined for well characterized intermediate spinels, NiAl₂O₄ and CuAl₂O₄, equilibrated over a range of temperatures. The absorption spectra were interpreted satisfactorily in terms of crystal field theory and were consistent with results of detailed X-ray analyses. Covalent bonding caused some band shifts and changes in the ratio Dq (tetr. coord.)/ Dq (oct. coord.) and the net stabilization energies of the transition metal cation.