Point Defects and Chromium(IV) Formation Mechanism in Gallia- and Alumina-Based Oxide Glasses

Point defects were found in as-quenched GeO₂, 65CaO35Al₂O₃, and 65SrO35Ga₂O₃ glasses on the basis of electron paramagnetic resonance (EPR) measurements. These defects were identified as Ge É centers in GeO₂ glass and O^-₂, O^-₃, and M-OHC (oxygen hole center) (where M = Al, Ga) in 65CaO35Al₂O₃ and 65SrO35Ga₂O₃ glasses. The formation of Ge É centers in as-quenched GeO₂ glass was due to the thermodynamic stability of GeO at the melting temperature. The latter oxygen-excess defects are supposed to be formed by excess oxygen ions derived from the modifiers in the aluminate and gallate glasses during the formation of these glasses. To investigate some of the properties of the oxygen-excess defects in the calcium aluminate and strontium gallate glasses, chromium ions were doped in these glasses as a probe and the relationship between the valency state of the chromium ion and the defects was determined. We conclude that the peroxy bonding (-O-O-) oxidizes the Cr³⁺ species to Cr⁴⁺. Similar defects have been identified in host compounds that are used for Cr⁴⁺ tunable lasers. These results reveal that the point defects are necessary to stabilize the Cr⁴⁺ ions in glasses and crystals.     

Reactions and Bonding of Sodium Disilicate Glass with Chromium

Reactions and reaction mechanisms occurring at sessile drop interfacial regions of sodium disilicate glass on chromium were identified at 1000°C and an ambient atmosphere of 2.7 × 10⁻⁴Pa with a p(O₂) of 1 × 10⁻¹⁵ and 1 × 10⁻⁵ Pa. Depending on the experimental conditions, the principal reactions were redox reactions of Cr⁰ with Na⁺ to form Cr²+ and Na⁰, Cr²+ with Na⁺ to form Cr³+ and Na⁺, Cr⁰ with Si⁴+ to form CrSi_x, alloy dendrites and Cr²⁺, and Cr²⁺ with Si⁴⁺ to form Cr³⁺ and SiO(gas). Adherence was developed when the interfacial region was saturated with Cr³⁺, i.e., Cr₂O₃.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Redox Equilibria of Multivalent Ions in Silicate Glasses

Experimental studies were made on the compositional dependence of the redox equilibrium of Eu in synthetic silicate liquids, together with an empirical model describing the observed compositional dependence. Electron paramagnetic resonance (EPR) was used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in various glasses formed by rapidly quenching silicate liquids. The compositional field studied comprised mixtures of SiO₂, TiO₂, Al₂O₃, CaO, MgO, and Na₂O. The proposed model describes the Eu²⁺/Eu³⁺ ratio over the entire compositional field in terms of parameters easily related to each glass composition. The general applicability and utility of the model is further demonstrated by its application to the Fe²⁺-Fe³⁺, Ce³⁺-Ce⁴⁺, and Cr³⁺-Cr⁶⁺ redox reactions in binary alkali oxide silicate glasses of Li, Na, and K.     

Optical and Magnetic Properties of Some Transition Metal Ions in Barium Phosphate Glass

In barium phosphate glass containing first-row transition metal ions, the ions appear to possess normal oxidation states: Cr³⁺, Mn³⁺, Fe^2+,3+, Co²⁺, Ni²⁺, and Cu²⁺. Room temperature and 77°K spectra for Cr³⁺-, Mn³⁺-, Fe^2+,3+-, Ni²⁺-, and Cu²⁺-containing glasses can be interpreted, on the, basis of octahedrally coordinated metal ions, whereas the spectrum of the Co²⁺ glass correlates with a tetrahedrally coordinated metal ion. Magnetic moments for the glasses are similar to those for the spin-only values of the various metal ions.     

Absorption Spectra of First-Row Transition Metal Ions in Glasses

Certain optical transitions were calculated for the first-row transition metal ions Ti³⁺, V³⁺, Cr³⁺, Fe²⁺ Co²⁺, and Ni²⁺ in a silicate matrix. The calculations were performed at 7 inter-ionic distances for each ion because the metal-ion-to-oxygen interionic distances are unknown; the silicate oxygen charge was varied from 1.00 to 2.00 in each case because the true charge on silicate oxygens is also unknown. Integrals used in the calculations were calculated using Slater and self-consistent field wave functions, and computer programs were used to evaluate all integrals. Equations were derived for calculating all optical transitions for these ions in octahedral fields; the extension to fields of other symmetry is trivial. Several optical transitions were computed to obtain the calculated orders of magnitude for the optical transition. The results for self-consistent field wave functions are lower than expected; they indicate that the transition metal ion in glasses is situated in fields near the size of aquo complexes.     

Stannous-Stannic Equilibrium in Molten Binary Alkali Silicate and Ternary Silicate Glasses

The stannous-stannic equilibrium in binary alkali silicate and ternary silicate glasses was studied by equilibrating glassmelts with air at 1400°C. The Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the oxidized state with increasing ionic radii of the alkali ions or with increasing concentration of the alkali ions in the same series of glasses. The slope of the straight lines obtained on plotting log (Sn⁴⁺)/(Sn²⁺)( pO₂ )^n/2 vs mol% R₂O increased in the order Li→Na→K. In ternary silicate glasses having the base glass composition 20Na₂O·10RO·70SiO₂, the Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the reduced state, with increasing bond strength between the divalent cations and the nonbridging oxygens. With increasing temperature, the equilibrium shifts more toward the reduced state.     

Compositional Dependence of Absorption Spectra of Ti3+ in Silicate, Borate, and Phosphate Glasses

Absorption spectra of Ti³⁺ were measured for silicate, borate, and phosphate glasses doped with 0.5 mol% Ti₂O₃.The absorption coefficient at the peak wavelength of the ₂T₂→₂E transition of Ti³⁺ is used as a parameter showing the relative content of Ti³⁺ions in glass samples. The effect of glass composition on Ti³⁺/Ti⁴⁺ redox was studied. For multicomponent glasses, a basicity parameter calculated from glass composition is proposed in terms of coulomb force between the cation and the oxygen ion. The value of the absorption coefficient depends on basicity in silicate and borate glasses; however, it is independent of composition in phosphate glasses.     

Electronically Conducting Doped Chromium Oxides

Samples of polycrystalline chromium oxide doped with Li₂O, MgO, NiO, and Ta₂O₅ were prepared by hot-pressing to test for suitability as current collector materials of the sulfur electrode in the sodium-sulfur cell. Density, grain size, dopant uniformity, and resistivity (to 350°C) were measured. X-ray photoelectron spectroscopy (XPS) tends to support the established model of Ni²⁺ on Cr³⁺ sites to explain the compositional dependence of electrical resistivity after air anneal. A higher level of Cr₂O₃ powder purity was required to obtain the low resistivities with the Li₂O dopant than with the MgO or NiO dopant. An observed increase in the bulk resistivity during several months of electrochemical cycling in sodium polysulfidelsulfur melts is attributed to loss of electronic carriers caused by equilibration of those carriers with the low oxygen partial pressure of the melt. Li₂O- and (to a lesser extent) MgO-doped Cr₂O₃ appear to be suitable as container coating materials.     

Ligand-Field Spectra and Structure of Nickel in Silicate Glasses

The spectra of nickel dissolved in silicate and aluminosilicate glasses with alkali and alkaline-earth modifiers were studied systematically as functions of composition, annealing and temperature, concentration of Ni²⁺, and redox conditions during melting. Although reducing conditions produced a gray scattering phase, only Ni²⁺ was observed as a dissolved species. Several glasses contained one of two Ni²⁺ species which corresponded to previously proposed 4- and 6-coordinated species. Therefore, both octahedral and tetrahedral calculations could be made from directly observed bands free of overlapping. The results of these calculations suggest that the postulated tetrahedral species may instead be cubic (8-coordinated). In addition, mixing of the Ni²⁺³ T _{1 g} ( P ) level with Ni²⁺ 4 p and/or ligand electron transfer bands was observed. Examination of the conditions that could produce the 8-fold site on the basis of stereochemical and crystal-chemical principles indicated that the packing requirements of the alkali-oxygen, alkaline-earth-oxygen, and silicate polyhedra may preclude sites other than a large 6-fold site or an even larger cubic (8) site. Thus, silicate glasses can be considered to consist of highly structured but disordered atomic arrangements.     

Compositional Dependence of Judd-Ofelt Parameters in Silicate, Borate, and Phosphate Glasses

Judd-Ofelt parameters Ω _t with t = 2,4, 6 for the rare-earth ions Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in alkali and/or alkaline-earth silicate, borate, and phosphate glasses have been determined. The variations of Ω _t with the number of 4 f electrons of the rare-earth ions are demonstrated, and factors affecting the Judd-Ofelt parameter Ω₆are discussed. The intensity parameter Ω₆ depends on the ionic packing ratio of the glass host by changing modifier type in silicate and borate glasses, and it is independent of that in a series of borate glasses as a function of modifier content and phosphate glasses. The peak wavenumbers of the transitions whose intensities are determined mainly by the Ω₆<| U ⁽⁶⁾|>² term—where <| U ⁽⁶⁾|> is one of the reduced matrix elements—shift systematically with the values of Ω₆ for all the rare-earth ions.     

Chromium-Doped Forsterite Nanoparticle Synthesis by Flame Spray Pyrolysis

Synthesis of chromium-doped forsterite (Mg₂SiO₄:Cr) nanoparticles by flame spray pyrolysis (FSP) was investigated. The morphology, crystalline phase, and photoluminescence of the products were evaluated. Crystalline Mg₂SiO₄:Cr nanoparticles of several 10 nm in diameter were obtained, although a small amount of the submicrometer-sized particles and the unreacted MgO phase existed. The product powder showed electron-spin resonance signals from Cr⁴⁺ and photoluminescence typical for Cr⁴⁺ in Mg₂SiO₄, suggesting that a part of the Cr⁴⁺ ions were incorporated into Si⁴⁺ sites by FSP. On the other hand, the effects of excess SiO₂ addition on the structural and optical characteristics of Mg₂SiO₄:Cr were examined. Addition of excess SiO₂ up to 20 mol% did not influence these characteristics of the products. Further addition of excess SiO₂ (60–100 mol%) enhanced the formation of the amorphous phase and resulted in the emission from Cr³⁺ in the amorphous phase in addition to an emission from Cr⁴⁺ in Mg₂SiO₄.     

Leaching of Lead and Connectivity of Plumbate Networks in Lead Silicate Glasses

The formation of a plumbate network in binary lead silicate glasses was examined based on the leaching behavior of Pb²⁺ in lead silicate glasses over a wide composition region. The effective diffusion coefficient of Pb²⁺ at 40°C was on the order of 10⁻¹⁷ m²/s for PbO<35 mol% glasses, and increased three orders of magnitude for 35–50 mol% PbO contents. Such a steep composition dependence is considered to be because of changes in the medium or longer range structure. That is, it is proposed that the plumbate network forms a percolative 3D network in the composition region to form diffusion paths for the lead ions. In addition, the present results indicate that the lead ion exists as a network former over the entire glass forming composition range of the binary silicate glasses.     

Effect of Codoping on the R-Line Luminescence of Cr3+-Doped Alumina

The frequency of the R-line luminescence from Cr³⁺ ions in aluminum oxide is known to be sensitive to chromium concentration, stress, and temperature. In this work, the effect of simultaneously doping aluminum oxide with both Cr³⁺ and other ions (Fe, Ti, Ni, or Y) on the R-line frequency and intensity at room temperature is investigated. It is concluded that ions soluble in aluminum oxide cause systematic shifts in the luminescence frequency and intensity whereas those ions that are insoluble have no effect on the R-line luminescence. In addition, it is shown that contrary to previous investigations, the ratio of the N1 to R line intensities is not simply related to the chromium concentration and hence cannot be used as a measure of the chromium concentration.     

Infrared Absorption Spectra of Sodium Silicate Glasses from 4 to 30μ

The infrared absorption spectra of sodium silicate glasses of molar composition Na₂O·xSiO₂ (where = 5, 4, 3, and 2) were determined with unpolarized radiation in the 4 to 30μ region. The glass spectra showed a definite similarity to the spectra of α-quartz, with lattice vibrational bands around 1066,786, and 464 cm⁻¹. As x decreased, some bands shifted toward lower frequencies indicating a decrease in the force constant. No new frequencies were found in the silicate glasses which did not have counterparts in α-quartz.     

Segregation of Aliovalent Solutes Adjacent Surfaces in MgO

The segregation of ferrous iron, ferric iron, chromium, and scandium solutes adjacent {100} surfaces of MgO crystals in the single-phase region was studied by ion microprobe mass spec-troscopy. Excess concentration in a region near the surface was found for Fe³⁺, Cr³⁺, and Sc³⁺, but none for Fe²⁺. This segregation is the type predicted as necessary to balance the negative surface charge.     

Electrical Resistivity of Titanium-Containing Glasses

Titanium-containing glasses were prepared by fusion of a base glass (BaO·B₂O₃SiO₂) and TiO₂ and/or Ti₂O₃ in Ar. Their resistivities did not vary with melting time and temperature. Interaction of Ti⁴⁺ and Ti³⁺ in the glasses was deduced by spectroscopy, but the valence states in the batch compositions were preserved in the glasses, according to the chemical analysis. Glasses containing either Ti⁴⁺ or Ti³⁺ had very high resistivities, whereas the glass prepared by melting a mixture of a Ti⁴⁺-containing and a Ti³⁺-containing glass had much lower resistivity. All results confirmed the possibility of controlling the resistivity by batch composition for these glasses.     

Transparent Cr4+-Doped YAG Ceramics for Tunable Lasers

Transparent Cr⁴⁺:YAG (Y₃Al_SO₁₂) ceramics doped with Ca and Mg as counterions and SiO₂ as a sintering aid were fabricated by a solid-state reaction method using high-purity powders of Al₂O₃, Y₂O₃, and Cr₂O₃. The mixed powder compacts were sintered at 1750°C for 10 h in oxygen, or 1750°C for 10 h under vacuum, and then annealed at 1400°C for 10 h in oxygen. Cr-doped YAG ceramics sintered in oxygen had a brown color and characteristic absorption by Cr⁴⁺ ions, whereas these YAG ceramics sintered under different conditions (vacuum + oxygen) had a green color and absorption at ∼590 and 430 nm by Cr³⁺ ions. The absorption behavior of YAG ceramics sintered in oxygen was almost equivalent to that of Cr⁴⁺:YAG single crystals fabricated by the Czochralski method.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Compositional Dependence of Infrared-to-Visible Upconversion in Yb3+- and Er3+-Codoped Germanate, Gallate, and Tellurite Glasses

Upconversion fluorescences of the green ⁴S_3/2→⁴I_15/2 and red ⁴F_9/2→⁴I_15/2 transitions of the Er³⁺ ion are studied for Yb³⁺- and Er³⁺-codoped sodium germanate, potassium tantalum gallate, and barium tellurite glasses by InGaAs laser-diode pumping. The phonon energies of the host glasses are determined by infrared-reflection measurements. Compositional effects on the Judd—;Ofelt parameters for the Er³⁺ ion, the spontaneous emission probability (SPE) of the ²F_5/2→²F_7/2 transition for the Yb³⁺ ion, and the phonon energy of the glass network are discussed in terms of glass structure. The factors that affect the upconversion fluorescence intensities of the Er³⁺ ion are discussed, using the phonon energy of the host glass and the SPE for the Yb³⁺ ion in the germanate, gallate, and tellurite glasses.