Optical Absorption Characteristics of Ni2+ in Mixed-Alkali Borate Glasses

Nickel-containing mixed-alkali borate glasses were prepared and their spectral absorption in the visible and near-ir were measured and used to determine the ligand field strength, Racah parameter, and state of coordination of Ni²⁺ ions. The change in these parameters with alkali content is discussed in terms of mixed-alkali effect. Nickel ions were found to occupy octahedral sites in the 20 mol% (Li₂O+Na₂O) glasses, whereas in the 30 mol% (Na₂O+K₂O) glasses Ni^Z+ ions are believed to occupy both octahedral and tetrahedral sites. In most cases slight deviations were observed which increased with increasing alkali content. The results obtained suggest that the substitution of one alkali for another in alkali borate glasses did not produce gross alterations of the glass network sufficient to induce deviations from linearity in property-composition relations.     

Optical Spectra of 3d Transition Metal Ions in MgO 3·52O3 Spinel

Large gem-quality boules of MgO·3.5A1₂, O₃ spinel, both pure and incorporating controlled amounts of Ti³, V₃₊, Cr³⁺, Mn₂, Fe₂₊, Co²⁺, or Ni₂₊ were synthesized by a modified verneuil technique. Each crystal was characterized by measuring the lattice parameter, chemical composition, and density. Using thin polished slices as specimens, the absorption spectrum of each crystal was measured from 400 to 50, 000 cm⁻¹, and band centers and order-of-magnitude oscillator strengths were calculated from the spectrum for each specimen. For these well characterized crystals, the absorption spectra were interpreted satisfactorily in terms of crystal field theory when the complete individual spectrum as well as the spectrum of the host matrix was considered. On this basis, the oxidation state and coordination of the 3d ion could be determined. Covalent bonding effects were of definite significance in these spinels and caused an inexact correspondence between observed band centers and those predicted by the Orgel diagrams. The ratio Dq (tetrahedral)/ Dg (octahedral) was 0.67 in the 1:3.5 spinel and verified the theoretical relation expressing this ratio in terms of a constant, 4/9, times the fifth power of the ratio of the site radii. Coordination preferences of the incorporated transition metal ions are also discussed.     

Preparation and Optical Properties of Transparent Glass-Ceramics Containing Cobalt(ll) Ions

Transparent glass-ceramics containing ZnAl₂O₄:Co²⁺ and LiGa₅O₈:Co²⁺ crystallites have been prepared by heat treatment of glasses in the zinc aluminosilicate and lithium gallate silicate systems, respectively. Crystalline LiGa₅O₈ was already precipitated in an as-prepared specimen, while ZnAl₂O₄:Co²⁺ precipitated from the glass upon heat treatment. The crystallite size varies from about 5 to 20 nm with increasing heat treatment temperature for both systems, and glass-ceramics containing crystallites of less than about 10 nm are transparent. The low-temperature optical absorption and emission spectra are compared with those of single crystals, indicating that almost all of the Co²⁺ ions replace Zn²⁺ ions in the ZnAl₂O₄ system, while some of the Co²⁺ ions are incorporated into the LiGa₃O₈ system, although the amount of Co²⁺ which remains in the glass matrix is rather large in the latter system.     

Hydrothermal Synthesis of Nanosized CoAl2O4 on ZnAl2O4 Seed Crystallites

ZnAl₂O₄-seeded CoAl₂O₄, with a core-shell structure, has been prepared under hydrothermal conditions when the Co²⁺ salt solution is substituted by 10% Zn²⁺ as a precursor. The ZnAl₂O₄ seed is generated during the synthesis process. The seeding process can decrease the synthesis temperature from 245° to 230°C and the particle size from 67 to 20 nm. The process can economize the consumption of Co²⁺ and control the particle size effectively.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

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.     

Crystal Field Studies of Co2+ in α-Zn3(VO4)2

Complete solid solubility was found in the system Cog-(VO₄)₂−α-Zn₃(VO₄)₂. Optical spectra of Co²⁺-containing α-Zn₃(VO₄)₂ samples are discussed in the light of crystal field theory. The calculated and theoretical frequencies were in good agreement for octahedral symmetry in the zinc sites. The crystal field parameter Dq was consistent with the ionic approximation rule. The nephelauxetic ratio did not show any relation to cation-anion distance. Most probably the expanding d-d electron clouds of Co²⁺ interacted.     

Property Variation in Alkali Alkaline-Earth Metaphosphate Glasses

Coefficients of helium migration, coefficients of thermal expansion, and glass-transition temperatures were measured for several series of alkali alkaline-earth phosphate glasses. The helium permeability decreases with the field strength of alkali or alkaline-earth ions in the order Na⁺ > Li⁺ and Mg²⁺ > Ca²⁺ > Ba2+. The glass-transition temperature increases and coefficient of thermal expansion decreases with increasing field strength of alkali or alkaline-earth ions. The relationship of lower molar volume, corresponding to decreased coefficients of helium diffusivity reported for other glass-forming systems, occurs in the phosphate glasses.     

Ferric Iron Species in Alkali Aluminosilicate Glasses During Fusion

The luminescence spectra of Fe³⁺ in alkali aluminosilicate glasses indicate that several species of Fe³⁺ are present during early stages of melting. When the glass has been heat-treated for periods of time on the order of 20 h at 1400°C, the luminescence spectra simplify to that of a single species which is interpreted as due to Fe³⁺ in the glass-forming framework.     

High-Temperature Defect Structure of Cobalt-Doped α-Alumina

Room-temperature optical absorption spectra, electron spin resonance spectra at 15° to 18°K, electrical conductivity, and emf measurements on concentration cells at 1620°C are analyzed and used to determine the defect structure of Codoped α-Al₂O₃. The crystals are mixed ionic and electronic conductors at 1620°C: ionic conduction occurs at 10^-8 atm< p O₂ < 10_-3 atm, with triply charged interstitial Al ions as the major charge carriers, and electronic conduction occurs at 10^-3 atm < p O₂ < 1 atm, with holes as the major charge carriers. A defect model based on the charge compensation of divalent Co at Al sites by triply charged Al interstitials is proposed. The mobilities and activation energy of ions and holes, the oscillator strengths of Co²⁺ and Co³⁺ absorption bands, parameters for electron spin resonance spectra, level positions of substitutional Co²⁺ and an unknown donor, and equilibrium constants for defect formation reactions are determined.     

States of Sulfur in Alkali Borate Glasses

Simple binary alkali borate glasses of differing alkali content (alkali = Li, Na, or K) containing sulfur were prepared and their absorption spectra were measured in the range of 200 to 700 nm. The absorption bands obtained were used to determine the stable states of sulfur in these glasses by directly comparing them with published data. The following states of sulfur were identified and their ranges of stability were determined: the S₂ molecule in glasses containing at least 15 mol% alkali oxide, the S₃⁻ and S₂⁻ in glasses containing 20 to 30 mol% Na₂O or K₂O, and the polysulfide ion (S x ²⁻) in glasses of 30 to 35 mol% Na₂O or K₂O.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Electrochemical Cells and Electrical Conduction of Pure and Doped Al2O3

Data concerning the types of charge carrier responsible for electrical conduction in single-crystal aluminum oxide are reviewed, and explanations are offered for discrepancies in terms of the experimental conditions. Measurements on undoped and Co²⁺ and Mg²⁺ doped crystals, made using a volume guard to avoid surface and gas conduction, are used to describe Al₂O₃ as an ionic conductor with Al interstitial ions as the principal charge carriers at high oxygen activity. A defect model is proposed and the mobility of Al _i ³⁺, the concentration of Al _i ³⁺ in undoped crystals, and the equilibrium constant for defect formation at high temperatures are estimated.     

Spectroscopic Analysis of the Structure and Properties of Alkali Tellurite Glasses

Structural development of tellurite glasses with the addition of Li₂O and Na₂O has been studied using infrared, Raman, and X-ray photoelectron spectroscopies. The increase in intensity of the peak at 755 cm⁻¹ in the infrared spectra as compared to the peak at 620 cm ⁻¹ suggests the transformation of TeO₄ building units to TeO₃ pyramids with the addition of alkali oxide. Proposed structural change is further supported by the strong compositional dependence of the 755-cm⁻¹ peak in the Raman spectra as well as by the formation of a shoulder in the O 1 s peak of X-ray photoelectron spectra. In contrast to alkali silicate glasses, formation of nonbridging oxygens with the addition of alkali oxide is not observed.     

Density Measurements of Glasses in the Series xNa2S + (1 −x)B2S3

Measurements of the densities of glasses in the series x Na₂S + (1 − x )B₂S₃ are reported for the first time. As has been found in the corresponding oxyborate series x Na₂O + (1 − x )B₂O₃, the addition of Na₂S to B₂S₃ causes the density to increase, from 1.80 g/mL for pure B₂S₃ to 1.99 g/mL at the limit of the low-alkali sulfide glass-forming range of x = 0.25. These data provide evidence for the formation of tetrahedral boron units (BS₄⁻) as alkali is added. Volumes for the trigonal BS₃ and tetrahedral BS₄⁻ groups of 57.55 and 51.79 ų, respectively, were determined. As has also been found in the oxyborates, the tetrahedral boron group occupies a smaller volume than the trigonal boron group and causes the increase in density with added Na₂S.     

Effect of Divalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect on the γ-Al₂O₃-to-α-Al₂O₃ phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu²⁺ and Mn²⁺), (2) those with little or no effect (Co²⁺, Ni²⁺, and Mg²⁺), and (3) those with a retarding effect (Ca²⁺, Sr²⁺, and Ba²⁺). The crystalline phase formed by reaction of the additive with γ-Al₂O₃ at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al³⁺ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al³⁺ and O²⁻ in γ-Al₂O₃, suppressing the grain growth of γ-Al₂O₃, and retarding the transformation into α-Al₂O₃.     

Temperature-Induced Structural Modifications Between Alkali Borate Glasses and Melts

High-temperature neutron diffraction and Raman spectra have been obtained on M₂O–2B₂O₃ (M=Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge-compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts.     

Copper ⇌ Alkali Ion Exchange of Alkali Aluminosilicate Glasses in Copper-Containing Molten Salt: II, Divalent Copper Salts, CuCl2 and CuSO4

The ion-exchange mechanism between copper and alkali ions, when 20R₂O · 10Al₂O₃· 70SiO₂ (R = Li, Na, and K) glasses are immersed in divalent copper-containing molten salts in air and nitrogen at 550°C, has been investigated. In molten CuCl₂, the ion-exchange behavior in both air and nitrogen was very close to that in molten CuCl in air reported previously. This is explained by assuming that CuCl₂ decomposes into CuCl and Cl₂ at 550°C and the Cu⁺ ions thus formed mainly diffuse in glasses to replace alkali ions, where Cl₂ acts as an oxidizing agent just like oxygen. In the case of molten CuSO₄─₂SO₄, a small amount of Cu⁺ which is present in the molten state plays a primary role in the Cu ⇌ R⁺ ion exchange process, although the contribution of direct Cu²⁺⇌ 2R⁺ ion exchange cannot be ignored.     

Preparation and Physical Properties of Radiofrequency-Sputtered Amorphous Films in the System AlPO4–TiO2

Amorphous films in the system AlPO₄–TiO₂ were prepared by an rf-sputtering method, and their physical properties, such as density, refractive index, and thermal expansion coefficient, and the infrared absorption spectra were measured. The thermal expansion coefficient increased linearly with increasing TiO₂ content. The results of the molar refractivity and the infrared absorption spectra indicated that the coordination number of titanium ions in these films is higher than that in SiO₂–TiO₂ glasses with a negative thermal expansion, in which Ti⁴⁺ ions are tetrahedrally coordinated. In order to confirm the coordination state of the titanium ions in these amorphous films, titanium K -band emission spectra were obtained by X-ray emission spectroscopy, revealing sixfold coordination. The higher coordination state of Ti⁴⁺ was considered to account for these amorphous films not exhibiting negative thermal expansion, as in the SiO₂–TiO₂ system.     

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.