Tin valence and local environments in silicate glasses as determined from X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn⁴⁺O₆ units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn⁴⁺O₆ and Sn²⁺O₄ sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å, that suggest Sn⁴⁺O₆ units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn⁴⁺O₆ units.     

Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.     

Vanadium contribution in different glasses in view of the ligand field theory

Spectrophotometric and magnetic studies were carried out on different glasses containing known amounts of V₂O₅ in order to throw some light on the valence states and coordination of vanadium in such glasses (in view of the ligand field theory).The results obtained were found to be in agreement with those of aqueous solutions, e.g. [V(H₂O)₆]³⁺, [VO(H₂O)₅]²⁺, and VO²⁺ which showed a slight difference in band positions due to the distortion as well as the difference between the ligand field strength of glass and water. Generally it may be concluded that vanadium is present in borate and silicate glasses in three possible valencies, V⁵⁺, V⁴⁺ and V³⁺, while in phosphate glasses it is generally present in two valencies V⁴⁺ and V³⁺.     

Spectroscopic and structural properties of Cr in silicate glasses: Cr does not probe the average glass structure

Cr³⁺-containing alkali, alkaline earth and mixed alkali-alkaline earth silicate glasses have been investigated using Cr K-edge extended x-ray absorption fine structure (EXAFS) and optical absorption spectroscopy. The local environment of Cr³⁺ appears similar in all glasses regarding EXAFS analysis, in particular for Cr-O distance (1.99 Å). By contrast, optical absorption spectra show variations of crystal field values and disorder effects with the nature of the modifier cation, revealing that some differences exist in the local surrounding of Cr³⁺ in glasses. In addition, in mixed alkali-alkaline earth glasses optical absorption parameters remain close to the values found in binary silicate glasses with the same alkali, which reveals a preference for alkalis in the surrounding of Cr³⁺. As a whole, these data show that Cr³⁺ is not probing the average glass structure and demonstrate its heterogeneous distribution at a local scale.     

Colorimetric study on the redox number of P2O5–PbO–Fe2O3 glasses

Semi-conducting property of lead iron phosphate glasses is assumed to be due to the hopping process. Redox number of glass is usually determined by Mössbauer spectroscopy. However, the technique is limited by low sensitivity in lower concentration of iron ions. Neither is suitable for the traditional method since Fe²⁺ in the glass is apt to oxidize and results in the uncertainty of the concentration. In this paper, a colorimetric method was performed to study the redox number of the glasses. Sample solutions have been prepared by using a vanadate as an oxidizing inhibitor for the equilibrium of Fe²⁺/Fe³⁺ pairs. The V⁵⁺/V⁴⁺ couple, which is strongly acid-dependent, oxidizes the Fe²⁺ to Fe³⁺ in the highly acidic solution, and it is regenerated when the solution was adjusted to pH 5 from V⁴⁺ preservative. The measurement of the electric conductivity of the glasses has indicated the fact that, other than the concentration of the electric conducting carriers, redox number of the glass plays a significant role for the semi-conducting phosphate glasses. The method was validated by analyzing a mixture of ammonium iron (II) sulfate and/or ammonium iron (III) sulfate solutions and has been proved to be simple, accurate and reliable.     

Oxidation state and local environment of selenium in alkali borosilicate glasses for radioactive waste immobilisation

The oxidation state and local environment of selenium in alkali borosilicate glasses for high level radioactive waste (HLW) immobilisation were studied by Se K-edge XANES and EXAFS spectroscopy. An inactive surrogate of the UK's “MW” HLW-loaded glass and two waste-free glasses were investigated. Results confirm that the predominant Se oxidation state in all air-melted glasses is Se⁴⁺. Low levels (< 10% each of Se⁰ and Se⁶⁺) were also detected in the simulated HLW-loaded glass. The presence of Se⁶⁺ is consistent with moderately oxidising melting conditions arising from decomposition of nitrates in the waste. Results also suggest small but measurable Fe-Se redox interactions. Imposed atmospheres of air, N₂, or H₂ during melting resulted in increasingly reduced average Se valence as expected. Linear combination XANES fitting to quantify the relative abundances of these species was restricted by the limited availability of appropriate XANES standards for reduced Se species. EXAFS of all air-melted glasses provided robust fits indicating Se-O bond lengths of 1.71 ± 0.1 Å and CN = 3 ± 0.3, consistent with Se⁴⁺ being present in SeO₃²⁻ selenite groups. Therefore, ⁷⁹Se in UK alkali borosilicate HLW glasses is expected to occur predominantly as Se⁴⁺ in SeO₃²⁻ selenite groups.     

Chromium phase behavior in a multi-component borosilicate glass melt

This paper reports the phase behavior of a multi-component borosilicate glass melt with 0–3 mass% Cr₂O₃ at 800–1500 °C in equilibrium with air. Both upper and lower liquidus temperatures were observed. When the temperature was between the upper and lower liquidus temperatures, eskolaite (Cr₂O₃) formed in melts with >2 mass% Cr₂O₃. Below the lower liquidus temperature, a dispersed chromate phase appeared in the melt that eventually became macroscopically segregated. The chemical durability of the glasses was virtually unaffected by chromium concentration. The particular glass studied was prototypic for vitrifying high-Cr high-level radioactive wastes stored in underground tanks at the Hanford site. The results suggest a significant potential cost benefit for Hanford tank waste cleanup through increased waste solubility in glass.     

Dehydration of Er-doped phosphate glasses using reactive agent bubble flow method

This paper investigated on the dehydration of Er³⁺-doped phosphate glasses using O₂ + CCl₄ bubble flow method. The influence of parameters of dehydration effect was studied systemically. It was found that the dehydration rate was rather rapidly at the initial stage of bubbling. Increasing gas flow rate and temperature could drive the dehydration process toward lower OH concentration in the melts. The dehydration process was carried out in open system and analyzed by thermodynamic theory of open system, which clarified the process of eliminating of OH groups from the glass melts with bubbling time and gas flow rate. A time-dependent empirical equation about reaction rate constant was derived, and the relationships between rate constant and bubbling flow rate, and temperature were also discussed. This analytical method could be applicable to the dehydration of glass melt using other dehydration agents.     

Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ ‐ irradiation

Recharging processes of chromium ions were investigated for Mg₂SiO₄:Mg, Cr single crystals using annealing in O₂ and in air and γ‐irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg₂SiO₄ :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O₂‐ vacancy existing in the crystal, but also to the external field such as γ‐irradiation. The additional absorption after γ‐irradiation shows the decrease in intensity of the absorption of Cr³⁺ and Cr⁴⁺ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr³⁺ and Cr⁴⁺. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr⁴⁺ giving rise to Cr⁴⁺ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ‐irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ‐irradiation suggests that γ‐irradiation ionizes only Cr ions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Line width measurements of Cr in a zinc borate glass

The temperature dependence of the homogeneous line width (Γ_h) of the R-line of Cr³⁺ in 4ZnO·3B₂O₃ glass in the region between 15 K and room temperature has been measured. We found that Γ_h has a T ² dependence down to approximately 30 K. Below 30 K the data do not follow the power law, T ². The results are compared with those of Cr³⁺ in mullite and ED2 silicate glasses where the crossover was observed at about 80 K and with the data for Eu³⁺ in zinc borate and other glasses. The comparison indicates that the local environment affects Γ_h. In fact, the Cr³⁺ ions appear to form more defined complexes with the oxygen ligands than do the Eu³⁺ ions, which have a larger distribution of sites in the glass.     

Redox freezing temperature and Bartenev equation in a 25Na2O-15B2O3-60SiO2 glass doped with chromium and manganese

In a melt with the base mol% composition 25Na₂O-15B₂O₃-60SiO₂, doped with chromium and manganese, a redox reaction takes place during cooling the melt. This reaction was studied using high temperature UV-vis spectroscopy. Above 600 °C, the reaction is in equilibrium and shifted during cooling to the Cr³⁺ and Mn³⁺ species. At temperatures between 500 and 600 °C, the kinetics of the redox reaction is decisive and the cooling rate plays an important part. At temperatures < 500 °C, the reaction is frozen in. The smaller the cooling rate, the smaller is the Cr⁶⁺ concentration and the lower is the fictive redox temperature.The kinetics of the reaction was described by a differential equation assuming Arrhenian behaviour. The equation was numerically solved and fictive temperatures were calculated. These temperatures depended on cooling rate similar to Bartenev equation. Activation energies calculated hereof were around 38 kJ?mol⁻¹ larger than those inserted into the kinetic equation. The experimentally determined activation energy is 565 kJ?mol⁻¹, a value much larger than the activation energies of diffusion of the polyvalent elements. The rate determining step in the case of the Cr³⁺/Cr⁶⁺/Mn²⁺/Mn³⁺ system is the electron transfer reaction, because a notable structural rearrangement is necessary during the course of the electron transfer reaction (Cr³⁺ and Cr⁶⁺ occur in octahedral and tetrahedral coordination, respectively). The latter leads to a large inner reorganisation energy and to an activation energy similar to that of the viscous flow. In the case of the redox reaction between copper and arsenic, the activation energy is much smaller (210 kJ?mol⁻¹), because here the coordination numbers do not change during the course of the redox reaction.     

State of the chromium ion in soda silicate glasses under various oxygen pressures

Optical absorption and ESR spectra of chromium in soda silicate glasses were measured to characterize the electronic environment of the chromium ion in these glasses. Glasses produced in very strongly reducing conditions showed a broad optical absorption in the wavenumber range of 10 000–25 000 cm^?1 without any ESR absorption, which suggested the formation of Cr²⁺ ions. Glasses produced in air or moderately reducing conditions showed ESR signals suggesting that there are three different states of Cr³⁺ ions, the strongly coupled Cr³⁺ ion pairs, the weakly coupled Cr³⁺ ion pairs and the isolated Cr³⁺ ions in elongated tetragonal sites or sites with lower symmetry. The presence of Cr⁵⁺ ions in glasses produced in air was also suggested. It was indicated that the critical partial oxygen pressure of the formation of Cr²⁺ ions is in the vicinity of P_O2 = 10^?8 atm and that Cr²⁺ ions do not coexist with Cr³⁺ ions homogeneously in soda silicate glasses.     

Magnetic behavior and microstructure of x%V2O5·(100−x)%As2O3 glasses

The magnetic properties and microstructure of x%V₂O₅·(100−x)%As₂O₃ glasses with x varying in the range 40 to 90 mol% were investigated in order to elucidate their magnetic ordering. Weak antiferromagnetic interactions between V⁴⁺ ions were observed. Glasses with x ⩾ 60 separated into two glassy phases. The effect of microstructure on magnetic properties of these glasses was investigated. Phase separation increases with increasing V₂O₅ content and produces a broadening of the EPR line width of glasses with high vanadium content (x > 70). The c = V⁴⁺/V_total ratio of x%V₂O₅·(100−x)%As₂O₃ glasses, determined from EPR and chemical analysis, are considerably greater than those usually reported for x%V₂O₅·(100−x)%P₂O₅ glasses.     

A square-wave voltammetric study on the diffusivities of polyvalent elements in CaO/BaO/Al2O3/SiO2 glass melts

Diffusion coefficients of various polyvalent ions (Sn²⁺, As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Cr³⁺, Ti⁴⁺, V⁴⁺, V⁵⁺ and Fe³⁺) were measured in melts with the basic compositions of 10CaO·10 BaO·10Al₂O₃·70SiO₂ and 10CaO·10BaO·15Al₂O₃·65SiO₂ by means of square-wave voltammetry. At temperatures in the range of 1300-1600 °C, linear correlations between logD and 1/T were observed. At 1400 °C, the diffusion coefficients obtained are compared with those obtained from other glass melt compositions.     

Ternary Colquiriite Type Fluorides as Laser Hosts

The Cr³⁺:LiCaAlF₆ (Cr:LiCAF) and Cr³⁺:LiSrAlF₆ (Cr:LiSAF) laser host crystals can be grown by the Czochralski technique. Problems result from the evaporation of lithium-aluminium-fluorides during the growth process and a corresponding stoichiometry shift of the melt towards the CaF₂ (SrF₂) corner of the ternary concentration triangle. Both Cr:LiCAF, and to a lower extend also Cr:LiSAF, contain micron-sized defects that disturb the lasing efficiency heavily. The defects are assumed to be related to water and/or oxygen impurities within the starting materials and growth atmosphere and to peculiarities of the LiF – CaF₂ – AlF₃ ternary phase diagram.     

The structural role of Zr within alkali borosilicate glasses for nuclear waste immobilisation

Zirconium is a key constituent element of High Level nuclear Waste (HLW) glasses, occurring both as a fission product and a fuel cladding component. As part of a wider research program aimed at optimising the solubility of zirconium in HLW glasses, we have investigated the structural chemistry of zirconium in such materials using X-ray Absorption Spectroscopy (XAS). Zirconium K-edge XAS data were acquired from several inactive simulant and simplified waste glass compositions, including a specimen of blended Magnox/UO₂ fuel waste glass. These data demonstrate that zirconium is immobilised as (octahedral) six-fold coordinate ZrO₆ species in these glasses, with a Zr-O contact distance of 2.09 Å. The next nearest neighbours of the Zr species are Si at 3.42 Å and possibly Na at 3.44 Å, no next nearest neighbour Zr could be resolved.     

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO₂ have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in T_d and/or O_h fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques.It turns out that the various metal ions have the following oxidation states: Ti³⁺ and Ti⁴⁺, V⁴⁺ and V⁵⁺, Cr³⁺ and Cr⁶⁺, Mn²⁺ and Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺. The coordination is 4-fold for Ti⁴⁺ and V⁵⁺ and 6-fold for Mn³⁺ and Cu²⁺. Moreover it has been found that Fe³⁺, Co²⁺ and Ni²⁺ are both 4-fold and 6-fold coordinated and that Cr³⁺ and Cr⁶⁺ oxidation states coexist in the same sample.     

Compositional dependence of ultraviolet fluorescence intensity of Ce in silicate, borate, and phosphate glasses

Fluorescence spectra of Ce³⁺ ions in silicate, borate, and phosphate glasses melted in Ar were measured. The relative fluorescence intensity of Ce³⁺ in the ultraviolet region increased in the order of R = Ba, Ca, Sr, and Mg in the 20Li₂O-20RO-60SiO₂ glass samples and with decreasing BaO content in the BaO-B₂O₃ glass samples, respectively. In contrast, the relative fluorescence intensity of Ce³⁺ did not change with varying the glass composition in phosphate glass samples. The compositional dependence of the relative fluorescence intensity of Ce³⁺ is discussed in terms of redox reaction of Ce³⁺-Ce⁴⁺ in oxide glasses.     

Silver valence and local environments in borosilicate and calcium aluminoborate waste glasses as determined from X-ray absorption spectroscopy

Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected and analyzed to characterize silver (Ag) environments in borosilicate and Ca-aluminoborate glass formulations developed as potential candidates for the immobilization of certain nuclear wastes. Silver is found in some nuclear waste streams and must be encapsulated in glass during waste vitrification processes. A related concern deals with phase separation within these glasses and whether colloidal silver would be present in the glass melt, which could present processing issues, or in the waste glass product. Characterization of the silver environments provides useful information for optimizing the silver incorporation ability of such glasses. Data were also gathered on four crystalline standards: Ag-foil, Ag₂O, argentojarosite (AgFe₃(SO₄)₂(OH)₆), and AgO. XANES data indicate Ag⁺ as the dominant species in the glasses. XANES and EXAFS data show that the average Ag environment in the Ca-aluminoborate glass is different compared with those in the two borosilicate glasses investigated. EXAFS analyses show that Ag in the borosilicate glasses is coordinated by two oxygens in a similar environment to that in crystalline Ag₂O, except that the associated Ag–O distances are approximately 0.10 Å longer in the glass. Silver in the Ca-aluminoborate glass may be within one highly disordered site, or possibly, several different sites, where the average Ag–O distance, coordination number, and Debye–Waller factor are larger than those determined for the borosilicate glasses. Despite their relatively high silver contents, there is no evidence from XANES or EXAFS of colloidal silver in the glasses investigated.     

The study of interactions between iron and vanadium ions in semiconducting barium-vanadate glasses doped with Fe2O3

Semiconducting barium-vanadate glasses doped with Fe₂O₃ ranging from 0.1 to 10 wt% were studied. We made attempts to understand features of an incorporation of the impurity ions into the host matrix. EPR, magnetic susceptibility, dc-conductivity and the Mössbauer effect were investigated.It was established that iron entered into the host as Fe³⁺·Fe³⁺ and V⁴⁺ ions formed associates coupled by dipole-dipole interactions for low Fe₂O₃ contents in the glass. The V⁴⁺?Fe³⁺ and Fe³⁺?Fe³⁺ pairs co-existed for all glasses. The contribution of Fe³⁺?Fe³⁺ interactions increased with increasing Fe₂O₃ content. The deviation from paramagnetic behaviour was observed for glasses with 8–9 wt% Fe₂O₃. It was attributed to presence of fine crystalline magnetic particles.The iron impurity induces no considerable changes in the dc-conductivity of the glass. The concentration dependence of dc-conductivity exhibits a minimum of about 5–6 wt% Fe₂O₃.