Vanadyl ion stability in silica gels

Silica gels doped with vanadium were prepared from tetra ethyl orthosilicate and an aqueous solution of NH₄VO₃, at a final sol pH of 1.5–2.5. Absorption and electron paramagnetic resonance (EPR) spectral studies establish that under ambient conditions the incorporated pentavalent vanadium is stabilized as tetravalent vanadyl ion in the gel monoliths. Dried gels were very stable under ambient conditions, however, transformation of V⁴⁺ to V⁵⁺ was noted during gel densification.     

Silica encapsulated hemoglobin and myoglobin powders prepared by an aqueous fast-freezing technique

Hemoglobin and myoglobin were encapsulated in silica gels and powders. Protein encapsulated powders were fabricated via the condensation of silicic acid around the protein, followed by a fast freezing with liquid nitrogen, and subsequent thawing. The fast-freezing technique led to high surface area stable silica encapsulated protein powders. Transmission UV-vis spectroscopy techniques were used to verify that neither protein was damaged during gelling or freezing processes. Both hemoglobin and myoglobin gels and powders retained their biological activity and were able to bind cyano ligands while in the oxidized Fe³⁺ state and carbon monoxy ligands while in the reduced Fe²⁺ state. Kinetics experiments showed that the rates of binding of CO and CN⁻ to the proteins in the silica gel versus a buffer solution are decreased by 30-45%. This result was likely due to mass transfer effects associated with diffusion through the gel network. Hemoglobin/silica powders were successfully stabilized in the Fe²⁺ oxidation state by addition of the amino acid l-cysteine.     

Probing the erbium ion distribution in silica optical fibers with fluorescence based measurements

Accurate determination of the rare earth dopant distribution in optical fibers enhances our understanding of the fiber manufacture process and enables further improvement in the design of fiber based products such as optical fiber lasers and amplifiers. Here a simple theoretical model consisting of an ensemble of rate equation systems, characteristic of the most likely electronic transitions that take place in the vicinity of erbium (Er³⁺) doped silica glasses, is developed and solved. Through this theoretical study it is established that information about the relative Er³⁺ ion distribution in fibers can be inferred by simply monitoring the backscattered fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. Following these theoretical studies a fluorescence intensity confocal optical microscopy (FICOM) scheme was employed to investigate the Er³⁺ ion distribution profiles in a range of silica optical fibers. The validity of the proposed theoretical model was confirmed through a comparison of the Er³⁺ ion distribution profiles acquired using the FICOM technique and those obtained from the application of a powerful analytical ion probe.     

Some features of EPR and optical spectra of vanadium in aluminophosphate glasses

The EPR and optical spectra of vanadium in glasses of ternary Al₂O₃P₂O₅SiO₂ and Al₂O₃P₂O₅B₂O₃ systems have been measured. The results were compared with earlier data for vanadium in binary phosphate, aluminophosphate and silicaphosphate glasses and with results of de-Biasi for V⁴⁺ in crystalline powder α-crystobalite AlPO₄. The superpositions of two hyperfine spectra (ASB-I and ASB-II) were found for many glasses of ternary systems. Both spectra can be attributed to VO²⁺ ions. The intensity ratio of the ASB-II spectrum to ASB-I depends on glass composition but is great (> 7) for all the glasses. Only the ASB-II spectrum was observed in glasses with low concentration of Al₂O₃. The spectral parameters of ASB-II spectrum are g_| = 1.916–1.921; g_⊥ 1.980–1.988; A_| = (188?190) × 10^?4cm^?1 and A_⊥ = (74–77) × 10^?4cm^?1. Three intense bands at 370, 455 and 700 ans 720 nm observed in these glasses can be attributed to V³⁺ ions. The excellent agreement of the parameters of the EPR spectrum of V⁴⁺ ions in crystalline α-crystobalite AIPO₄ and ASB-II spectra in the glasses under study suggest the identical electron structure of the paramagnetic species. This species is believed to be characterized by optical bands at 680 and 790 nm which have been observed by de Biasi. The orbital mixing coefficients indicate strong tetragonal distortion of vanadyl complexes responsible for the ASB-II spectrum. It is assumed that VO²⁺ ions responsible for this spectrum act as modifiers fitting into the relatively small holes of the three-dimensional networks of phosphate glasses containing no cations of large radii. The microscopic basicity of oxygens in such holes must be about 0.48.     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.     

Growth and characterization of Mn2+ doped calcium l‐tartrate crystals

Single crystals of Mn²⁺ doped calcium levo‐ tartrate tetrahydrate (CLTT) were grown by single diffusion gel growth technique in silica hydro‐gel media. The doping of Mn²⁺ was varied by mixing 0.001M, 0.005M, 0.01M, 0.05M, and 0.1M solutions of MnCl₂ with 1M CaCl₂ solution in equal volumes in the supernatant solutions. The actual amount of Mn²⁺ doping in CLTT crystals was estimated by ICP (Inductively Coupled Plasma) technique. The powder XRD of the samples suggested no significant change in the unit cell dimensions and the presence of any extra phase. The FT‐IR spectra indicated the presence of water molecule, O‐H bond, C‐O bond and carbonyl C=O bond. The EPR spectra confirmed the presence of Mn²⁺ ions in the crystals. The variation of the dielectric constant with temperature confirmed the earlier results of pure calcium tartrate crystals and indicated the ferroelectric nature of the doped crystals. As the amount of doping of Mn²⁺ increased the value of dielectric constant increased. The results are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EPR study in the SiO2 system

The EPR spectra of Fe³⁺ and V⁴⁺ ions, and radiation centres have been studied in α-tridimite. Consideration of the EPR data in all crystalline and vitreous forms of the SiO₂ system is made. It is shown that the crystal field strength and symmetry did not change considerably in different polymorphs of silica, the changes in hyperfine structure constants being significant. The relation between electronic structure of vitreous silica and different crystalline forms is established.     

EPR study of defect centers in Zn2InV3O11

A multicomponent vanadate compound Zn₂InV₃O₁₁ has been synthesized and investigated using the electron paramagnetic resonance (EPR) technique. The Zn₂InV₃O₁₁ compound is isostructural with a previously studied magnetic system M₂FeV₃O₁₁ (M = Mg, Zn, Co, Ni). According to the nominal stoichiometry of the Zn₂InV₃O₁₁ compound all ions are non-magnetic, but the registered EPR spectra in the 4–300 K temperature range have revealed the presence of a rich variety of paramagnetic centers involving predominantly monomeric and dimeric units, as well as clusters of V⁴⁺ ions. The most intense EPR component, attributed to the VO²⁺ vanadyl ion in an axial symmetry, has displayed a well resolved hyperfine structure. The presence of a complicated spectrum at low magnetic field (g  4.4) and a broad line at g  2 has confirmed the existence of vanadium dimers. A spectral feature attributed to vanadyl spin clusters has been observed at temperatures above 80 K in the 270–310 mT magnetic field range. The role of oxygen deficiency in the appearance of the observed magnetic centers has been discussed. Comparison with the previously studied Mg₂InV₃O_11?δ compound has been made and the influence of specific cations on magnetic defect centers has been considered.     

Magnetic resonance study of the crystallization behavior of InF3-based glasses doped with Cu2+, Mn2+ and Gd3+

The crystallization of fluoroindate glasses doped with Gd³⁺, Mn²⁺ and Cu²⁺ heat treated at different temperatures, ranging from the glass transition temperature (T_g) to the crystallization temperature (T_c), are investigated by electron paramagnetic resonance (EPR) and ¹⁹F nuclear magnetic resonance (NMR). The EPR spectra indicate that the Cu²⁺ ions in the glass are located in axially distorted octahedral sites. In the crystallized glass, the g-values agreed with those reported for Ba₂ZnF₆, which correspond to Cu²⁺ in a tetragonal compressed F⁻ octahedron and to Cu²⁺ on interstitial sites with a square-planar F⁻ co-ordination. The EPR spectra of the Mn²⁺ doped glasses exhibit a sextet structure due to the Mn²⁺ hyperfine interaction. These spectra suggest a highly ordered environment for the Mn²⁺ ions (close to octahedral symmetry) in the glass. The EPR spectra of the recrystallized sample exhibit resonances at the same position, suggesting that the Mn²⁺ ions are located in sites of highly symmetric crystalline field. The increase of the line intensity of the sextet and the decrease of the background line in the thermal treated samples suggest that the Mn²⁺ ions move to the highly ordered sites which contribute to the sextet structure. The EPR spectra of the Gd³⁺ doped glasses exhibit the typical U-spectrum of a s-state ion in a low symmetry site in disordered systems. The EPR of the crystallized glasses, in contrast, have shown a strong resonance in g ≈ 2.0, suggesting Gd³⁺ ions in environment close to cubic symmetry. The ¹⁹F NMR spin–lattice relaxation rates were also strongly influenced by the crystallization process that takes over in samples annealed above T_c. For the glass samples (doped or undoped) the ¹⁹F magnetization recoveries were found to be adjusted by an exponential function and the spin–lattice relaxation was characterized by a single relaxation time. In contrast, for the samples treated above T_c, the ¹⁹F magnetization-recovery becomes non-exponential. A remarkable feature of our results is that the changes in the Cu²⁺, Mn²⁺, Gd³⁺ EPR spectra and NMR relaxation, are always observed for the samples annealed above T_c.     

Synthesis by sol–gel process,characterization and catalytic activity of vanadia–silica mixed oxides

This paper describes a new route for the preparation of V₂O₅–SiO₂ mixed oxides with high vanadium content and high surface area by the sol–gel processing method. Dry samples were characterized by powder X-ray diffraction, electron paramagnetic resonance, Fourier-transform infrared, and energy dispersive X-ray spectrometry. Our results show that the incorporation of vanadium pentoxide into the silica matrix was obtained by intimate mixing of the two different inorganic polymers (Si–O and V–O based polymers), and the overall structure is held together by a Si–O–Si network interpenetrated with V–O–V polymeric chains. The catalytic activity of the V₂O₅–SiO₂ mixed oxides was evaluated in the oxidation of the cyclooctene and styrene in liquid phase.     

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn²⁺-doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm⁻¹ showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn²⁺ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn²⁺ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm⁻¹), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).     

Fe3+ doped SiO2 nanostructured gel-glasses: Structural,optical and magnetic properties

Transparent crack-free and bubble-free Fe³⁺ doped SiO₂ nanostructured gel-glasses were obtained by the sol–gel process. The process involves the hydrolysis and condensation of an appropriate molar ratio of tetraethoxysilane (TEOS), absolute ethanol, nitric acid and ferric nitrate, followed by stepwise annealing at temperatures ranging from 110 °C to 1000 °C. The structural variation of the gel-glasses and their influence on physical properties during annealing has been studied. It has been observed that monolithicity and chemical environment around Fe³⁺ in the gel-glasses are strongly dependent on the annealing temperatures. The colour of gel-glass samples is different for different annealing temperatures, mainly due to the different co-ordination state of Fe³⁺ and the generation of Fe₂O₃ colloids of size 20–60 nm in the silica matrix. The annealing process facilitates the tuning of the UV–visible transmission cut-off edge in high optical quality Fe³⁺ doped silica gel-glasses. A marked difference in the magnetic properties of these glasses is also observed with annealing temperatures.     

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.     

Growth and characterization of pure and Co2+‐doped Li2B4O7 single crystals

Pure and Co‐doped Li₂B₄O₇ (LBO) single crystals were grown by the Czochralski method. Starting concentrations of Co₂O₃ in the melt were: 0.5, 0.85 and 1 mol% relative to Li₂CO₃. Technological factors affecting the quality of both crystals were discussed. Optical absorption and EPR spectra were analyzed to define the oxidation states and lattice sites of cobalt ions. It was shown that Co²⁺ ions enter LBO crystal at octahedral Li⁺ site positions. Low‐temperature EPR measurements revealed that two types of Co²⁺ complexes can be distinguished in the Li₂B₄O₇:Co crystals. Additional absorption calculated for γ‐irradiated crystals showed V_k type defects suggesting the creation of cation vacancies during growth. The concentration of the defects decreases with an increase of intentional Co concentration. Introduction of cobalt ions to LBO crystal is limited probably by the formation of cobalt ion pairs or by the entrance of cobalt as Co⁺. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Growth and characterization of pure and doped nonlinear optical l‐arginine acetate single crystals

Single crystals of organic nonlinear optical material of pure, Cu²⁺ and Mg²⁺ doped L‐arginine acetate (LAA) were successfully grown by slow evaporation method at room temperature. The UV‐Vis‐NIR spectra of pure and doped LAA indicate that these crystals possess a wide optical transmission window from 240‐1600 nm. Non‐linear optical studies reveal that the SHG efficiency of LAA is nearly three times that of KDP. The dielectric response of the samples was studied in the frequency region 100 Hz to 2 MHz and the influence of Cu²⁺ and Mg²⁺ substitution on the dielectric behaviour had been investigated. Photoconductivity study proves that both pure and Cu²⁺ and Mg²⁺ doped LAA crystal exhibit positive photoconductivity. It is evident from the Vickers hardness study that the hardness of the crystal decreases with increasing load both for pure and doped samples. ESR studies confirmed the incorporation of Cu²⁺ into LAA and the value of g‐factor was found to be 2.1654. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of doped lithium aluminate nanocrystalline particles by sol‐gel method

Nanocrystalline particles of Co²⁺doped lithium aluminate (Co²⁺:LAO) and Ni²⁺‐doped lithium aluminate (Ni²⁺:LAO) were synthesized by sol–gel method. The crystalline nature and particle size of the samples were characterized by X‐ray diffraction analysis (XRD). The morphology and the presence Co²⁺ and Ni²⁺ in the synthesized samples were analyzed by scanning electron microscope (SEM) and energy dispersive X‐ray analysis (EDAX). The presences of functional groups in the samples were analyzed using FT‐IR analysis. The optical absorbance of the synthesized samples were observed using UV absorption spectral analysis. The frequency dependent dielectric behaviour of the synthesized nano materials was analyzed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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⁺.     

Relationship between structure and optical properties in rare earth-doped hafnium and silicon oxides: Modeling and spectroscopic measurements

The SiO₂–HfO₂ binary system is recognized as a promising candidate for Erbium-doped waveguides amplifiers fabrication. Recently, it was demonstrated that Er³⁺-activated 70SiO₂–30HfO₂ planar waveguides with valuable optical and structural properties can be prepared by sol–gel technique with dip-coating processing. The important role played by hafnium in the silica network was evidenced by the particular spectroscopic properties presented by Er³⁺-ions in the silica–hafnia planar waveguides. In this work we present preliminary results on HfO₂–SiO₂ bulk xerogels doped with Eu³⁺ ions, with the aim to go inside the role of hafnium on the rare earth ions local environment. Spectroscopic measurements of the Eu³⁺ photoluminescence emission are given. Numerical simulations by the molecular dynamics method have been performed showing clearly a phase separation for the HfO₂ richer samples. Moreover it is found than the rare earth-doping ions stay preferentially in hafnium rich domains, thus explaining why the rare earth spectroscopic properties are strongly modified.     

EPR and optical spectra of gamma-irradiated sodium-silicate glasses containing copper oxide

The effects of an addition of CuO on the intrinsic and induced EPR and optical absorption spectra of γ-irradiated sodium-silicate glasses melted under different redox conditions are studied. It is shown that the CuO impurity blocks the formation of radiation-induced centers associated with the intrinsic defects of glass matrix. A new paramagnetic centre appears in the sodium-silicate glasses containing CuO after γ-irradiation. This center is the hole trapped at the Cu¹⁺ ion, and its spectral parameters are different from ones of the Cu²⁺ ions obtained by chemical oxidation of copper.