Electron spin resonance and magnetic studies on CaO-SiO2-P2O5-Na2O-Fe2O3 glasses

Room temperature electron spin resonance (ESR) spectra and temperature dependent magnetic susceptibility measurements have been performed to investigate the effect of iron ions in 41CaO · (52 − x)SiO₂ · 4P₂O₅ · xFe₂O₃ · 3Na₂O (2 ? x ? 10 mol%) glasses. The ESR spectra of the glass exhibited the absorptions centered at g ≈ 2.1 and g ≈ 4.3. The variation of the intensity and linewidth of these absorption lines with composition has been interpreted in terms of variation in the concentration of the Fe²⁺ and Fe³⁺ in the glass and the interaction between the iron ions. The magnetic susceptibility data were used to obtain information on the relative concentration and interaction between the iron ions in the glass.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

The effect of composition on UV-vis-NIR spectra of iron doped glasses in the systems Na2O/MgO/SiO2 and Na2O/MgO/Al2O3/SiO2

Glasses with the compositions xNa₂O · 10MgO · (90 − x)SiO₂, 10Na₂O · xMgO · (90 − x)SiO₂, 5Na₂O · 15MgO · xAl₂O₃ · (80 − x)SiO₂, xNa₂O · 10MgO · 10Al₂O₃ · (80 − x)SiO₂, 10Na₂O · 10MgO · xAl₂O₃ · (80 − x)SiO₂, 10Na₂O · 5MgO · 10Al₂O₃ · (80 − x)SiO₂ were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm⁻¹. At [Al₂O₃] > [Na₂O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm⁻¹ attributed to Fe²⁺ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm⁻¹ increases. This is explained by the incorporation of iron in the respective glass structures.     

Spectroscopic and magnetic behavior of xNd2O3(1−x)(3Bi2O3 · PbO) glasses

Glasses of the xNd₂O₃(1−x)(3Bi₂O₃ · PbO) system with 0?x?0.30 were obtained and studied by IR spectroscopy, X-ray photoelectron spectroscopy (XPS), density and magnetic susceptibility measurements. IR and density measurements show that the addition of neodymium ions produces structural changes and the neodymium ions play a network modifier role in the host glass matrix. XPS investigation permitted following the evolution of the structural disorder, of the degree of polymerization of bismuthate chains and of the fraction of bridging oxygens with respect to the neodymium ion concentration of the studied glasses. Magnetic susceptibility data show that the Nd³⁺ ions are present as isolated species for x?0.05 and as both isolated and exchange coupled species for higher x values.     

Investigation of near constant loss contribution to conductivity in lithium bismo-silicate glasses

Glasses having compositions 20Li₂O · (80 − x)Bi₂O₃ · xSiO₂ (x = 55, 60, 65, 70 mol%) were investigated using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 543 to 663 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. The increase in conductivity with increase in SiO₂ content is attributed to the change in the structural units of bismuth. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Similar values of activation energy for dc conduction and for conductivity relaxation time indicates that the ions overcome same energy barrier while conducting and relaxing. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Near constant losses (NCL) characterize this linearly dependent region of conductivity spectra. A deviation from ‘super curve’ for various isotherms of conductivity spectra was also observed in high frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.     

Structure and electrical conductivity of new Li2O-CeO2-B2O3 glasses

Fourier transformation infrared spectra, density and DC electrical conductivity of 30Li₂O · xCeO₂⋅(70 − x)B₂O₃ glasses, where x ranged between 0 and 15 mol%, have been investigated. The results suggested that CeO₂ plays the role of network modifier up to 7.5 mol%. At higher concentrations it plays a dual role; where most of ceria plays the role of network former. The density was observed to increase with increasing CeO₂ content. The effect on density of the oxides in the glasses investigated is in the succession: B₂O₃ < Li₂O < CeO₂. Most of CeO₂ content was found to be associated with B₂O₃ network to convert BO₃ into B O₄⁻ units. The contribution of Li⁺ ions in the conduction process is much more than that due to small polarons. The conductivity of the glasses is mostly controlled by the Li⁺ ions concentration rather than the activation energy for CeO₂ > 5 mol%. Lower than 5 mol% CeO₂ the conductivity is controlled by both factors. The dependence of W on BO₄⁻ content supports the idea of ionic conduction in these glasses.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Effect of boron addition on the structure and properties of iron phosphate glasses

Effects of boron addition on the glass forming characteristics, structure and properties of iron phosphate glasses with nominal compositions of xB₂O₃-(40−x)Fe₂O₃-60P₂O₅ (x = 2-20, mol%) and xB₂O₃-(100−x)[Fe₂O₃-60P₂O₅] (x = 2-20, mol%) have been investigated by DTA, XRD, IR and Mössbauer spectroscopy. Although there were some weak local surface crystallizations on especially most of the glasses in group B, all of the compositions formed glass. DTA spectra showed two exothermic peaks corresponding to crystallizations along with an endothermic glass transition peak. T_g increased with increasing B₂O₃ content for the glasses in the first series which indicates that the addition of B₂O₃ increases the thermal stability of glasses in this series while the opposite is observed in the second series. The dissolution rates of boron containing bulk glasses were found to be around 10⁻⁹ gr/cm² min which are comparable to that of the base iron phosphate glass. When the B₂O₃ content was above 14%, new bands related to BO₄ tetrahedral groups have been observed in the IR spectra. The Mössbauer isomer shift values of boron doped glasses were found to be a little lower than that of base glass but both iron ions had distorted octahedral coordination in all glasses. The fraction of Fe²⁺ ions in glasses (Fe²⁺/∑(Fe²⁺ + Fe³⁺)) was found to be 23% for the base glass while it was 10-22% for the boron doped glasses.     

Influence of alkali and alkali-earth metal oxide substitutions on the properties of lithium-iron-phosphate glasses

The influences of different alkali and alkali-earth oxide substitutions on the properties of lithium-iron-phosphate (LIP) glasses have been studied. Na₂O, K₂O, MgO, CaO and BaO were used to substitute Li₂O to prepare LIP glasses with molar compositions of (20 − x)Li₂O − xR₂O(RO) − 30Fe₂O₃ − 50P₂O₅ (x = 2.4, 4, 5.6 and 7.2). The glass transition temperature (T_g) was determined by the differential thermal analysis technique. The density and chemical durability of the prepared glasses were measured based on the Archimedes principle and the weight losses after the glasses were boiled in water. The results show that T_g decreases with the initial substitutions, whereas the density and chemical durability increase. The diminution of the aggregation effect of Li⁺ ions on the glass structure due to the decrease in Li⁺ concentration, the larger molecule weights of the substitutes, the mixed-alkali and depressing effects as well the slower mobility of substitute ions mainly contribute to the initial changes in T_g, density and chemical durability of the LIP glasses, respectively. Further increasing the amounts of substitutes brings about increasing diminution of the aggregation effect of Li⁺ ions and breakage of the glass network on the one hand and increasing amounts of substitutes with larger molecule weights and ion radii on the other hand. Both aspects influence the glass properties oppositely and consequently non-monotonic variations in the properties of LIP glasses with the substitutions are observed.     

Structural and optical properties of the gadolinium-lead-germanate glasses

Glasses in the system xGd₂O₃(100 − x)[7GeO₂·3PbO] with 0 ≤ x ≤ 40 mol% have been prepared from melt quenching method. The influence of gadolinium ions on structural behavior in lead-germanate glasses has been investigated using FTIR, UV-VIS and EPR spectroscopy. The structural changes have been analyzed with increasing rare earth concentration.FTIR data suggest that the glass network modifications has taken place mainly in the germanate part whereas the lead part remained unmodified and its network consists mainly from the [GeO₄], [GeO₆], [Ge₂O₇] structural units and with interconnected through Ge-O-Ge bridges in [GeO₄] structural units. The changes in amplitude and bandwidth of the UV-VIS bands ranging from 200 nm to 350 nm depend on the content of Gd₂O₃.By increasing the Gd₂O₃ content in the glass matrix, the optical band gap energy increases, indicating changes of the lattice parameters and that no non-bridging-oxygens form upon the addition of gadolinium oxide. The decreasing trend has been observed both in optical gap band energy and refractive index of oxide glasses at x = 10 mol% Gd₂O₃ indicating breaks up the [GeO₄] tetrahedral units bonds and create of non-bridging oxygen atoms. For sample with x ≥ 20 mol%, the gadolinium ions having a behavior of network formers (g ≈ 4.8) will coordinate more with the excess of oxygen. Accordingly, the gadolinium ions are generally suspected to improve their environment of network formers.     

The structure and optical properties of GeO2-GeS2 glasses

Oxy-chalcogenide glasses with compositions of xGeO₂-(100 − x)GeS₂, where 0 ? x ? 100 mol%, have been prepared and studied in terms of their structures and optical properties. X-ray fluorescence spectroscopy shows that Ge:S ratio can deviate from GeS₂ by ∼10 at.%, depending critically upon the preparation conditions. Raman scattering spectroscopy suggests that stoichiometric GeO₂-GeS₂ glasses have a heterogeneous structure in the scale of 1-100 nm. The optical gaps are nearly constant at 3.0-3.5 eV for glasses with 0 ? x ? 80 mol% and abruptly increase to ∼6 eV in GeO₂. This dependence suggests that the optical gap is governed by GeS₂ clusters, which are isolated and/or percolated. Composition-deviated glasses appear as orange and brown, and these glasses seem to have more inhomogeneous structures.     

Ge and Ga K-edge EXAFS analyses on the structure of Ge-Ga-S-CsBr glasses

The local structure of Ge and Ga ions in (1 − x)(Ge_0.25Ga_0.10S_0.65)-xCsBr glasses (x = 0.00, 0.05, 0.10 and 0.12) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. CsBr formed [GaS_3/2Br]⁻ structural units in glass while Ge ions remained in GeS_4/2 tetrahedra, unaffected by CsBr addition. Rare-earth ions can be surrounded by Br ions only when CsBr/Ga ratio in glass became larger than unity.     

FTIR and ultrasonic investigations on modified bismuth borate glasses

Studies on xRO · 30Bi₂O₃ · (70−x)B₂O₃ glasses have been carried out (0 ? x ? 30 mol%, R = Zn, Ba). Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to explore the role of divalent cations in the structure of the studied glasses. Analysis of infrared spectra indicates that RO is preferentially incorporated into the borate network, forming BO₄ units. It is assumed that Bi₂O₃ enters the structure in the form of BiO₆ only. The change of density and molar volume with RO content reveals that BO₄ units linked to R²⁺ cations are denser than those linked to positive sites in the Bi₂O₃ network. Predicted values of four co-ordinated boron put forward questions about the reliability of assignment of structural units that Bi₂O₃ may form.     

Characterization of the structure of calcium alumino-silicate and calcium fluoro-alumino-silicate glasses by magic angle spinning nuclear magnetic resonance (MAS-NMR)

Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by ²⁹Si, ²⁷Al and ¹⁹F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO₂ · Al₂O₃ · 2CaO (ART1) and the mineral anorthite 2SiO₂ · Al₂O₃ · CaO (ART2). The observed chemical shifts for ²⁹Si and ²⁷Al agreed with previous studies. The fluorine containing glasses were based on 2SiO₂ · Al₂O₃ · (2−X)CaO · XCaF₂. The ²⁹Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The ²⁷Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0-0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The ¹⁹F spectra indicated the presence of F-Ca(n) in low fluorine content glasses and both F-Ca(n) and Al-F-Ca(n) in high fluorine content glasses. We speculate here that the Al-F-Ca(n) species are oxyfluorides [AlO_xF_y]ⁿ⁻, where x=1-6, y=1-6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca²⁺ to F-Ca(n).     

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe₂O₃–60P₂O₅ (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe₂O₃ content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe₂O₃ content from 10 to 30 mol%.     

Sodium tracer diffusion in glasses of the type (Na2O)0.2[(BO1.5)x(SiO2)1 − x]0.8

Sodium tracer diffusion coefficients, D_Na*, have been measured in sodium borosilicate glasses of the type (Na₂O)_0.2[(BO_1.5)_x(SiO₂)_1 − x]_0.8 as a function of temperature and the composition parameter x. In these glasses, which can alternatively also be described by using the formula Na₂O·(2B₂O₃)_x·(4SiO₂)_1 − x, one network former unit, SiO₂, is replaced by another one, BO_1.5, while keeping the sodium concentration constant. At constant temperature, the tracer diffusion coefficient of sodium as a function of x has a shallow minimum at about x = 0.7. At temperatures below about 310 °C the temperature dependences of the measured tracer diffusion coefficients are of Arrhenius-type; at higher temperatures one observes an increase in the temperature dependence with increasing temperature. The activation energy derived from sodium tracer diffusion data for temperatures up to about 310 °C increases about linearly with increasing x from about 70 to 80 kJ/mol. The pre-exponential factor as a function of x varies by about one order of magnitude and has a minimum at about x = 0.4. Values derived for the Haven-ratio are smaller than one and show a shallow minimum as a function of x at around x = 0.75. Furthermore, it was investigated whether there is a significant, directly measurable uptake of water during annealing in moist atmospheres and whether water taken up from moist atmospheres can influence the diffusion of sodium.     

Nano- and micro-indentation studies on lithium borate-barium bismuth niobate glasses

The structure and mechanical properties of multifunctional lithium tetra-borate based glasses and glass-ceramics of the system (100 − x) Li₂B₄O₇₋x(BaO-Bi₂O₃-Nb₂O₅) with x = 10, 20 and 30 in molar ratio, have been characterized. Nano- and micro-indentation techniques were employed to evaluate the elastic modulus, hardness and toughness of the as-cast and annealed glasses. These were complemented with detailed structural investigations using X-ray diffraction, microscopy (optical, scanning electron and high resolution transmission electron microscopies) and nuclear magnetic resonance spectroscopy. These investigations reveal a smooth variation of the mechanical properties with composition except for the composition corresponding to x = 20. This deviation has been attributed to subtle changes in the glass-structure due to amorphous phase-separation and heat-treatment-assisted nano-crystallization.     

Structural and magnetic investigations of Fe2O3–TeO2 glasses

A series of tellurite glasses containing Fe₂O₃ with the nominal composition x(Fe₂O₃)–(1−x)(TeO₂), where x = 0.05, 0.10, 0.15, and 0.20, have been synthesized and investigated using X-ray photoelectron spectroscopy (XPS) and magnetization techniques. The Te 3d core level spectra for all glass samples show symmetrical peaks at essentially the same binding energies as measured for TeO₂ indicating that the chemical environment of the Te atoms in these glasses does not vary significantly with the addition of Fe₂O₃. Furthermore, the full-width at half-maximum (FWHM) of each peak does not vary with increasing Fe₂O₃ content which suggests that the Te ions exist in a single configuration, namely TeO₄ trigonal bipyramid (tbp). The O 1s spectra are narrow and symmetric for all compositions such that oxygen atoms in the Te–O–Te, Fe–O–Fe and Te–O–Fe configurations must have similar binding energies. The analysis of the Fe 3p spectra indicates the presence of Fe³⁺ ions only, which is consistent with the valence state of the Fe ions determined from magnetic susceptibility measurements.