The role of coordination and valance states of tungsten ions on some physical properties of Li2O–Al2O3–ZrO2–SiO2 glass system

A series of Li₂O–Al₂O₃–ZrO₂–SiO₂ glasses doped with different concentrations of WO₃ (0 to 5.0 mol.%) have been synthesized. Differential thermal analysis of the samples indicated increasing glass forming ability with the increasing concentration of WO₃ in the glass matrix. A variety of spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties (over a range of frequency and temperature) of these glasses have been investigated. The optical absorption and ESR spectral studies have pointed out that a part of tungsten ions do exist in W⁵⁺ state in addition to W⁶⁺ state especially in the samples containing low concentration of WO₃. The IR and Raman spectral studies have suggested that there is a decreasing degree of disorder in the glass network with increase in the concentration of WO₃. The values of dielectric parameters viz., dielectric constant, loss and ac conductivity at any frequency and temperature are observed to decrease as the concentration of WO₃ is increased. Such changes have been attributed to decrease of redox ratio or decreasing proportions of W⁵⁺ ions that act as modifiers in the glass network. The quantitative analysis of the results of ac conductivity and dielectric properties have indicated an increase in the insulating character of the glasses with the concentration of WO₃; this is attributed to the presence of tungsten ions largely in W⁶⁺ ions that participate in the glass network forming with WO₄ structural units.     

Dielectric properties,conductivity, UV–visible and infrared spectroscopy of PbO P2O5 NaF glasses containing WO3

Homogeneous (50P₂O₅–(30 ? x)PbO–20NaF–xWO₃ where x = 0.0, 5, 10 and 15 mol%) glasses were synthesized using a melt-quenching method. The short range structures of the phosphate samples were examined by Fourier transform infrared spectroscopy. The infrared spectral studies have pointed out the existence of conventional PO₄, WO₄ and WO₆ structural units in the glass network, the number of WO₄ tetrahedra decreases as WO₃ concentration increases. The optical transmittance and reflectance spectrum of the glasses have been recorded in the wavelength range of 190–1100 nm. The values of the optical band gap E_op for all types of electronic transitions and refractive index have been determined and discussed. The real and imaginary parts ε₁ and ε₂ of the dielectric constant have been determined. The type of electronic transitions in the present glass system is indirectly allowed and the high values for the refractive index and dispersion are recorded due to the high polarizability of tungsten ions. The results of refractive indices as determined reveal the homogeneity of samples and were found to depend on the glass composition. The electrical properties of the glasses were investigated by ac conductivity from 0.12 to 100 kHz for temperatures ranging from room temperature to 600 K. The study of dielectric properties suggested increase in the insulating character of the glass system with increase in the content of WO₃. The ac conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from W⁴⁺ to W⁵⁺ ions.     

Glasses in the SbPO4–WO3 system

Glasses in the binary system (100 − x)SbPO₄–xWO₃ (20 ⩽ x ⩽ 60, x in mol%) have been prepared and characterized. Differential thermal analysis (DTA) shows that the glass transition temperature, T_g, increases from 412 °C, for samples containing 20 mol% of WO₃, to 481 °C observed for glass containing 60 mol%. Sample containing 40 mol% in WO₃ were observed to be the most stable against devitrification. The structural organization of the glasses has been studied by using Fourier transform infra-red (FTIR), Raman, ³¹P Magic angle spinning (MAS) and spin echo nuclear magnetic resonance (NMR) spectroscopies. Results suggest two distinct networks comprising the glass structure, one with high SbPO₄ content and the other characteristic of the highest WO₃ content samples. The glasses present photochromic properties. Colour changes are observed for samples after exposure to ultraviolet or visible laser light. XANES, at L₁ absorption edge of tungsten, suggests partial reduction from W⁶⁺ to W⁵⁺ species during the laser irradiation. The photochromic effects and the colour changes, promoted by laser excitation, are reversible and easily removed by heat for during 1 h at 150 °C. Subsequent ‘write/erase’ cycles can be done without degradation of the glasses.     

Time-resolved fluorescence measurements on Dy3+ and Sm3+ doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Dy³⁺ (4f⁹) and Sm³⁺ (4f⁵) between 10¹⁸ and 10²¹ cm^?3 and their time resolved fluorescence in the visible range in combination with characteristic physical properties were studied. Different fit procedures were carried out. Although both ions differ in their intrinsic fluorescence lifetime, with 1.5 ms for Dy³⁺ and 6.5 ms for Sm³⁺, their dependence on glass matrix is remarkable similar. Fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100), and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) were used for investigations. A strongly ionic surrounding by fluorine ligands, as in fluoroaluminate glass samples, provides the longest fluorescence lifetime. It decreases with increasing phosphate content by increasing oxygen surrounding and with increasing RE³⁺ doping. Large differences were detected in the two borosilicate glasses depending on their optical basicity mainly due to differences in the Na₂O/B₂O₃ ratio. Duran-like samples with low Na₂O content have shown phase separation with higher doping concentration. The RE³⁺ ions are accumulated in the borate-rich droplets. Surprisingly only very low concentration-quenching effects were observed. In the opposite of NBS1 samples with high Na₂O content this generated extremely high quenching effect.     

Synthesis and characterization of tin containing molybdophosphate and tungstophosphate glasses

Two series of tin containing molybdophosphate and tungstophosphate glasses and glass-ceramics were achieved by means of a domestic microwave oven under air. During melting, a redox reaction takes place between Sn²⁺ and Mo⁶⁺ generating the crystallization of a Na(Sn^IV,Mo^IV)(PO₄)₃ solid solution with a NZP (NaZr₂P₃O₁₂) type structure. Such reactivity was not underlined in the case of the W-series. All the samples were characterized from a thermal and mechanical point of view as a function of the RO₃ (R = Mo, W) for SnO substitution. Two types of behaviors were identified. For the Mo-series, all the characteristics, except density, present an extremum value for the chemical composition with a 1:1 SnO-MoO₃ molar ratio. This is strongly correlated to the amount of NZP crystals present in the glass ceramic, the different behavior observed for the density being due to the low compactness of the NZP phase. For the W-series, all these different characteristics varies monotonously according to a progressive strengthening of the network by replacement of a low field strength ion (Sn²⁺) by a higher field strength ion (W⁶⁺). In addition, the solid state reactivity of a 1:1 SnO-RO₃ mixture was examined confirming the absence of any redox process between SnO and WO₃ during the glass synthesis.     

Thermal and optical properties of TeO2–BaO–SrO–Nb2O5 based glasses: New broadband Raman gain media

We systematically added WO₃ (up to 10 mol%) and P₂O₅ (up to 16 mol%) in TeO₂–BaO–SrO–Nb₂O₅ (TBSN) glass system and studied thermal and optical properties of the resultant glasses. The dependences of the additive concentration on glass transition (T_g) and crystallization (T_x) temperatures are presented. The TBSN glass added with ⩾4 mol% WO₃ and P₂O₅ showed high stability against crystallization. The changes in optical band gap energy due to WO₃ and P₂O₅ addition was studied using UV–VIS–NIR absorption spectrometry. The WO₃ addition shifted the optical band gap to longer wavelengths, whereas P₂O₅ addition shifted that to shorter wavelengths. Effects of the WO₃ and P₂O₅ addition on the Raman spectra of TBSN glass are clarified. New Raman bands due to WO₄ and PO₄ tetrahedra formed in the resultant glasses broadened their Raman spectra. Present glasses are characterized by higher thermal stability and wider Raman spectra, therefore, they are promising candidates for fiber Raman amplifiers in photonics systems.     

A study of nonlinear optical properties in Bi2O3-WO3-TeO2 glasses

A series of bismuth tungsten tellurite glasses were prepared and their densities, linear refractive indices and transmission spectra were measured. The optical bandgaps E_opt and Urbach energies E_e of glasses were obtained from ultraviolet absorption edges. Both the optical gap (E_opt) and the band tail (E_e) are behaving oppositely. As the value of E_opt decreases with increasing WO₃ content, the degree of disorder increases which causes more defects or localized states resulting in deep localized in the bandgap with the tailing increased. Z-scan technique was carried out to investigate the third-order nonlinear optical properties of Bi₂O₃-WO₃-TeO₂ glasses. The third-order optical nonlinearity increases with decreasing the optical bandgap E_opt, since a increase of WO₃ content can provide the non-bridging oxygen ion content.     

Raman and luminescence studies of alkali borate tungstate glasses

Raman and luminescence spectroscopy were used to determine the structure of alkali borate tungstate glasses: M₂O(B₂O₃)₂·xWO₃, M = Li or Na (0 < x < 1). Raman scattering results showed the dominant tungstate species in these photochromic glasses to be tetrahedral WO₄⁼. At high concentrations of WO₃, WO₃·H₂O, and W₂O₇⁼ are also present. Luminescence measurements provided evidence for an octahedral WO₃ structure not identified by the Raman results. The results also revealed a possible change in the structure of the glasses similar to that observed in alkali borate glasses and associated with the “borate anomaly”. In addition, preliminary measurements are reported on the variation of the band gap, density, index of refraction, and the elastic coefficient C₁₁ determined by Brillouin scattering with composition.     

Structure and non-linear optical performance of TeO2-Nb2O5-ZnO glasses

The non-linear optical performance and structure of TeO₂-Nb₂O₅-ZnO glasses was investigated as a function of ZnO content. The third-order non-linear optical susceptibility (χ⁽³⁾) as measured by a Degenerate Four Wave Mixing (DFWM) method, initially increased with increasing ZnO content to about 8.2 × 10⁻¹³ esu for a glass containing 2.5 wt% ZnO, and then decreased to 5.9 × 10⁻¹³ esu as the ZnO content increased to 10 wt%. There was no noticeable change as the ZnO content increased from 10 to 15 wt%. The non-linear optical response time, which caused electron cloud deformation, was from 450 to 500 fs. The structure of these glasses as analyzed by Raman spectroscopy and FT-IR spectra, was affected by the addition of ZnO up to 5 wt%, when, it is believed, the Zn²⁺ ions occupied the interstitial positions in the glass network by replacing the Nb⁵⁺ ions. The replaced Nb⁵⁺ ions occupied the network forming positions as the Te⁴⁺ ions. Increasing ZnO > 5 wt% did not have any further effect on the glass structure.     

Vibrational, EPR and optical spectroscopy of the Cu doped glasses with (90-x)TeO2-10GeO2-xWO3 (7.5 ≤ x ≤ 30) composition

Infrared, EPR and optical absorption studies on (90-x)TeO₂-10GeO₂-xWO₃ (7.5 ≤ x ≤ 30) glasses containing Cu²⁺ spin probe have been carried out. The Infrared spectral studies show that the structure of glass network consists of [TeO₄], [TeO₃]/[TeO_3 + 1], [WO₄], [WO₆] and [GeO₆] units in the disordered manner. Physical parameters such as density (ρ), molar volume (V_m), oxygen packing density (OPD), oxygen molar volume (V_o), optical basicity (Λ), oxide ion polarizability (α_O²⁻), inter ionic distances and the concentration of ions per unit volume of Te, Ge, W, Cu and O have been determined. The spin-Hamiltonian parameters (g_||, g_⊥ and A_||) of Cu²⁺ ions in the present glasses have been estimated from EPR spectra at 300 K. Bonding parameters such as α², β₁², β², Γ_σ, and Γ_π have been calculated from both optical absorption and EPR data. The observed variations in spin-Hamiltonian parameters and bonding parameters have been correlated to the structural modifications due to the WO₃ incorporation into the TeO₂ glass network at constant 10 mol% GeO₂ content.     

Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Eu³⁺ (4f⁶) at doping concentrations between 10¹⁸ and 10²¹ cm^?3. By applying reductive melting conditions Eu³⁺ could partially be transformed to Eu²⁺ (4f⁷). Four fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100) and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) have been used for these investigations. The time-resolved fluorescence in the visible range has been studied for both ions. Although static and dynamic fluorescence of Eu³⁺ and Eu²⁺ are dramatically different (f–f-transitions for Eu³⁺; d–f-transitions for Eu²⁺), glass structure changes have a similar influence on the dynamic fluorescence behavior of both ions. A strongly ionic surrounding due to fluorine ligands as in fluoroaluminate glass samples provides the longest fluorescence lifetime (about 7 ms for Eu³⁺; about 1.3 μs for Eu²⁺). Increasing phosphate content decreases the fluorescence lifetime due to more oxygen ligands. Interesting differences have been found for the two borosilicate glasses due to the difference in their optical basicity (Na₂O/B₂O₃ ratio). Measurements indicate a homogeneous distribution of europium ions in most FP samples. NBS1 measurements suggest that two different local europium sites are formed. For Duran-like samples only one specific europium site was found, although these samples show phase separation at high doping concentrations into a SiO₂-rich phase and borate- and europium-rich droplets. Fluorescence quenching due to energy transfer from Eu²⁺ to Eu³⁺ could be found for co-doped samples; Eu³⁺-doped samples show no fluorescence quenching.     

Glass formation and structure of glasses in the ZnO―Bi2O3―WO3―MoO3 system

The glass formation region in the ternary ZnO―Bi₂O₃―WO₃ system is determined by melt quenching technique (cooling rates 10¹-10² K/s). New original glasses are obtained in a narrow concentration range with high WO₃ content (60-75 mol%). Homogeneous glasses of the composition (100 − x)[0.2ZnO·0.3Bi₂O₃·0.5WO₃]xMoO₃, were obtained between 20 and 60 mol% MoO₃. Characterization of the amorphous samples was made by x-ray diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR). The thermal stability of glasses decreases with the increasing of MoO₃ content. The glass transition temperature, T_g, varies between 340-480 °C, while the crystallization temperature, T_x, varies between 388-531 °C. The tungstate glasses possess higher crystallization temperature (T_x over 500 °C) in comparison with the other vanadate and molybdate non-traditional glasses. The glass network is realized by transformation of three-dimensional structure of WO₃ into a layered one, consisting mainly of WO₆ units. We supposed that the network of quaternary glasses is built up by MoO₄, MoO₆ and WO₆. At low concentration ZnO and Bi₂O₃ facilitate the disorder in the supercooled melts, while at high concentration stimulate crystallization processes. These oxides belong to the intermediate ones.     

Local structure and redox state of copper in tellurite glasses

The local glass structure of tellurite glasses containing CuO with the nominal composition x(CuO) · (1−x)(TeO₂), where x=0.10, 0.20, 0.30, 0.40, and 0.50, as well as the valence state of the copper ions have been investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. The Te 3d core level spectra for all glass samples show symmetrical peaks (Te 3d_5/2 and Te 3d_3/2) at essentially the same binding energies as measured for TeO₂ indicating that the chemical environment of the Te atoms in the glasses does not vary significantly with the addition of CuO. The O 1s spectra, however, show slight asymmetry for all glass samples which results from two contributions, one from the presence of oxygen atoms in the Te-O-Te environment (bridging oxygen BO) and the other from oxygen atoms in an Te-O-Cu environment (non-bridging oxygen NBO). The ratio of NBO to total oxygen was found to increase with CuO content and to be in good agreement with calculated values for the TeO₄ trigonal bipyramid structure. Moreover, the appearance of a satellite peak in the Cu 2p spectra provides definitive evidence for the presence of Cu²⁺ ions in these glass samples where the asymmetry and broadening of the Cu 2p_3/2 and Cu 2p_1/2 peaks are indicative of the presence of both Cu²⁺ and Cu⁺ ions. The relative concentration Cu²⁺ determined from XPS is in good qualitative agreement with the determinations of Cu²⁺ from magnetic susceptibility measurements on the same glass samples. Furthermore the susceptibility data follow a Curie-Weiss temperature-dependent behavior (χ=C/(T−θ)) with negative Curie temperatures indicating that the predominant magnetic interactions between the Cu²⁺-Cu²⁺ exchange pairs are antiferromagnetic in nature.     

Optical properties of Er3+-singly doped and Er3+/Yb3+-codoped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂–Al₂O₃–Na₂O–ZnF₂ doped with Er³⁺ and Er³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Er³⁺ were determined by measurement of the absorption spectrum of Er³⁺-singly doped glass. According to the Judd–Ofelt theory, the experimental intensity parameters were calculated, from which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. The fluorescence lifetime and quantum efficiency for the near-infrared emission of Er³⁺-singly doped glass were determined to be 3.0 ms, and 42%, respectively. Visible upconversion luminescence was observed under 980 nm diode laser excitation. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms are discussed.     

Characterization of the reversible photoinduced optical changes in Sb-based glasses

Changes occurring in absorption coefficients when glasses in the SbPO₄–WO₃ binary system were irradiated by light, at the edge of the absorption band, were measured in real time. These glasses present good thermal and optical properties and photoinduced changes in the absorption coefficients are reversible by heat treatment around 150 °C. Subsequent recording/erasing cycles could be made without sample degradation. The sensitivity of the induced optical changes was studied for different wavelengths, light powers and energy of light dose exposures, and for different compositions of the glasses. The changes in the absorption coefficients of the glass samples were accompanied by a color change from yellow to blue, and were also characterized by visible spectroscopy. The color changes occurred through the entire volume of the glass (∼2 mm thickness) for the Ar-ion laser lines at the edge of the absorption band.     

Correlation between physical and structural properties of Co doped mixed alkali zinc borate glasses

The physical and structural properties of Co²⁺ doped 20ZnO + xLi₂O + (30 − x)Na₂O + 50B₂O₃ (5 ≤ × ≤ 25) (ZLNB) glasses have been studied and correlated. The physical and structural parameters of all the glasses are evaluated and a non-linear behavior is observed. No sharp peaks are observed in XRD patterns of the glass samples which confirm the amorphous nature. FT-IR spectra of ZLNB glasses reveal diborate units in borate network. The optical absorption spectra suggest the site symmetry of Co²⁺ in the glasses is near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibit the mixed alkali effect. All the samples are found to be strong and stable in structure with low values of Urbach energy which lie between 0.027 eV and 0.039 eV. The correlation between densities and Urbach energies of Co²⁺ doped ZLNB glasses with respect to Li₂O content suggest a changeover conduction mechanism from electronic to ionic, with a diffusivity crossover point at x = 15 mol%.     

Local order around tungsten atoms in tungstate fluorophosphate glasses by X-ray absorption spectroscopy

X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO₃–BaF₂–WO₃ glasses and the results were compared with crystalline references Na₂WO₄ and WO₃. XANES measurements at the W-L₁ edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO₃. Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO₃ concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO₃. With increasing WO₃ concentration, it was found that R₂ decreases from 1.96 to 1.92 Å whereas R₃ increases from 2.07 to 2.12 Å.     

Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses

In this paper, optical properties of 75TeO₂-20ZnO-5Na₂O host glass doped with concentration of Tm³⁺ up to 10 %mol were studied in order to assess the most suitable rare earth content for short cavity fiber lasers. Raman spectroscopy revealed a change in the glass structure while increasing Tm³⁺ content, similar to the well known addition of alkali ions in a glass. Influence of the fabrication process on the OH⁻ content was determined by FTIR measurements. Refractive index of Tm³⁺ doped tellurite glasses was measured at five different wavelengths ranging from 533 nm to 1533 nm. Lifetime and emission spectra measurements of the Tm³⁺ doped tellurite glasses are reported.     

Study of CaO-WO3-P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Dielectric constant ε^′, loss tanδ and ac conductivity σ_ac of 40CaO-xWO₃-(60−x)P₂O₅ (with 0?x?15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., ε^′, tanδ and σ_ac of CaO-P₂O₅ glasses are found to decrease with the introduction of WO₃ up to 3 mol% and increase with further increase in the concentration of WO₃; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tanδ with temperature at different frequencies of CaO-P₂O₅ glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO₃ from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO₃ beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole-Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.