Structural and physical properties of a novel TeO2-BaO-SrO-Ta2O5 glass system for photonic device applications

A detailed study on a novel TeO₂-BaO-SrO-Ta₂O₅ glass system developed for photonic device applications is reported in this paper. The glass transition and crystallization temperatures could be selected by varying the Ta₂O₅ content in this glass system. This glass system is found to have good thermal stability among tellurite glasses. Raman spectroscopy has been used as a tool to analyze the structural details of this technologically important glass system. In addition to the TeO₄ trigonal bipyramid and TeO₃ trigonal pyramid structural units, glasses in this system revealed the presence of an additional Raman band attributed to TaO₆ octahedra. The Raman bandwidth of the present glasses are broader compared to the conventional tellurite glasses by 35%. The influence of a gradual addition of the modifier oxides on the coordination geometry of tellurium atoms has been elucidated. Unlike the other tellurite glasses, even at higher modifier concentrations the TeO₄ structural units dominate in the glass network compared to TeO₃ trigonal pyramids. The ratio of TeO₄/TeO₃ structural units was discussed for different series of glass compositions.     

Structure and properties of glasses in ZnO-P2O5-TeO2 system

Glasses in the ternary ZnO-P₂O₅-TeO₂ system were prepared and studied in two compositional series (100 − x)[0.5ZnO-0.5P₂O₅]-xTeO₂ (X-series) and 50ZnO-(50 − y)P₂O₅-yTeO₂ (Y-series) within the concentration range of x = 0-60 and y = 0-40 mol% TeO₂. Their structure was studied by Raman and ³¹P MAS NMR spectroscopies. The incorporation of TeO_x units into the structural network is associated with the depolymerisation of phosphate chain structure as revealed by both methods. At a high TeO₂ content isolated PO₄ tetrahedra are formed in the structure of glass series Y, while diphosphate O₃P-O-PO₃ groups are present in the structure of the glass series X. In the structure of glass series Y tellurium atoms form predominantly TeO₃ trigonal pyramids, whereas in the X glass series TeO₄ trigonal bipyramids prevail in the glass structure. The addition of TeO₂ to the parent zinc metaphosphate glass results in a decrease of glass transition temperature in both compositional series associated with the replacement of stronger P―O bonds by weaker Te―O bonds.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

X-ray absorption and Raman characterizations of TeO2-Ga2O3 glasses

The thermodynamic properties of transparent glasses prepared in the TeO₂-Ga₂O₃ system were investigated by differential scanning calorimetry. The change of the thermal parameters as a function of the chemical composition is discussed. Raman and both Te L_III and Ga K edge X-ray absorption spectroscopies at room temperature were used to examine the short range order. Analyses of the spectra suggest that the addition of Ga₂O₃ content to the TeO₂ glass matrix induces the transformation of trigonal bipyramids (TeO₄E, E=lone electronic pair 5s² of Te) to trigonal pyramids (TeO₃E) with formation of Te-O-Ga bridging bonds. Furthermore, Ga K edge XANES and EXAFS studies show that Ga atoms exhibit both tetrahedral (GaO₄) and octahedral (GaO₆) environments.     

Optical and spectroscopic properties of germanotellurite glasses

In this work, new glass compositions in the TeO₂-GeO₂-Nb₂O₅-K₂O system have been prepared and studied. The germanotellurite glasses were prepared by melt-quenching and their density, refractive index and characteristic temperatures have been determined. The structure of these glasses has been studied by infrared and Raman spectroscopies.The progressive replacement of TeO₂ by GeO₂ led to an increase of the glass transition and crystallisation temperatures of the glasses and a simultaneous decrease of their density and refractive index. Typical density and refractive index values of these glasses ranged from 4.98 to 3.85 g cm^− 3 and 2.08 to 1.79, respectively, with increasing GeO₂ content. The infrared spectra are dominated by a band ~ 640 cm^− 1 in the tellurite glass and ~ 800 cm^− 1 in the germanate glass. The Raman spectra of the germanotellurite glasses present an intense boson peak between ~ 34 and 47 cm^− 1, together with high frequency peaks at ~ 670 cm^− 1 and ~ 470 cm^− 1 for high tellurite and high germanate glass compositions, respectively. The vibrational spectra of these germanotellurite glasses indicate that the glass network consists basically of TeO₄ and [TeO₃]/[TeO_3 + 1] units, mixed with GeO₄ and NbO₆ polyhedra.     

On the structure of tungstate–tellurite glasses

A continuous network model of xWO₃–(1 − x)TeO₂ glasses is developed, based on quantum-chemical calculation and Raman spectra analysis, in order to relate the structural and vibrational properties with glass composition. The tungstate–tellurite glass network is shown to be formed mainly by structural units of three types, TeO₄ trigonal bipyramids, OTeO₂ trigonal pyramids, and WO₆ octahedra with OW double bonds. Most of the W atoms are found to be incorporated into single OWO₅ octahedra, with no more than several percents of these atoms occurring in 2[OWO₅] paired tungstate centers. The structural and vibrational properties of tungstate–tellurite glasses of several compositions are analyzed by application of the model and a novel interpretation of the Raman spectra is suggested.     

Remark on the optical gap in ZnO-Bi2O3-TeO2 glasses

Four ZnO-Bi₂O₃-TeO₂ glasses were prepared from high purity (4N5) oxides. From measurements of the optical transmission on very thin bulk samples the optical gap was determined at around 3.55 eV for the glasses studied. The temperature dependence of the optical gap was also determined from the room temperature close up to 500 K. Preliminary Raman scattering measurements indicate that with a decrease in TeO₂ content, TeO₄ trigonal bipyramid transformation proceeds into TeO₃ trigonal pyramids.     

The Raman coupling function in v-GeO2 and v-SiO2: A new light and neutron scattering study

We report inelastic Raman and neutron scattering spectra for the network glass formers vitreous silica (v-SiO₂) and vitreous germania (v-GeO₂) measured at temperature from 10 to 300 K. The ability to determine the temperature dependence of the luminescence background in Raman scattering has allowed to obtain the Raman coupling function C(ω) and in particular, its low-frequency limit. This study indicates that C(ω) has a linear behavior near the Boson peak maximum and below.     

Raman spectra and thermal analysis of a new lead–tellurium–germanate glass system

Differential scanning calorimetry (DSC) and Raman scattering studies of a new glass system, lead–tellurium–germanate glasses in the form of (90−x)GeO₂·xTeO₂·27PbO·10CaO with x=0, 10, 20, 30, and 40, are reported. The glass samples were fabricated using a conventional melt-quenching method. The Raman spectra and possible glass structures are discussed for different TeO₂ contents. The results indicate that increasing TeO₂ content up to 40 mol% in the glass system decreases the glass transition temperature and melting temperature, and suppresses the crystallization tendency in the fiber pulling temperature range. The lead–tellurium–germanate glass, GTPC, possesses a larger refractive index and a smaller maximum phonon energy than that of a lead–germanate glass, 63GeO₂·27PbO·10CaO, and shows a better thermal stability compared to a tellurite glass, 75TeO₂·20ZnO·5Na₂O (TZN). These improved properties could be beneficial for fabricating rare-earth doped fiber devices.     

The preparation and structural studies in the (80 − x)TeO2–20ZnO–(x)Er2O3 glass system

Er³⁺-doped tellurite glasses of the (80 ? x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ? x ? 2.5 mol%) have successfully been made by melt-quenching technique and their structure has been investigated by means of DTA and Raman spectroscopy. The DTA results show the thermal parameters; such as the glass transition temperature (T_g) and crystallization temperature (T_c) were determined. It is found that this system provides a stable and wide glass formation range in which the glass stability around 99–140 °C may be obtained. The Raman spectroscopy used the structural studies in the glass system. Two Raman shift peaks were observed around 640–670 cm^?1 and 720–740 cm^?1, which correspond to the stretching vibration mode of TeO₄ tbp and TeO₃ tp, respectively. It is found that the spectral shift in Raman spectra is depending on the Er₂O₃ content. This evolution is an indication of the changes in the basic unit of the glass structure.     

Calculation and analysis of vibrational spectra of PbCl2-Sb2O3-TeO2 glass from first principles

Ternary PbCl₂-Sb₂O₃-TeO₂ system provides promising materials for photonic applications. Glasses in this system are thermally stable, their refraction index is about 2.2 and they are transparent from 400 nm up to 6 μm. The ab initio molecular dynamics (MD) simulations of xPbCl₂-10SbO_3/2−(90-x)TeO₂ glasses (x = 10, 20, 30, 40, and 50) were carried out using density functionals, the plane-wave basis set expansion, and the projector-augment waves (PAW). The resulting glassy structures were analyzed with the help of partial radial distribution functions (RDF) and partial coordination numbers. Good agreement with the available experimental data was confirmed. The axial and equatorial oxygen atoms of TeO₄ trigonal bi-pyramids were distinguished. Vibrational frequencies and their corresponding eigen-modes were found by diagonalization of the dynamical matrix. The simulated vibrational spectra were decomposed to determine the contributions from individual atomic species and some more complex structural features and compared with experimental Raman spectra. The spectral decomposition was carried out by projecting vibrational displacement vectors onto various normal modes of typical symmetric structural units. The method proved to be able to successfully interpret the experimental Raman spectra.     

Structure of TeO2 · B2O3 glasses inferred from infrared spectroscopy and DFT calculations

Glasses in the system (1 ? x)TeO₂ · xB₂O₃ glasses (with x = 0.3 and 0.4) have been prepared from melt quenching method. The structural changes were studied by FTIR spectroscopy and DFT calculations. From the analysis of the FTIR spectra it is reasonable to assume that when increasing boron ions content the tetrahedral [BO₄] units are gradually replaced by trigonal [BO₃] units. The increase in the number of non-bridging oxygen atoms would decrease the connectivity of the glass network, would depolymerize of borate chains and would necessite quite a radical rearrangement of the network formed by the [TeO₆] octahedral. This is possible considering that tellurium dioxide brings stoichiometrically two oxygen atoms in [TeO₄] and needs an additional oxygen atom for the formation of [TeO₆] octahedra. This additional oxygen atom is evidently taken off from the boron co-ordination and thus boron atoms transfer their [BO₄] co-ordination into [BO₃] co-ordination. We used the FTIR spectroscopic data in order to compute two possible models of the glasses matrix. We propose two possible structural models of building blocks for the formation of continuous random network glasses used by density functional theory (DFT) calculations.     

Pressure dependence of the Boson peak in glassy As2S3 studied by Raman scattering

A detailed pressure dependence study of the low energy excitations of glassy As₂S₃ is reported over a wide pressure range, up to 10 GPa. The spectral features of Boson peak are analyzed as a function of pressure. Pressure effects on the Boson peak are manifested as an appreciable shift of its frequency to higher values, a suppression of its intensity, as well as a noticeable change of its asymmetry leading to a more symmetric shape at high pressures. The pressure-induced Boson peak frequency shift agrees very well with the predictions of the soft potential model over the whole pressure range studied. As regards the pressure dependence of the Boson peak intensity, the situation is more complicated. It is proposed that in order to reach proper conclusions the corresponding dependence of the Debye density of states must also be considered. Comparing the low energy modes of the crystalline counterpart of As₂S₃, as well as the experimental data concerning the pressure dependencies of the Boson peak frequency and intensity, a structural or glass-to-glass transition seems to occur at the pressure ∼4 GPa related to a change of local dimensionality of the glass structure. Finally, the pressure-induced shape changes of the Boson peak can be traced back to the very details of the excess (over the Debye contribution) vibrational density of states.     

Computational and Raman studies of phospho-tellurite glasses as ultrabroad Raman gain media

Molecular orbital calculations of two phospho-tellurite model clusters were performed to clarify the origins of the Raman bands in the Stokes region of over 1000 cm^− 1 in phospho-tellurite glasses. The Raman bands could be attributed to two components of 900-1050 cm^− 1 of symmetrical stretching vibrations of PO₄ units and 1050-1200 cm^− 1 of anti-symmetrical stretching vibrations of PO₄ units. It was also clarified that the top of the valence band of phospho-tellurite glasses consists of the lone pair electrons in a TeO_4 + 1 unit and the bottom of the conduction band of the glass consists of the antibonding hybrids of Te 5p and O 2p orbitals in the equatorial plane of a TeO₄ unit.We have developed new phospho-tellurite glasses which have the Raman gain peak of 30 times as large as silica glass or the Raman gain bandwidth of more than 1200 cm^− 1.     

The vibrational spectra of B2O3-GeO2 glasses

Infrared and Raman spectra were measured for a series of glass compositions in the binary B₂O₃-GeO₂ system at room and elevated temperatures. No coordination changes were detected from spectral analysis for either boron or germanium with concentration or temperature changes. Interpretation of the vibrational spectra indicated that both boroxol rings and BO₃-triangular units were present as the basic units in glasses with higher B₂O₃ concentrations. Only BO₃-triangular units were detected as the basic units in glasses with lower B₂O₃ concentrations. Raman spectra of glasses measured at higher temperatures (>T_g) indicated that their boroxol ring concentrations decreased as a function of temperature. The enthalpy for ring rupture in 35GeO₂–65B₂O₃ glass was 7.7 kcal/mol. Also, spectral analysis indicated that the boroxol ring concentrations of these glasses at room temperature were dependent upon their previous heat treatments.     

Anomalous physical properties and Raman spectra of glassy B2O3BaTiO3Na2O materials

Anomalous physical properties (refractive index and density) of B₂O₃BaTiO₃Na₂O ternay glasses are determined and discussed on the basis of the structure present in the glasses and evidenced by vibrational Raman spectroscopy.These glasses behave in a manner analogous to the alkali B₂O₃X₂O binary glasses for molar ratio R = basic oxide/B₂O₃ up to 0.3, with oxygen binding by means of bridging bonds while boron coordination changes from the trigonal to tetrahedral type. The phenomenon is indicated by a progressive weakening of the 806 cm^?1 peak (attributable to a breathing vibration of the boroxol unit) and by a concomitant strengthening of the ～775 cm^?1 peak (attributable to a vibrational mode of boroxol units, or derived units, containing at least one 4-coordinate boron atom). For higher R values the Raman spectra bring to light the progressive demolition of the structural units responsible for the 775 cm^?1 Raman peak, which gives rise (the transformation is complete for R ～ 1) to two new main structural units, orthoborate [BTi₄O₁₀]^?1 (peak at 845 cm^?1) and metaborate BO₂^? (peak at 715 cm^?1).     

A low-frequency Raman study of glassy,supercooled and molten silica and the preservation of the Boson peak in the equilibrium liquid state

A temperature dependent Raman spectroscopic study of the Boson peak in silica is being presented. For the first time experimental data are obtained not only above the glass transition temperature but also above the melting point. The intriguing temperature dependencies exhibited by the Boson peak frequency and intensity are examined in detail. A comparison of the above features between the frequency-reduced and the true excess density of vibrational states revealed that the spectrum of the latter does not exhibit the anomalous trend of the Boson peak as regards the temperature dependence of its frequency, and further the number of the excess modes decreases with increasing temperature. The observability of the Boson peak in the normal liquid state as a well-resolved peak is also discussed in the context of recent theoretical and simulation works that relate the Boson peak with the details of the potential energy landscape of the supercooled liquid.     

Optical properties of selenite glasses

The aim of this work is to obtain multicomponent selenite glasses containing other non-traditional glass formers such as V₂O₅, TeO₂ and MoO₃ and to verify their optical properties in the visible spectral region. Glasses containing MoO₃ and TeO₂ are transparent in the visible range and near IR region from 400 to 2300 nm. Transparent coloured glasses were obtained due to the electron transfer charge processes. Using IR spectroscopy it was determined the main building units of the amorphous network. It was found the presence of TeO₄, SeO₃ and MoO₄ units.     

The elastic behaviour of TeO2 glass under uniaxial and hydrostatic pressure

TeO₂ glass of purity exceeding 98.5 mol.% has been made, despite earlier suggestions that some 7.5–10 mol.% of modifier is required to form vitreous telluride networks. It is argued that in view of the high purity of the glass obtained, TeO₂ may well be able to form glass by itself, given an appropriate thermal history in preparation. The hydrostatic and uniaxial pressure dependences of ultrasonic waves propagated in this glass at room temperature have been measured. The results provide the second and third order elastic constants of the glass. The bulk modulus is consistent with a ring diameter averaging about 8 atoms (Te₄O₄ rings) suggesting that the glass is a disordered version of paratellurite; however, if the TeO bending force constant were to be unusually strong, then a larger ring diameter (as in tellurite) would be indicated. Although the anion coordination number is only 4, the pressure derivatives of the second order elastic constants are positive and the third order elastic constants are negative, in marked contrast to the anomalous behaviour of silica-based glasses. These findings suggest that bond bending motions of bridging atoms between the trigonal bypyramidal groups (which are the structural units) do not play an important role in the elastic properties of TeO₂ glass. In consequence the shear and oongitudinal acoustic mode Grüneisen parameters are both positive (γ₁ = +2.14, γ_s = +1.11): the long wavelength acoustic modes stiffen under hydrostatic pressure.     

The glass transition temperature and infrared absorption spectra of: (70−x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Glasses of the system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (T_g) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO₂ by Li₂O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm⁻¹. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure.