Structure of lead germanate glasses by Raman spectroscopy

Lead germanate glasses of the xPbO(1−x)GeO₂ compositions with x ranging from 0.200 to 0.625 have been synthesised and their crystallisation behaviour and vibrational properties studied by differential thermal analysis (DTA) and Raman spectroscopy. Raman spectra of the Pb₅Ge₃O₁₁ ceramics also have been measured and compared to lead germanate glass spectra and literature data on β-PbO. Ge atoms in low lead glasses are supposed to be surrounded by four to six oxygen atoms, while the high-lead glass (x=0.625) corresponding to stoichiometry of the Pb₅Ge₃O₁₁ ferroelectrics contains predominantly fourfold coordinated germanium. Structural resemblence of high-lead germanate glasses to the Pb₅Ge₃O₁₁ crystal is proposed rather than to β-PbO. The `boson' peak range in the spectra of lead germanate glasses is discussed taking into account the similarities of the spectra of glassy and crystalline Pb₅Ge₃O₁₁.     

Neodymium ion environment in germanate crystals and glasses

Absorption and fluorescence spectra are presented for neodymium-doped alkali germanate and bismuth germanate glasses and crystals. Narrower spectral components and a shift in intensity distribution toward shorter wavelengths for the composite bands corresponding to the principal neodymium absorption and fluorescence transitions are observed as the radius of the glass modifier ion is increased (Li⁺Na⁺K⁺). These changes in component band width and intensity distribution become more pronounced as the concentration of large radius modifier ion is increased, and result in effectively narrower widths for the fluorescent bands involved in laser emission. Optical spectra for neodymium in a sodium germanate single crystal show broad bands characteristic of those of glasses. Electron microprobe scans reveal non-uniform distribution and clustering of neodymium in alkali germanate crystals. Narrow, temperature sensitive bands similar to those of neodymium in other crystals are found for single crystal and polycrystalline neodymium-doped bismuth germanate, while the glass exhibits broad bands. Substitutional replacement of Bi³⁺ with Nd³⁺ in the bismuth germanate crystal lattice is indicated from the optical spectra but the environment of neodymium in crystalline sodium germanate appears to have only short-range order. The difficulty encountered in introducing neodymium into substitutional positions in alkali germanate crystals may result from the charge difference between neodymium and the alkali cations as well as the oxygen: germanium ratio of these compounds, which favors glass network formation rather than crystallization.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Optical properties of Tm3+ ions in alkali germanate glass

Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 μm diodes because of low phonon energies. Spectroscopic analysis indicates low non-radiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.     

Optical properties of bismuth in germanate,borax and phosphate glasses

Absorption, emission, excitation spectra and quantum efficiencies of luminescence of bismuth were obtained in germanate, borax and phosphate glasses. Decay times were measured for fluorescence of bismuth in germanate glass at room temperature and liquid air temperature. A non-exponential decay of fluorescence at room temperature is observed. A thermal equilibrium between the ³P₁ and ³P₀ states is discussed. The dependence of fluorescence intensity and decay times on temperature is presented. The amount of covalency between bismuth and glass matrix was calculated from the nephelauxetic effect.     

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.     

Refractive index and material dispersions of multi-component oxide glasses

Refractive index dispersion curves in the wavelength region of 0.40 to 5.03 μm are presented for multi-component oxide glass systems: borate, silicate, aluminate, germanate, tellurite, antimonate and heavy metal gallate. The material dispersion was determined using the refractive index data. Reflection spectra in the ultraviolet and infrared regions were measured to investigate the effects of electronic transitions and lattice vibrations on the material dispersion. Thallium tellurite, thallium antimonate and lead gallate glasses exhibit zero material dispersion wavelengths (ZMDWs) over 2.4 μm. The factors affecting the ZMDW are discussed.     

Germanate anomaly in heavy metal oxide glasses: an EXAFS analysis

An EXAFS analysis of the spectra collected at the germanium K-edge for a series of lead–germanate and bismuth–germanate glasses is presented. The EXAFS-extracted structural parameters have allowed us to reconstruct the local neighbourhood of Ge atoms and verify current opinions on the germanate anomaly in the considered glasses.     

A model for the Ge–O coordination in germanate glasses

A successful model for the composition-dependence of the coordination number in borate glasses has been adapted for germanate glasses. The predicted coordination number has then been compared with neutron diffraction results for caesium and sodium germanate glasses, which show strong evidence of a significant growth and decline in the coordination number as modifier is added to the glass. The comparison with the predictions of the model support an interpretation in which the predominant higher coordination of germanium is five-fold, not six-fold.     

Study on photoluminescence properties of oxyfluoride germanate Glass

The oxyfluoride germanate glass and glass-ceramics were prepared by the melting–annealing method. The composition of the glass was chosen as 45GeO2–30BaF2–5B2O3–15AL2O3–5R2O–xSm2O3 (R = Li, Na, and K. x = 1.5, 2.0, and 2.5, mol%). Differential thermal analysis, X-ray diffraction, and photoluminescence spectra were carried out to study the effects of the concentration of Sm ions, the kinds of alkali ions and heat treatment on the photoluminescence properties of Sm ion doped oxyfluoride germanate glass. The results showed that the luminescence intensity of the oxyfluoride germanate glass increased when x changed from 1.5 to 2.0 and then decreased when x changed from 2.0 to 2.5. Among various R2O, the Li2O included glass has the strongest luminescence intensity among all the glasses. Heat treatment at 660 °C for 4 h resulted in the formation of the Ba3AlF9 nanocrystal and enhanced the luminescence intensity of the sample. The mechanism of luminescence intensity increase was discussed.     

Sub-critical crack growth in sodium germanate glasses

Sub-critical crack growth in binary sodium germanate glass was investigated over a wide range of the crack velocities, 10⁻⁷-10⁻² m s⁻¹, by using small-size specimens with double cleavage drilled compression configuration. For evaluating the intrinsic sub-critical crack growth, crack initiation and subsequent propagation of the crack were performed in heptane. With increasing Na₂O content in sodium germanate glass, sub-critical crack growth curve shifted toward higher stress intensity factors first up to 10 mol% Na₂O, but more addition of Na₂O caused the curve to shift to lower stress intensity factor regions. In other words, fracture toughness shows a maximum at the composition of 10Na₂O·90GeO₂, whose value is 1.07 MPa m^1/2. This compositional dependence of fracture toughness originates from the so-called germanate anomaly. On the other hand, the slope of sub-critical crack growth curve for the glass containing >10 mol% Na₂O was much shallower than that for soda-lime glass. These glasses are very fatigable even in inert condition. It is considered that this fatigue behavior can be caused by the microscopic structural variation, which is the presence of GeO₆ units in GeO₂ glass network, and that these units can be the fatigue crack path.     

Optical hole burning studies in europium doped oxide glasses

Europium doped sodium borate, silicate, borosilicate, germanate and tellurite glasses have been prepared by melt quenching technique. Optical spectroscopy techniques were used to characterize the divalent and trivalent ions. Optical hole burning technique was used to characterize rare-earth ion doped glasses for optical data storage. The hole burning mechanisms, which depend on the glass composition and the preparation methods used, are discussed.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.     

Study of fluorine losses in oxyfluoride glasses

Glasses in PbGeO₃–PbF₂–CdF₂ and GeO₂–PbO–PbF₂–CdF₂ systems were studied and the fluorine losses during synthesis were investigated. Samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and Raman scattering spectroscopy. The use of stoichiometric germanate glass, PbGeO₃, instead of introducing individual oxides (GeO₂ + PbO) lead to decreasing fluorine losses, as detected by a fluorine ion selective electrode. The main structural features obtained from vibrational spectroscopy could be described by a metagermanate basic structure permeating fluorine rich regions.     

Effect of optical basicity on Er3+ up-conversion luminescence in GeO2–R2O glasses

The effect of optical basicity on Er³⁺ up-conversion luminescence in germanate glasses is investigated under 980 nm excitation. The intensity of green and red up-conversion luminescence decreased with the increase in radius of alkali ion or Li₂O content, implying that up-conversion luminescence strongly relates to the optical basicity of glass host. On the other hand, as increasing the optical basicity, the red emission intensity decreased significantly, while the green emission intensity decreased slightly. It has been proposed that the up-conversion luminescence intensity was dominated by the optical basicity, which theoretically estimated from glass composition. The interaction mechanism between up-conversion process and optical basicity was proposed.     

Structure and electric conductivity of reduced lead–germanate,bismuth–germanate and bismuth–silicate glasses modified with potassium

Lead–germanate, bismuth–germanate and bismuth–silicate glasses, as potential materials for detectors production, have been modified with potassium with the purpose of improvement of mechanical properties. Studies on the influence of potassium modification on the structure and electric properties have been conducted with AFM, DSC and conductivity measurements. It has been observed that potassium modification improves the mechanical and technological properties of glasses and simultaneously changes slightly their electric properties. The studies have confirmed that bismuth–silicate and bismuth–germanate glasses are good materials for microchannel plates or channeltrons.     

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.     

Switching Behaviour of Pb5Ge3O11 Single Crystals

Single crystals of lead germanate have been grown by Czochralski method. Hysteresis and switching pulses could only be measured after treating the crystal samples at elevated temperature and electrical a.c. field. Hysteresis is strongly frequency dependent. We observed the motion of the domain walls in polarized light simultaneously. The hysteresis loop is distorted at elevated temperature because of electric conductivity of the crystals. Temperature dependence of electric conductivity has been measured.     

The structure of germanosilicate glasses,studied by X-ray photoelectron spectroscopy

O1s photoelectron spectra of several alkali silicate, germanate and germanosilicate glasses are given. The concentration ratio of bridging and non-bridging oxygen atoms is determined from these spectra. In contrast to silicate glasses, in germanate glasses non-bridging oxygen atoms are only observed when the alkali oxide mole fraction exceeds 0.18. The introduction of SiO₂ in the germanate glasses drastically alters the ratio of fourfold coordinated to sixfold coordinated germanium atoms.     

An estimation of the distribution functions of doped Tb and Nd in glasses by fluorescence measurements

The distribution functions of doped Tb³⁺ and Nd³⁺ in silicate, germanate, borate and phosphate glasses, in which cross-relaxation among active ions plays an important role, have been estimated by fluorescence measurements. In order to analyze both the steady state and the transient fluorescence characteristics, a non-linear model for the resonant energy transfer has recently been developed. The experimental fluorescence intensity and decay curves, measured as a function of acceptor concentration, suggested a modified distribution function, which included the effect of the segregation of donors and acceptors. The number of segregated Tb³⁺ ions at the second nearest neighbor position around a Tb³⁺ ion was estimated to be 0.1–3.4, depending on the host glass.