On the Urbach rule in non-crystalline solids

A comparative analysis of temperature behavior of optical absorption edge is performed for non-crystalline materials where the Urbach behavior is observed (SiO₂), with deviations from the Urbach behavior (20PbO · 80SiO₂) and with a mixed behavior (As₂S₃). For glassy As₂S₃, the Urbach behavior of the absorption edge in a limited temperature range is explained from the point of view of two components of structural disordering - static and dynamic. A parallel red shift of the absorption edge in As₂S₃ and, consequently, the temperature-independent Urbach energy at T < 300 K, are related to the lack of medium-range order in the atomic arrangement in the presence of short-range order.     

Crystallization kinetics determination of Pb15Ge27Se58 chalcogenide glass by using the various heating rates (VHR) method

The kinetics of crystallization of Pb₁₅Ge₂₇Se₅₈ was studied by differential scanning calorimetry non-isothermally. Various experimental methods are currently employed for determining the kinetic parameters of crystallization in a glassy system. These parameters include the activation energy of crystallization E (kJ/mol), the kinetic exponent n and the frequency factor K_o (s⁻¹). Recently, a new method (VHR method) has been derived from Johnson-Mehl-Avrami (JMA) transformation rate equation to calculate - in sequence - the crystallization kinetic parameters of a glassy system. The VHR technique has been used to estimate the crystallization parameters of Pb₁₅Ge₂₇Se₅₈ chalcogenide glass under non-isothermal conditions. The average value of E, n and K_o are found equal to 181.74 ± 0.58 (kJ/mol), 1.085 ± 0.023 and (9.196 ± 0.716) × 10¹² (s⁻¹), respectively. The kinetic exponent, n ≈ 1 indicates a surface nucleation mechanism.     

On a qualitative model for the incorporation of fluoride nano-crystals within an oxide glass network in oxy-fluoride glass-ceramics

We report on a Raman scattering study of oxy-fluoride glass-ceramics with a typical composition 32SiO₂:9AlO_1.5:31.5CdF₂:18.5PbF₂:5.5ZnF₂:3.5ErF₃ (in mol%), which indicates a narrow size distribution of β-PbF₂ fluoride nano-crystals, typically of 13 ± 1 nm, in the silica-based glass network and gives insight on how then nano-crystals are incorporated into the glassy network. The Raman spectra indicate the presence of Q⁰, Q¹ and Q²-like units, which are SiO_xF_4−x tetrahedra with zero, one and two bridging oxygens, respectively. The frequency, width and depolarization ratio of Raman bands corresponding to these Qⁿ-like units (n=0, 1, 2) indicate that Q⁰ units are mostly symmetric SiO₄ tetrahedra, Q¹ are SiO₄ tetrahedra where one F may substitute for O and Q² are SiO₄ tetrahedra where one or two F may substitute for O. We argue that the non-bridging O atoms belonging to Qⁿ tetrahedra, mostly to Q⁰ tetrahedra, are located near the interface between the nano-crystalline and glassy phases, allowing an easy accommodation of fluoride nano-crystals with a spherical shape, in the oxide glass network. This was in agreement with the very low-frequency Raman features found between ∼4 and 8 cm⁻¹, due to acoustic vibrations of the nano-crystals embedded in the glassy matrix.     

Interplay of ballistic and chemical effects in the formation of structural defects for Sn and Pb implanted silica

The electronic structures of Sn and Pb implanted SiO₂ are studied using soft X-ray absorption (XAS) and emission (XES) spectroscopy. We show, using reference compounds and ab initio calculations, that the presence of PbO and SnO interactions can be detected in the pre-edge region of the oxygen K-edge XAS. Via analysis of this interaction-sensitive pre-edge region, we find that Pb implantation results primarily in the clustering of Pb atoms. Conversely, with Sn implantation using identical conditions, strong SnO interactions are present, showing that Sn is coordinated with oxygen. The varying results between the two ion types are explained using both ballistic considerations and density functional theory calculations. We find that the substitution of Pb into Si sites in SiO₂ requires much more energy than substituting Sn in these same sites, primarily due to the larger size of the Pb ions. From these calculated formation energies it is evident that Pb requires far higher temperatures than Sn to be soluble in SiO₂. These results help explain the complex processes which take place upon implantation and determine the final products.     

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.     

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.     

Thermodynamic mixing properties of liquids in the system Na2O-SiO2

Enthalpies of fusion have been measured by differential scanning calorimetry for a Na₂O-SiO₂ system at 50, 66.6, and 74.4 mol% SiO₂. Enthalpies of mixing of liquids obtained from different calorimetry techniques are critically evaluated. The data on calorimetric enthalpy, activity of Na₂O, cristobalite liquidus, and immiscibility gap are used to determine the enthalpy and entropy of mixing of sodium-silicate liquids are determined as a function of composition by the least squares method. The derived mixing properties are based only on the experimental data and are independent of any assumption about the structure and chemical species in liquids. The enthalpy of mixing has a minimum value of −120 kJ/mol at 35-40 mol% SiO₂ and is convex upward around 80-90 mol% SiO₂. The entropy of mixing have a maximum value of + 6 J/K-mol at 75 mol% SiO₂, and it decreases with the SiO₂ content to −5 J/K-mol at 40 mol% SiO₂. This decrease in entropy can be accounted for by ideal mixing of Q⁴, Q³, and Q^{0 + 1 + 2} (= Q⁰ + Q¹ + Q²) species in the liquids and is responsible for the negative temperature dependence of the partial molar Gibbs energy of mixing of Na₂O, observed in activity measurements. Comparison of the present results with previous values suggests that a quasi-chemical model and the Adam-Gibbs model overestimate the configurational entropy of mixing of liquids.     

Structure study of multi-component borosilicate glasses from high-Q neutron diffraction measurement and RMC modeling

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65 − x)SiO₂ · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5-15 mol%. The structure factor has been measured up to a rather high momentum transfer value of 30 Å⁻¹, which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si-O network consists of tetrahedral SiO₄ units with characteristic first neighbor distances at r_Si-O = 1.60 Å and r_Si-Si = 3.0 Å. The boron environment contains two well-resolved B-O distances at 1.40 and 1.60 Å and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including ^[4]B-O-^[4]Si and ^[3]B-O-^[4]Si chain segments. The O-O and Na-O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions.     

Vitrification and crystallization in the system of K2O-B2O3-TiO2

The vitrification and crystallization behavior of melts produced at 1400 °C in the ternary system of K₂O-B₂O₃-TiO₂ is investigated. It is shown that there are two fields of compositions (indicated in mol%) which allow obtaining the glass-ceramic materials with continuous glassy matrix after the cooling of molten compositions. In the first field [TiO₂] = 25-57, [K₂O] = 30-50 and [B₂O₃] = 0-25, the glass-ceramics consisted of the potassium-titanium-borate glassy phase and different crystalline potassium titanates (K₄Ti₃O₈, K₂Ti₂O₅, K₂Ti₄O₉, K₂Ti₆O₁₃). The ratio of TiO₂:K₂O in the obtained titanates increases with [TiO₂] and [B₂O₃]. In the second field, [TiO₂] = 7-37, [K₂O] = 0-25 and [B₂O₃] = 52-93, the obtained glass-ceramics consisted of a similar vitreous phase, as mentioned above, and TiO₂ crystals. During the cooling of the melts, short whiskers-like crystals of anatase formed in the compositions with relatively low [TiO₂] and relatively high [K₂O], whereas long fiber-shaped crystals of rutile appeared with the compositions characterized with relatively high [TiO₂] and relatively low [K₂O]. The possible application of the obtained glass-ceramic materials as a source of fibrous TiO₂, for composite reinforcement, and as solid lubricants is discussed.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Structural investigations of magnesium silicate glasses by Si 2D Magic-Angle Flipping NMR

The two-dimensional Magic Angle Flipping Nuclear Magnetic Resonance (2D MAF NMR) experiment on ²⁹Si nuclei is used to determine the distribution of Q⁽ⁿ⁾ sites in two ²⁹Si-enriched magnesium silicate glasses with compositions 2MgO·SiO₂ and MgO·SiO₂. A significant degree of polymerization is observed in the 2MgO·SiO₂ glass, supporting previous studies using Raman and ²⁹Si NMR spectroscopy. Relative abundances of 0.629 ± 0.001 for Q⁽⁰⁾ and 0.371 ± 0.001 for Q⁽¹⁾ were obtained from spectral fits of the 2D MAF spectrum of the 2MgO·SiO₂ glass. Mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species were calculated and used in a thermodynamic model of Q⁽ⁿ⁾ disproportionation to calculate an equilibrium constant of k₀ = 0.04 ± 0.02 in 2MgO·SiO₂. In the MgO·SiO₂ glass relative abundance of 0.014 ± 0.001 for Q⁽⁰⁾, 0.191 ± 0.003 for Q⁽¹⁾, 0.530 ± 0.004 for Q⁽²⁾, 0.252 ± 0.003 for Q⁽³⁾, and 0.014 ± 0.001 for Q⁽⁴⁾ were measured. The mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species in MgO·SiO₂ were used to calculate corresponding disproportionation equilibrium constants of k₁ = 0.19 ± 0.02, k₂ = 0.174 ± 0.009, and k₃ = 0.11 ± 0.01. A comparison of k₃ values from previous MAF studies of various alkali and alkaline earth silicate glasses indicate an exponential increase in k₃ with the increasing modifying cation potential. Using the van't Hoff relation, we show that differences in both thermal history and modifier cation potential contribute to this spread in k₃ values. Nuclear shielding tensor anisotropy, ζ, and asymmetry, η, values of ζ = 0.0 ppm and η = 0.0 for Q⁽⁰⁾ and ζ = 33.0 ± 0.1 ppm, and η = 0.4 ± 0.1 for Q⁽¹⁾ in 2MgO·SiO₂ glass were determined from its 2D MAF spectrum. These values were used in obtaining the remaining values of ζ = − 36.0 ± 0.5 ppm and η = 0.99 ± 0.01 for Q⁽²⁾, and ζ = − 27.5 ± 0.5 ppm and η = 0.45 ± 0.11 for Q⁽³⁾, ζ = 0.0 ppm and η = 0.0 for Q⁽⁴⁾ in the MgO·SiO₂ glass from its 2D MAF spectrum. The magnitude of ζ values observed are lower than those reported in previous MAF studies of alkali and alkaline earth silicate glasses containing different modifier cations, consistent with previously reported trends in ζ versus modifying cation potential.     

The effect of alumina on the Sn/Sn redox equilibrium and the incorporation of tin in Na2O/Al2O3/SiO2 melts

Glasses with the basic compositions 10Na₂O · 10CaO · xAl₂O₃ · (80 − x)SiO₂ (x=0, 5, 15, 25) and 16Na₂O · 10CaO · xAl₂O₃ · (74 − x)SiO₂ (x=0, 5, 10, 15, 20) doped with 0.25-0.5 mol% SnO₂ were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na₂O concentration the peak potentials get more negative. Mössbauer spectra showed two signals attributed to Sn²⁺ and Sn⁴⁺. Increasing alumina concentrations did not affect the isomer shift of Sn²⁺; however, they led to increasing quadrupole splitting, while in the case of Sn⁴⁺ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mössbauer parameters.     

Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 2. Medium-range order

Characteristics of the medium-range order (MRO) of K₂O-GeO₂-P₂O₅ (KGP) glasses are obtained from X-ray and neutron scattering data. The behavior of the MRO is expressed in changes of pre-peaks and smooth contributions in the structure factors, S(Q), for Q < 12 nm^-1. Peaks at Q = (7.5 ± 0.5) nm^-1 (Q - magnitude of the scattering vector) are the outstanding features and reach maximum intensity for a glass of ∼25/50/25 mol% K₂O/GeO₂/P₂O₅. The pre-peaks are explained by a structural model in which K-rich and Ge-rich regions are separated by PO₄ units. The distance of repetition of similar regions (∼1 nm) is responsible for the pre-peak at ∼7.5 nm^-1. The intensity of this pre-peak is reduced and finally eliminated for glass compositions approaching the binary GeO₂-P₂O₅ system. The pre-peak is changed to a smooth scattering contribution and finally shifted to ∼9 nm^-1 for glass compositions approaching the binary K₂O-GeO₂ system. The strong tendency of the PO₄ units to coordinate K⁺ and Ge neighbors at their four corners is the source for the special MRO of the KGP glasses.     

Nucleation and grain growth in as deposited and ion implanted GeTe thin films

The crystallization dynamic of amorphous GeTe 50 nm thick films deposited on a SiO₂/Si substrate by RF magnetron sputtering, either ion implanted by Ge⁺ ions or not, has been analyzed in situ by optical microscopy during annealing in the 143-155 °C temperature range. Raman spectroscopy has been also performed in as deposited, ion implanted (i.i.) and melt quenched (m.q.) amorphous samples to compare the local order among the different amorphous structure. Nucleation and growth rates, for i.i. and as deposited samples, have been observed and directly compared by optical microscopy in a region of about 5 × 10⁴ μm². From these data, the activation energy and pre-exponential terms of each process have been calculated. The nucleation rate and growth velocity of the i.i. films increased by a factor thirteen and a factor three with respect to the as deposited samples. This evidence, in agreement with Raman spectroscopy data, suggests that implantation, providing kinetic energy by collision cascade, induces a local atomic rearrangement towards more relaxed amorphous states. As a result the crystallization kinetic is enhanced by the reduction of wrong bonds formed during sputter deposition, a process which occurs far from equilibrium conditions.     

Thermal stability, spectra and laser properties of Yb: lead-zinc-telluride oxide glasses

A series of new glasses of 70TeO₂-(20 − x) ZnO-xPbO − 5La₂O₃-2.5K₂O-2.5Na₂O (mol%) doped with Yb³⁺ is presented. Thermal stability, spectra and laser properties of Yb³⁺ ions have been measured. It found that 70TeO₂-15PbO-5ZnO-5La₂O₃-2.5K₂O-2.5Na₂O composition glass had fine stability ((T_x−T_g)>190 °C), high-stimulated emission cross-section of 1.25 pm² for the ²F_5/2 → ²F_7/2 transition and existed measured fluorescence lifetime of 0.94 ms and the broad fluorescence effective linewidth of 72 nm. Evaluated from the good potential laser parameters, this system glass is excellent for short pulse generation in diode pumped lasers, short pulse generation tunable lasers, high-peak power and high-average power lasers.     

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.     

Diffusion of the S isotope in soda-lime-silica and sodium trisilicate glass melts

The diffusivity of sulphur in nominal 10 Na₂O-16 CaO-74 SiO₂ (NCS) and 26 Na₂O-74 SiO₂ (NS3) melts was investigated in the temperature range 1273-1473 K using the ³⁵S radioactive isotope in a sandwich setup. Samples were sealed in platinum capsules and run with vertical alignment at 100 MPa confining pressure in an internally heated gas pressure vessel. Using the lowest diffusion coefficient D (m² s^− 1) for each temperature the Arrhenian relations logD = −(4.6 ± 0.3) − (216 ± 7) kJ mol^− 1/RT for NCS and logD = −(6.3 ± 0.6) − (167 ± 17) kJ mol^− 1/RT for NS3 were determined. Viscosity of the melts was used to test the applicability of the Stokes-Einstein (SE) and the Eyring (EY) equations to sulphur diffusion. The SE equation yields unrealistically low radii of the diffusing particles, a consequent of the non-molecular structure of the silicate melts. On the other hand, the Eyring relation yields reasonable jump distances of 550 pm (NCS) and 750 pm (NS3) by fitting the diffusion data to the EY equation. These large values imply that sulphate ions (ionic diameter = 290 pm) migrate as large entities through the silicate network.     

Preparation and optical properties of samarium doped sol-gel materials

Transparent crack-free glassy monoliths containing up to 0.2 Sm/Si are prepared at room temperature by sol-gel technique. The sol-gel preparation conditions are: acid catalyzed hydrolysis followed by basic catalyzed gelation and controlled drying. The prepared xerogels are characterized by X-ray diffraction, thermogravimetry with Fourier transforming infrared spectroscopy, scanning electron microscopy and optical spectroscopy. The values of the phenomenological Ω-intensity parameters of Sm(III) ions in the gels are (in 10⁻²⁰ cm²) Ω₂ = 76 ± 16.71, Ω₄ = 3.01 ± 0.21, Ω₆ = 2.99 ± 0.41. The oscillator strengths of the Sm(III) ions in the gels are calculated. The results show a linear concentration dependence of Sm(III) in UV/Vis absorption spectra and formation of a Sm(NO₃)₃ · 6H₂O complex at high samarium concentrations.     

Sodium tracer diffusion in glasses of the type (Na2O)0.2(B2O3)y(SiO2)0.8 − y

Sodium tracer diffusion coefficients, D^?_Na, have been measured in sodium borosilicate glasses of the type (Na₂O)_0.2(B₂O₃)_y(SiO₂)_0.8−y as a function of temperature and the composition parameter y. At constant temperature, the tracer diffusion coefficient of sodium decreases as y increases. The activation enthalpy derived from sodium tracer diffusion data for temperatures up to about 350 °C increases about linearly with increasing values of y from about 70 to 100 kJ/mol. The pre-exponential factor of the sodium tracer diffusion coefficient as a function of y varies by about one order of magnitude and has a minimum at near y = 0.3.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.