Three-dimensional structure of fast ion conducting 0.5Li2S + 0.5[(1 − x)GeS2 + xGeO2] glasses from high-energy X-ray diffraction and reverse Monte Carlo simulations

A high-energy X-ray diffraction study has been carried out on a series of 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses with x = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8. Structure factors were measured to wave vectors as high as 30 Å⁻¹ resulting in atomic pair distribution functions with high real space resolution. The three dimensional atomic-scale structure of the glasses was modeled by reverse Monte Carlo simulations based on the diffraction data. Results from the simulations show that at the atomic-scale 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses may be viewed as an assembly of independent chains of (Li^+-S)₂GeS_2/2 and (Li^+-O)₂GeO_2/2 tetrahedra as repeat units, where the Li ions occupy the open space between the chains. The new structure data may help understand the reasons for the sharp maximum in the Li⁺ ion conductivity at x ∼ 0.2.     

Study of the alkaline environment in mixed alkali compositions by multiple-quantum magic angle nuclear magnetic resonance (MQ–MAS NMR)

45S5 Bioglasses of the composition 46.1 SiO₂–2.6 P₂O₅–26.9 CaO–(24.4 ? x) Na₂O–xMe₂O (Me = Li or K) have been investigated using MAS NMR and MQ–MAS NMR methods. The analysis of the ²⁹Si MAS NMR spectrum revealed two lineshapes whose chemical shift is consistent with two silica Q_n=2,3 species. The ³¹P MAS NMR spectrum reveals the effect of both Na and Ca ions. The chemical shift of the observed ³¹P signal is intermediate between those of Na₃PO₄ (near 10 ppm) and Ca₃(PO₄)₂ (near 3–0 ppm) species. The ²³Na MAS NMR spectra were observed in the alkali oxide composition: 24.4 Na₂O, 12.2 Na₂O–12.2 K₂O and 12.2 Na₂O–12.2 Li₂O. The substitution of Na with Li or K was done to determine the extend of alteration of the glass structure. This goal was best accomplished by ²³Na MQ–MAS NMR. The two-dimensional spectra revealed three sites in the 24.4 mol% Na₂O glass. These sites were not resolved in the 1D MAS NMR spectroscopy. In the mixed glasses, only two sites were obtained.     

Crystal structure of Li2MgSiO4

The crystal structure of Li₂MgSiO₄ was established by single-crystal X-ray diffraction analysis. The crystals are monoclinic, a = 4.9924(7) Å, b = 10.681(2) Å, c = 6.2889(5) Å, β = 90.46(1)°, Z = 4, sp. gr. P2₁/n, V = 335.54 ų, R = 0.062. In a Li₂MgSiO₄ crystal, four types of independent T(1–4) tetrahedra share vertices to form a three-dimensional framework. Three of these tetrahedra are occupied simultaneously by Li and Mg cations, which corresponds to the crystallochemical formula (Li_0.98Mg_0.02)(Li_0.80Mg_0.20) · (Li_0.22Mg_0.78)SiO₄. In slightly distorted SiO₄ tetrahedra denoted as T(1), the average Si-O distance is 1.635(2) Å. The distortions of other tetrahedra and the average (Li _x Mg_{1 ? x} )-O distances increase with an increase in lithium content. These distances in the T(2), T(3), and T(4) tetrahedra are 1.955(2), 1.971(4), and 2.019(6) Å, respectively. The structure of the new compound is compared with the crystal structures of other Li₂ M ²⁺SiO₄ compounds and the luminescence spectra of Cr⁴⁺: Li₂MgSiO₄.     

Photoluminescence and structural studies on Na2O–PbO–SiO2 glasses

PbO_x[(Na₂O)_0.25(SiO₂)_0.75]_1−x glasses (with x = 0.1–0.30) were prepared by a conventional melt-quench method and characterized by UV–visible optical absorption, photoluminescence and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) studies. It has been confirmed that the increased number of non-bridging oxygen atoms brought about by the increase in Q² structural units of silicon is responsible for the shifting of the wavelength corresponding to the onset of absorption, as PbO concentration increases in the glass. Emission in the visible region (400–450 nm), from these glasses has been attributed to the presence of L-centers, whose the peak maxima and line widths systematically shift to higher values by incorporating Pb²⁺ along with Na⁺ in the network modifying positions. All the Pb²⁺ ions in these glasses occupy only the network modifying positions, as revealed by the identical chemical shift values of Qⁿ structural units of silicon with increase in PbO concentration. In contrast to binary lead silicate glasses, no luminescence has been observed for comparable concentration of PbO, possibly due to the significant quenching of Pb²⁺ ions in the excited state.     

Electrical transport properties of 0.5Li2O-0.5M2O-2B2O3 (M = Li, Na and K) glasses

Transparent glasses in the system 0.5Li₂O-0.5M₂O-2B₂O₃ (M = Li, Na and K) were fabricated via the conventional melt quenching technique. The amorphous and glassy nature of the samples was confirmed via the X-ray powder diffraction and the differential scanning calorimetry, respectively. The frequency and temperature dependent characteristics of the dielectric relaxation and the electrical conductivity were investigated in the 100 Hz-10 MHz frequency range. The imaginary part of the electric modulus spectra was modeled using an approximate solution of Kohrausch-Williams-Watts relation. The stretching exponent, β, was found to be temperature independent for 0.5Li₂O-0.5Na₂O-2B₂O₃ (LNBO) glasses. The activation energy associated with DC conduction was found to be higher (1.25 eV) for 0.5Li₂O-0.5K₂O-2B₂O₃ (LKBO) glasses than that of the other glass systems under study. This is attributed to the mixed cation effect.     

X-ray photoelectron spectroscopy of erbium-activated-silica–hafnia waveguides

X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er³⁺-activated xHfO₂–(100 − x)SiO₂ (x = 10, 20, 30, 40, 50 mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO₂ content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO₂ and SiO₂ rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO₂ domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.     

Molecular dynamics simulation of ternary glasses Li2S–P2S5–LiI

The structure of superionic glasses in ternary systems x(0.4Li₂S–0.6P₂S₅)–(1 − x)LiI and x(0.5Li₂S–0.5P₂S₅)–(1 − x)LiI (x = 0.9, 0.75) has been studied by molecular dynamics (MD) simulations. The configurations obtained by MD were analyzed by graph theory. Phosphorus is surrounded by sulfur and iodine atoms. Li is surrounded by sulfurs alone and LiI clusters are not present as speculated by earlier spectroscopic reports. The equilibrium configuration is made up of (Li, S) and (P, S, I) rich regions which creates wide channels for Li⁺ movement. Reported variations of glass transition temperature (T_g) and ionic conductivity (σ) with LiI addition are explained based on the simulation results.     

Fabrication of a high lithium ion conducting lithium borosilicate glass

A lithium ion conducting glass, Li₂O-B₂O₃-SiO₂, is fabricated by the conventional melt and quenching technique from a mixture of Li₂CO₃, B₂O₃ and SiO₂ powders. It appears that B₂O₃ decreases the crystallization tendency of the Li₂O-SiO₂ binary glass, resulting in an expanded glass forming region in the Li₂O-B₂O₃-SiO₂ ternary glass. The maximum conductivity is 2 × 10^− 6 S cm^− 1 at 25 °C for the 50Li₂O-38B₂O₃-12SiO₂ glass sample. The observed high conductivity is due to the mixed former effect. The conductivity strongly depends on the Li₂O content, but not on K (SiO₂/B₂O₃) in the Li₂O-B₂O₃-SiO₂ ternary glass.     

Raman and NMR study of bioactive Na2O–MgO–CaO–P2O5–SiO2 glasses

The results of a structural study combining NMR and Raman spectroscopy of several melt-derived glasses in the system Na₂O–MgO–CaO–P₂O₅–SiO₂ are presented. The Raman spectra show clear changes in the Si–O–Si vibrational modes (related to the bridging oxygen atoms, BO) and also verify the presence of non-bridging oxygen atoms (NBO), also named terminal oxygens. The intensity of the Si–O–NBO stretching mode depends on the cation concentration. It can be concluded from the NMR studies that the MgO-containing samples have orthophosphate units charge-compensated by Ca²⁺ and Mg²⁺. The silicate matrix also contains both types of two-valent cations and consists of Q² and Q¹ units. Similarly, the Na₂O-containing samples contain isolated orthophosphate units in a silicate matrix (Q² and Q³ units), both charge-compensated by mixed cations Ca²⁺ and Na⁺. These experimental data were compared with theoretical parameters given by the Stevels model, which is a suitable tool for understanding bioactive behavior of these glasses. Furthermore, results of the in vitro tests carried out in simulated body fluids are presented and compared with both Raman and NMR structural data.     

Potential energy landscape of Li+ ions in lithium silicate glasses: Implications on ionic transport

The potential energy landscapes of Li⁺ ions in Li₂O–SiO₂ glasses containing 3.3–15 mol% Li₂O have been studied using molecular dynamics simulation. It is shown for the first time that the densities of states for Li⁺ ions follow a nearly universal logarithmic distribution irrespective of the Li concentration. Such a functional form of the ionic density of states is shown to provide an explanation for the experimentally observed logarithmic dependence of the activation energy of dc conductivity on the modifier ion concentration in a wide variety of glasses.     

The short-range structure of alkali disilicate glasses by pulsed neutron total scattering

Time-of-flight total neutron scattering experiments using pulsed neutrons with short wavelength produced by an electron linear accelerator have been carried out to measure the structure factors of SiO₂ and M₂O · 2 SiO₂ (M = Li, Na and Li_0.63Na_0.37) glasses over a wide range of the scattering vector up to Q (=4π sin θ/λ) ～480 nm^?1. The Placzek correction has been considered up to the second order in the energy transfer. The Fourier transform of the structure factors yield the pair distribution functions highly resolved in real space, from which the stretching of the SiO and OO atomic distances and the number of bridging and non-bridging oxygen atoms in the alkali disilicate glass have been obtained.     

Formation and structural characteristics of Pd–Ag/SiO2 and Pd–Cu/SiO2 catalysts synthesized by cogelation

Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of either Pd and Ag or Pd and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). After an extensive study of the influences of synthesis operating variables over structural characteristics of gels, highly dispersed bimetallic Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts were obtained. These samples are then composed of completely accessible Pd–Ag and Pd–Cu alloy crystallites with sizes of 2–3.5 nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears also that the metal complex acts as a nucleation agent in the formation of silica particles.     

Structure of hybrid (organic/inorganic) TiO2–SiO2 xerogels II: thermal behavior as monitored by temperature-programmed techniques and spectroscopy

Titanium-containing xerogels made by acid pre-hydrolysis of tetraethylorthosilicate (TEOS) and dimethyl-dimethoxysilane (SiMe₂(OMe)₂), followed by reaction with dichloro-diisopropyl titanium (TiCl₂(i-OPr)₂) were characterized by means of several techniques. The decomposition patterns of these xerogels, both in the presence and in the absence of oxygen are discussed in terms of the gaseous products' evolution under temperature-programmed (TP) reaction conditions, and diffuse reflectance infrared spectroscopy (DRIFTS). The primary decomposition routes of residual alkoxide groups are: (i) auto-redox (CO, CO₂) and dehydration (to olefins) under pyrolysis conditions; (ii) oxidative (CO, CO₂) under flowing air. The decomposition of the Si–C bond upon heating, as monitored by all three techniques, follows a pattern independent of that of (CH) species from −OR groups. The ²⁹Si MAS/NMR of these materials is dominated by the so-called D² (Me₂Si(OX)₂), Q³ ((OR)Si(OX)₃) and Q⁴ (Si(OX)₄) resonances. Fairly homogeneous proton envelopes around Si centers are evident from the NMR spectra of these materials in the proton cross-polarization mode.     

Bond character,optical properties and ionic conductivity of Li2O/B2O3/SiO2/Al2O3 glass: Effect of structural substitution of Li2O for LiCl

A glass of composition (20 ? x)Li₂O–xLiCl–65B₂O₃–10SiO₂–5Al₂O₃ where 0 ? x ? 12.5 wt% is prepared using the normal melt-quenching technique. The optical constants and electrical conductivity and their correlation are investigated, furnished and discussed with the substitution of Li₂O for LiCl. The mechanism of the optical absorption and the calculated Urbach energy follow the rule of phonon-assisted transitions. The ionic conduction mechanism is determined by activation energy process. Substitution up to 10 wt% LiCl provides high ionic conductivity (1.9 × 10^?2 Ω^?1 m^?1) due to the high average electronegativity of LiCl which increases the polarizability of lithium ions. The small cation–anion distance approach confirmed the enhancement in ionic conductivity of LiCl containing glass compared to that of Li₂O. Due to the large size of Cl^? ions, there is an expansion of the lattice which in turn broadens the available path windows. For 12.5 wt% LiCl, anomalous density behavior is observed and a reduction in conductivity is occurred, σ = 5.4 × 10^?3 Ω^?1 m^?1. Owing to the model of bond fluctuation, the reduction is attributed to the increase in the alkali halide concentration which creates bottlenecks that hinder the motion of Li⁺ ions. The ionic conductivity character is strongly supported by the behavior of the glass ionicity factor, density, molar volume, refractive index, average boron–boron separation, molar refraction, metallization criterion and non-bridging oxygen concentration of the studied glass.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

Compositional dependence of ultraviolet fluorescence intensity of Ce in silicate, borate, and phosphate glasses

Fluorescence spectra of Ce³⁺ ions in silicate, borate, and phosphate glasses melted in Ar were measured. The relative fluorescence intensity of Ce³⁺ in the ultraviolet region increased in the order of R = Ba, Ca, Sr, and Mg in the 20Li₂O-20RO-60SiO₂ glass samples and with decreasing BaO content in the BaO-B₂O₃ glass samples, respectively. In contrast, the relative fluorescence intensity of Ce³⁺ did not change with varying the glass composition in phosphate glass samples. The compositional dependence of the relative fluorescence intensity of Ce³⁺ is discussed in terms of redox reaction of Ce³⁺-Ce⁴⁺ in oxide glasses.     

Studies on binary silicate glasses based on the SiKα and SiKß emission X-rays

The SiKα and Kβ X-ray emission spectra of binary silicate glasses of the Li₂O–SiO₂, Na₂O–SiO₂, K₂O–SiO₂, Cs₂O–SiO₂, B₂O₃–SiO₂, and GeO₂–SiO₂ systems were measured with an X-ray fluorescence spectrometer to examine the chemical effects on the spectra. The SiKα peak wavelengths for all the glasses agreed with that for SiO₂ glass, which corresponded to the concept that the coordination number of Si shouldbe four in all the glasses examined in this study. The SiKβ peak wavelength decreased with increasing alkali content in the alkali silicate glasses, indicating that the Si–O bond weakend in average as the alkali oxide was added to SiO₂ glass. On the other hand, no drastic shift in the SiKβ peak wavelength was observed in the B₂O₃–SiO₂ and GeO₂–SiO₂ systems, and this was interpreted as showing the constancy of Si–O bond strength in these glasses.     

X-ray spectroscopic investigation of the structure of silica,silicates and oxides in the crystalline and vitreous state

The Si Kα_1,2 emission lines and their satellite lines α′, α₃ and α₄ were measured for several samples of vitreous silica (Suprasil W, Infrasil, Suprasil), sodium silicate glasses (8, 15, 20 and 25 wt% Na₂O), and crystalline Mg₂SiO₄ (Forsterite). The observed shifts of the peak positions indicate a systematic increase of the electron density on the silicon atoms with increasing break-up of the SiO₂ network by OH^? or alkali ions. These results are compared with information from the corresponding Si Kβ and O K emission bands and also with the O K emission bands from quartz, MgO and Al₂O₃. They are discussed on the basis of the MO theory and are compared with the characteristic physical properties and structure of silica and silicate glasses. Both the O K and Si Kβ emission bands are closely related to the electronegativities of the relevant metal atoms of the oxides and glasses.     

Structural rules of phase separation in alkali silicate melts analyzed by high-temperature Raman spectroscopy

The structure of alkali silicate melts with the compositions around the immiscibility dome, i.e. xR₂O-(100 − x)SiO₂ (R = Li, Na; x = 15, 25, 33) were investigated by high-temperature Raman spectroscopy. The distributions of structural units of Qⁿ were estimated as a function of temperature and composition through the curve fitting of spectra based on the Qⁿ equilibration 2Q³ ↔ Q² + Q⁴. The Qⁿ distributions of lithium silicate melts were insensitive to temperature, while those of sodium silicate melts showed a temperature dependence especially in the high sodium concentration region. By using the temperature and composition dependences of the Qⁿ distribution, a new expression of the non-ideal entropy of mixing (ΔS_mix) for the silicate system is proposed. In order to clarify the relationship between the Qⁿ equilibration and the entropy change of alkali silicate melts, the Qⁿ distributions were summarized as isothermal Qⁿ curves on the Q²–Q³–Q⁴ ternary diagram (Qⁿ diagram) on a contour map of ΔS_mix. The Qⁿ diagrams reveal unique characteristics of the melt under the immiscibility condition. It is suggested that phase separation takes place in a specific composition range, where the system cannot decrease entropy by structural change via the equilibration of Qⁿ species.