Multifractal nature of heterogeneous dynamics and structures in glass forming ionic liquids

Complex dynamics and structures of ionic liquids exemplified in 1-ethyl-3-methylimidazolium nitrate (EMIM-NO₃) are examined by molecular dynamics simulations. Correlation length of the radial charge distribution function and density distribution function show different temperature dependence. Density profiles are obtained from the accumulated positions visited by ions during the MD runs. The profile originating from the coexistence of layered structures of density (density wave) and those of charges (charge density wave), shows complicated heterogeneity, which is proven to be multifractal in nature. Thus, present is more than one characteristic length scale together with their mixing. The multifractal density profiles are formed by the multifractal walks with fast and slow ions. Generally, the coexistence of different length scales due to the different species or the different local structures can be the mechanism to form similar multifractal dynamics and structures.     

Ionic jump processes and high field conduction in glasses

A theory of multiple-ion jumps is proposed to explain the large apparent jump distances which have been calculated from high field conduction in glasses. An analysis of published data on correlation coefficients suggests that multiple ion jumps up to about four ions at a time may occur in single-alkali glasses. In mixed-alkali glasses such processes are shown to be less probable. Measurements of high field conductivity were made on both single- and mixed-alkali glasses, using blown membranes between N/10 acid solutions. The onset of departures from Ohm's law occurred at fields of about 10⁷ V m^?1 from which a jump distance of about 25–30 Å is calculated. This is too large for the proposed multiple-ion jump processes suggested, and, furthermore, the high field currents followed the Poole-Frenkel (PF) law. An alternative theory is given in which the PF law is derived for ionic migration: shallow coulombic potential basins are assumed to be distributed throughout the volume of the glass, owing to non-bridging oxygens whose charge is not balanced by cations on adjacent sites.     

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu₂O₃) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.     

Structure and lithium ion diffusion in lithium silicate glasses and at their interfaces with lithium lanthanum titanate crystals

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Lithium containing glasses such as lithium silicate glasses have been widely studied due to their high ionic conductivity. Recently, lithium silicate glasses were introduced in polycrystalline lithium lanthanum titanate (LLT) ceramics as intergranular thin films between the crystalline grains to achieve higher lithium ion conductivities in these solid state electrolytes. In this work, we present investigations of the structure and diffusion behavior of lithium silicate glasses and their interfaces with LLT crystals using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces, as well as the effect of atomic structure on diffusion behaviors, were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. The interface lithium ion diffusion energy barrier was found to be dominated by the glass phase.     

Spectroscopy of transition metal ions in inorganic glasses

Transition metal (TM) ions have been used as colouring agents in the glass industry for a long time. Recently, great attention has been paid to the TM ion doped glasses for the development of new lasers or luminescence materials. The absorption spectra of TM ions in different kinds of glasses have been studied extensively, but little work has been done for fluorescence and relaxation spectra. In this paper emphasis is laid on analysing the influence of chemical bond characteristics of the base glass on the spectra and the site structure of transition metal ions in glasses. Recent experimental results about the luminescence characteristics of low valence ions (such as Ti³⁺, Cr³⁺, V²⁺, Mn²⁺, Cu⁺, Mo³⁺) in glasses are also reported.     

Distributions of copper and CuBr in CuBr nanocrystallite-dispersed glasses prepared by copper staining

CuBr nanocrystallite-dispersed glasses were prepared by incorporation of copper into bromide ion-containing borosilicate glass using the technique of copper staining. The copper ion incorporation process was mainly controlled by ionic diffusion from the surface to the interior of the glass. The depth profiles of Cu and CuBr concentration were examined by energy dispersive X-ray analysis and by reference to the change in absorption intensity assigned to the CuBr exciton band along the depth. While the Cu concentration was found to decrease monotonically, the CuBr concentration profile showed a maximum at a distance of 10-50 μm from the glass surface. Although the depth reached by the copper ions became greater with increasing heat-treatment time, the depth at which CuBr was precipitated was found to be saturated. This means that regions were found in the glasses in which no CuBr crystallites precipitated, although migration of Cu ions to these regions had taken place.     

Fast and slow dynamics in single and mixed alkali silicate glasses

A molecular dynamics (MD) simulation for single and mixed alkali metasilicates has been performed. The mixed alkali effect (MAE) in the diffusion coefficients is reproduced. The motion of lithium ions in Li₂SiO₃ can be divided into slow (type A) and fast (type B) categories clearly in the glassy state. It turns out that the slow dynamics are caused by localized jump motions with a long tail of the waiting time distribution. On the other hand, fast dynamics of the lithium ions are caused by successive cooperative jump motions and are identified as Lévy flights. The MAE in LiKSiO₃ occurs due to mutual interception of jump paths of both kinds of mobile ions. In such paths, the fast component is considerably suppressed. Namely, the cooperative forward-correlated jump process is sensitive to the blockage of the jump paths. Propagation of the immobilization in the MAE is discussed in relation with the mechanism of the cooperative jump motion. The ratio of the fast and slow dynamics plays an important role to determine the transport properties in the glassy state.     

Compositional dependence of the first sharp diffraction peaks in alkali silicate glasses: A molecular dynamics study

The compositional dependence of the first sharp diffraction peaks (FSDPs) of lithium, sodium and potassium silicate glasses were studied by calculating the neutron static structure factors. The advantage of using molecular dynamics simulations is that the contributions of partial structure factors to the FSDPs can be easily determined and provides the basis for detailed analysis. Examination of the correlations of the FSDP with the short and medium range structure reveal that the position and the shape of FSDP strongly depend on the type and concentration of alkali oxide in alkali silicate glasses. The characteristic repeating distances and the characteristic correlation lengths both decrease with increasing alkali oxide concentration indicating a decrease in the intermediate range order. In the lithium silicate glasses, the characteristic correlation length increases with lithium oxide concentration that is anomalous in that the trend is opposite to the other alkali silicate glasses. This anomaly is explained by the high field strength of lithium ions that increases the intermediate range order of the silicon oxygen network.     

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 Debye-Falkenhagen theory of electrical relaxation in glass

The Debye-Falkenhagen-Tomozawa theory (DFT theory) of electrical relaxation in glass is reviewed. This theory interprets the principal electrical relaxation in ionically conducting glasses in terms of relaxation of Debye-Huckel ion atmospheres. Electrical relaxation data is presented to demonstrate that in typical alkali silicate glasses the concentration of free ions is too great for the DFT theory to hold. Further data in a series of borosilicate glasses containing Na₂O at the 0.01 mol% level offers evidence for the validity of the theory in the dilute alkali regime. However, a quantitative fit to the theory can only be made under the assumption that less than 8% of the sodium in the glass is present as free ions at 400°C. This result is discussed in terms of the Bjerrum theory of ionic association.     

Correlation of ion dynamics and structure in superionic glasses and nanocomposites

Ion dynamics of AgI-doped Ag₂O–TeO₂ glasses and Ag₂S doped glass nanocomposites have been studied using impedance spectroscopy and correlated with their structures investigated using Fourier transform infrared spectroscopy. The composition dependences of the dc conductivity and the activation energy of these glasses and nanocomposites have been compared with those of AgI-doped silver phosphate and borate glasses. We have studied the ion dynamics in the framework of the power-law and the electric modulus formalisms. We have established a correlation between the crossover rate of the mobile silver ions and the rearrangement of the structural units of glassy networks. The scaling of the conductivity spectra has been used to interpret the temperature and composition dependence of the relaxation dynamics. Analysis of the dielectric relaxation in the framework of modulus formalism indicates an increase in ion–ion cooperation in the glass compositions with increasing AgI content.     

Elasticity of ion stuffing in chemically strengthened glass

The chemical strengthening of glass involves the stuffing of large ions into network sites previously occupied by smaller ions. Typically this involves an exchange of Li⁺ or Na⁺ ions in the glass for larger K⁺ ions from a molten salt bath. This swapping of ions creates compressive stress in the surface of the glass, significantly increasing the strength of the final glass product. The magnitude of this compressive stress is governed by the linear network dilation coefficient (LNDC), which defines the amount of linear strain per unit of ion substitution. However, the amount of strain attainable through ion exchange is much smaller compared to what is expected from as-melted versions of the same final glass composition. This effect, which we have termed the “network dilation anomaly,” is a result of the different local environment around the invading ion species in as-melted versus ion-exchanged glasses. A remaining question concerns the nature of the network strain due to ion stuffing. Using molecular dynamics simulations, we show that the strain induced by ion stuffing is entirely elastic. In other words, when a reverse ion exchange is performed to swap the original ions back into the glass, the initial volume of the as-melted glasses is entirely recovered. Moreover, we show that the local structural environment around the alkali ions is restored to the as-melted conditions. The elastic nature of ion stuffing demonstrates that the network dilation anomaly is not a result of plasticity, but rather a failure to achieve the full amount of expected elastic strain during ion exchange. The elasticity itself consists of both instantaneous and delayed contributions.     

Atomistic understanding of the network dilation anomaly in ion-exchanged glass

Chemically strengthened glasses are of increasing technological importance for personal electronic devices, tablet computers, and a variety of other applications. However, there are many unexplained phenomena associated with the physics of the ion exchange process used for strengthening. One of the most puzzling of these is the anomalous behavior of the network dilation coefficient, i.e., the parameter governing the amount of linear strain of the glass per unit of alkali ions exchanged, which is inevitably a factor of 2–4 higher for as-melted glasses as compared to chemically strengthened versions of the same glass compositions prepared via ion exchange. In this paper, we investigate the atomistic origin of this discrepancy between as-melted and ion-exchanged glasses based on molecular dynamics simulations of a series of alkali tetrasilicate glasses, viz., xNa₂O·(20 ? x)K₂O·80SiO₂ (mol%). The network dilation coefficient of the ion-exchanged glasses is dependent on composition and ranges from 30% to 54% of the ideal value obtained from the as-melted glasses. This anomalous behavior of the network dilation coefficient is explained in terms of different local environments between sodium and potassium sites in the glass network and a two-stage relaxation process of the local potassium environment after ion exchange.     

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.     

Chromium doping of silicate glasses by field-assisted solid-state ion exchange

Field-assisted solid-state ion exchange (FASSIE) is a suitable way to dope glasses with metallic ions. This approach is a promising technique for the production of glass waveguides containing either bivalent or trivalent ions, allowing the doping of glass surfaces with multivalent ions which could not diffuse into the glass matrix by means of the usual thermal ion-exchange process in molten salt baths. In this paper, results on the diffusion of chromium in silicate glasses are presented. A metallic chromium film deposited on the top of the glass substrates was used as the metal ions supplier. The doped layers were investigated by secondary ion mass and Rutherford backscattering spectrometries, as well as by micro-Raman spectroscopy. Chromium entered the glass matrices for some hundreds of nanometers, depending on the process temperature and the applied electric field. Strong compositional modification of the treated glasses was detected, related to alkali and alkali-earth elements distribution. For field-assisted solid-state diffusion, borosilicate glasses seem to be more stable matrices than the soda-lime silicate ones.     

The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study

A series of 27 silicate glasses of various compositions containing 0.2-2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.     

A neutron diffraction investigation of the structure of vitreous zinc chloride

A survey is presented of the AX₂ glasses and the corresponding crystalline polymorphs. The relationship between ZnCl₂ and the other members of the series is discussed and it is predicted that the number of shortest path seven and higher-membered rings in the non-chalcogenide AX₂ glasses is likely to be small. A neutron diffraction investigation of vitreous ZnCl₂ using the D4 diffractometer at ILL Grenoble shows that the structure of vitreous ZnCl₂ comprises a distorted random close packed array of Cl^? ions with the Zn²⁺ ions in tetrahedral holes, arranged in such a way as to maximise corner sharing of the resulting ZnCl₄^2? tetrahedra at the expense of edge and face sharing. The inability of molecular dynamics simulations, using purely ionic potentials, to predict the ZnZn component correlation function is taken as an indication of covalent character to the ZnCl bond. The crystalline polymorph formed on devitrification of dry ZnCl₂ glass is δ-ZnCl₂ and not the α-form as previously reported.     

State of the chromium ion in soda silicate glasses under various oxygen pressures

Optical absorption and ESR spectra of chromium in soda silicate glasses were measured to characterize the electronic environment of the chromium ion in these glasses. Glasses produced in very strongly reducing conditions showed a broad optical absorption in the wavenumber range of 10 000–25 000 cm^?1 without any ESR absorption, which suggested the formation of Cr²⁺ ions. Glasses produced in air or moderately reducing conditions showed ESR signals suggesting that there are three different states of Cr³⁺ ions, the strongly coupled Cr³⁺ ion pairs, the weakly coupled Cr³⁺ ion pairs and the isolated Cr³⁺ ions in elongated tetragonal sites or sites with lower symmetry. The presence of Cr⁵⁺ ions in glasses produced in air was also suggested. It was indicated that the critical partial oxygen pressure of the formation of Cr²⁺ ions is in the vicinity of P_O2 = 10^?8 atm and that Cr²⁺ ions do not coexist with Cr³⁺ ions homogeneously in soda silicate glasses.     

Hydration of soda-lime glass

The hydration of soda-lime glass is studied using resonant nuclear reactions to measure the hydrogen and sodium profiles of hydrated glasses. The rate of growth of the surface layer of hydrated glass is initially proportional to the square root of time as is characteristic of diffusion controlled processes. After longer exposure a steady-state hydration profile is observed, which indicates that in addition to the diffusion controlled reaction there is a slow etching of the glass surface. The measured hydration profiles are discussed in relationship to the Doremus model of interdiffusing ions, which is found to be in good agreement with the data. This model is also discussed in relationship to measured hydration profiles of vacuum heated samples of hydrated glass.     

A quantitative explanation of the difference between nuclear spin relaxation and ionic conductivity relaxation in superionic glasses

Electrical conductivity relaxation (ECR) experimental data represented by either the complex conductivity or the complex electric modulus are macroscopic in nature. In contrast to ECR, nuclear spin relaxation is a more direct probe of ionic movement and from its result we can infer the microscopic dynamics of the ions. Combined studies of ionic motion using ECR and nuclear spin-lattice relaxation (SLR) in several glassy ionic conductors have shown a large difference between the ECR and SLR times. Any theory that attempts to explain quantitatively the difference faces the dilemma of how to compare the SLR time with the ECR time. In this work we use a recent result [Phys. Rev. B 60 (1999) 9396] to find the ion hopping correlation time from the experimental ECR time. Next, we use the coupling model to calculate the SLR time from the ion hopping correlation time. Good agreements are obtained in three glassy ionic conductors and a crystalline ionic conductor.