A structural basis for ionic diffusion in oxide glasses

Electronic, mechanical, optical and piezoelectric properties of glass-like sodium silicate (Na2SiO3) under compressive pressure

The structural, mechanical, electronic, optical and piezoelectric properties of Na₂SiO₃ are studied under varying compressive unidirectional pressure (0–50 GPa with a difference of 10 GPa) using density functional theory (DFT). The calculated structural properties agree well with previously reported results. At 12 GPa, our calculation shows a structural phase transition from orthorhombic Cmc2₁ to triclinic P1. The mechanical profile of Na₂SiO₃ structures under different compressive unidirectional pressures are analysed by calculating the elastic moduli, Poisson’s ratio and eigenvalues of stiffness matrix. Our study shows the mechanical stability of the system up to a pressure of 40 GPa. Herein, we have obtained an indirect band gap of 2.97 eV at 0 GPa. Between 0–50 GPa, the band gaps are within the range 2.62 to 3.46 eV. The system in our study possesses a wide band gap and high optical absorption in the UV-Vis range of electromagnetic radiation. The calculated static refractive indices η^x,y,z(0) are close to unity suggesting its transparency. For piezoelectric properties, we have reported the total Cartesian polarization. Our calculations have revealed that Na₂SiO₃ is a promising candidate for optoelectronic devices while its application in ferroelectric and piezoelectric devices could be improved with further research.

Structural phase transitionoccurs from space group Cmc2₁ to C11 at 12 GPA under compressive unidirectional pressure in Na₂SiO₃.

Electronic, mechanical and piezoelectric properties of glass-like complex Na2Si1−xGexO3 (x = 0.0, 0.25, 0.50, 0.75, 1.0)

Motivated by our previous work on pristine Na₂SiO₃, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na₂Si_1−xGe_xO₃ (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT). Interestingly, the optimized bond lengths and bond angles of Na₂SiO₃ and Na₂GeO₃ resemble each other with high similarity. On doping we report the negative formation energy and feasibility of transition of Na₂SiO₃ → Na₂GeO₃ while the structural symmetry is preserved. Analyzing the electronic profile, we have observed a reduced band gap on increasing x = Ge concentration at Si-sites. All the systems are indirect band gap (Z–Γ) semiconductors. The studied systems have shown mechanical stabilities by satisfying the Born criteria for mechanical stability. The calculated results have shown highly anisotropic behaviour and high melting temperature, which are a signature of glass materials. The piezoelectric tensor (both direct and converse) is computed. The results thus obtained predict that the systems under investigation are potential piezoelectric materials for energy harvesting.

Motivated by our previous work on pristine Na₂SiO₃, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na₂Si_1−xGe_xO₃ (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT).

Statistical learning of NMR tensors from 2D isotropic/anisotropic correlation nuclear magnetic resonance spectra

Many linear inversion problems involving Fredholm integrals of the first kind are frequently encountered in the field of magnetic resonance. One important application is the direct inversion of a solid-state nuclear magnetic resonance (NMR) spectrum containing multiple overlapping anisotropic subspectra to obtain a distribution of the tensor parameters. Because of the ill-conditioned nature of this inverse problem, we investigate the use of the truncated singular value decomposition and the smooth least absolute shrinkage and selection operator based regularization methods, which (a) stabilize the solution and (b) promote sparsity and smoothness in the solution. We also propose an unambiguous representation for the anisotropy parameters using a piecewise polar coordinate system to minimize rank deficiency in the inversion kernel. To obtain the optimum tensor parameter distribution, we implement the k-fold cross-validation, a statistical learning method, to determine the hyperparameters of the regularized inverse problem. In this article, we provide the details of the linear-inversion method along with numerous illustrative applications on purely anisotropic NMR spectra, both synthetic and experimental two-dimensional spectra correlating the isotropic and anisotropic frequencies.

Ionic conductivity of solid polyelectrolyte complexes with varying water content: application of the dynamic structure model

We present a systematic study of the dc conductivity of solid polyelectrolyte complexes (PEC) of type x_PSS PSS·(1 − x_PSS) PDADMA as a function of temperature, and of PSS and water content, respectively. The data are discussed in the framework of the dynamic structure model.

Facile Method for the Investigation of Temperature-Dependent C60 Diffusion in Conjugated Polymers

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C₆₀ molecules from a C₆₀ layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C₆₀ molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C₆₀ can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C₆₀ diffusion is thermally activated and not driven by WFL-type collective motion above T_g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.

Ab Initio Modeling of Structure and Properties of Single and Mixed Alkali Silicate Glasses

A density functional theory (DFT)-based ab initio molecular dynamics (AIMD) has been applied to simulate models of single and mixed alkali silicate glasses with two different molar concentrations of alkali oxides. The structural environments and spatial distributions of alkali ions in the 10 simulated models with 20% and 30% of Li, Na, K and equal proportions of Li-Na and Na-K are studied in detail for subtle variations among the models. Quantum mechanical calculations of electronic structures, interatomic bonding, and mechanical and optical properties are carried out for each of the models, and the results are compared with available experimental observation and other simulations. The calculated results are in good agreement with the experimental data. We have used the novel concept of using the total bond order density (TBOD), a quantum mechanical metric, to characterize internal cohesion in these glass models. The mixed alkali effect (MAE) is visible in the bulk mechanical properties but not obvious in other physical properties studied in this paper. We show that Li doping deviates from expected trend due to the much stronger Li-O bonding than those of Na and K doping. The approach used in this study is in contrast with current studies in alkali-doped silicate glasses based only on geometric characterizations.

Lithium conductivity in glasses of the Li2O–Al2O3–SiO2system

An Arrhenius plot for DC electrical conductivity of fully polymerized and depolymerized Li aluminosilicate glasses is one way to reflect the effect of local structurevs.lithium content.

Internal friction and vulnerability of mixed alkali glasses

Based on a hopping model we show how the mixed alkali effect in glasses can be understood if only a small fraction c(V) of the available sites for the mobile ions is vacant. In particular, we reproduce the peculiar behavior of the internal friction and the steep fall ("vulnerability") of the mobility of the majority ion upon small replacements by the minority ion. The single and mixed alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange processes. If c(V) is small, they can become comparable in height even at small mixing ratios. The large vulnerability is explained by a trapping of vacancies induced by the minority ions. Reasonable choices of model parameters yield typical behaviors found in experiments.

Complete identification of alkali sites in ion conducting lithium silicate glasses: a computer study of ion dynamics

The available sites for ions in a typical disordered ionic conductor are determined. For this purpose we devised a straightforward algorithm which via cluster analysis identifies these sites from long time ionic trajectories below the glass transition. This is exemplified for a lithium silicate glass (Li2O)(x)(SiO2)((1-x)) for x=0.5 and x=0.1. We find for both concentrations that the number of sites is only slightly bigger than the number of ions. This result suggests a theoretical description of the dynamics in terms of mobile vacancies as most appropriate. Furthermore, identification of the ionic sites allows one to obtain detailed characteristics of the ionic motion, e.g., quantification of correlated forward-backward jumps.

23Na nuclear magnetic resonance spin echo decay spectroscopy of sodium silicate glasses and crystalline model compounds

The possibility of using Hahn spin echo decay spectroscopy for measuring 23Na-23Na dipole-dipole couplings in crystalline solids and glasses is investigated. Although the presence of nuclear electric quadrupolar splittings complicates the situation compared to the case of dipole-dipole couplings among spin-1/2 nuclei, experimentally measured second moments lie within approximately 10-20% of the calculated values, if the following conditions are fulfilled: (1) selective excitation of the central 1/2-->-1/2 transition, and (2) restriction of the analysis to short evolution times (2t1 < or = 200 microseconds). At longer evolution times, partial suppression of the spin-exchange term in the dipolar Hamiltonian due to magnetic inequivalencies between the interacting nuclei produces substantial differences between experimental and calculated spin echo decays, the extent of which depends on the compound considered. These results signify the potential utility of 23Na spin echo nuclear magnetic resonance for testing the distribution of Na-Na distances in structurally disordered solids and glasses. The method is shown to be applicable to sodium silicate glasses, where it reproduces anticipated compositional trends in 23Na-23Na couplings. Details and inherent limitations of this methodology are discussed.