On the nature of the heterogeneous dynamics of ions in ionic conducting glasses

Molecular dynamics simulations were performed to study the complex and heterogeneous dynamics of ions in ionically conducting glasses. The dynamics of Li ions in lithium silicate glasses have been characterized by van-Hove functions, fractal dimension of random walks, multifractal analysis of density profile, and principal component analyses of time series. Hierarchy dynamics with local and successive jumps, which are followed by cooperative jumps involving more ions, are observed in the self-part of the van-Hove functions and in trajectories where they form a multifractal density profile. Fractal dimensions of the random walks can be used to characterize the motions of ions in both the short and long length-scale regions. Difference between them becomes clearer in the glassy state, and the corresponding power law dependences of the mean squared displacements can be well explained by these exponents. Phase-space plot using the de-noised data obtained by principal component analysis reveals that the trajectory extending from the short time to the long time dynamics in the plot is continuous and has long term memory. The motion examined by the principal component analysis of the time series shows the deterministic character of the single particle ion dynamics. Each ion moves among domains of collective motion. Recurrence plot and mutual information function shows long correlation of the dynamics. Therefore, the cause of the dynamic heterogeneity is rather deterministic in nature.     

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.     

Charge Density Wave Conduction Of Small Samples

The charge density wave dynamics of samples with dimension near the Lee-Rice coherence length were studied. In the high frequency and current limits the conductivity of such small samples approaches that of infinite samples. Finite size effects change the response at low frequency and current. A “kink” is introduced in dV/dI like that found experimentally in some NbSe₃ samples at low temperatures.     

X-Ray Diffuse Scattering Study of Some (TMTSF)2X and (TMTTF)2X Salts

X-ray diffuse scattering experiments on the superconducting salts (TMTSF)₂X with X = PF₆, AsF₆ and ClO₄ reveal no charge density wave instability, in contrast with the poorly conducting (TMTTF)₂PF₆ salt. New structural phase transitions coinciding with conductivity anomalies are found in (TMTSF)₂NO₃, (TMTSF)₂ReO₄, and (TMTTF)₂ClO₄. It is suggested that they correspond to order-disorder transitions involving the non-centrosymmetric counter ions. Relationships between these structural instabilities and superconductivity are also discussed.     

Investigation on the blue photorefractive properties varied with MgO codoping in Ru:Fe:LiNbO3 crystals

Mg:Ru:Fe:LiNbO₃ crystals with various doping concentration of MgO have been grown by Czochralski method. The type of charge carriers and photorefractive properties in Mg:Ru:Fe:LiNbO₃ crystals were measured by two‐wave coupling method using Kr⁺ laser (476 nm) and He‐Ne laser (633 nm) as light sources. We found that holes were the dominant charge carriers under blue light irradiation while electrons were the dominant charge carriers under red light irradiation. Mg²⁺ ions behaved no longer as damage resistant, but promoter to the photorefractive properties at 476 nm wavelength. The photorefractive properties under blue light improved with the increase concentration of Mg²⁺ ions. The enhancement mechanisms of the blue photorefractive were suggested. Experimental results definitely showed that Mg‐doped two‐centre Ru:Fe:LiNbO₃ was a promising blue photorefraction material for holographic volume storage.     

Effect of strontium substitution on the structure,ionic diffusion and dynamic properties of 45S5 Bioactive glasses

Strontium ions can promote bone growth and enhance bioactivity in bioactive glasses that find critical biomedical applications. In this paper, the effect of SrO/CaO substitution on the structure, ionic diffusion, and dynamic properties of 45S5 bioactive glasses has been studied using constant pressure molecular dynamics simulations with a set of effective partial charge potentials. The simulated structure models were validated by comparing with experimental neutron diffraction results. It was found that the SrO/CaO substitution leads to an increase of glass density and decrease of oxygen density, a measure of compactness of the glass, in excellent agreement with available experimental data. On the other hand, the substitution does not significantly change of the medium range glass structures as characterized by silicon and phosphorous Q_n distributions, network connectivity, and ring size distributions. The diffusion and dynamic behavior of these glasses and their melts were also determined by calculating the mean square displacements and velocity autocorrelation functions. It was found that the diffusion energy barriers of sodium, calcium and strontium ions remain nearly constant with respect to the level of substitution. However, strontium ions do influence the diffusion behaviors of calcium and sodium ions at high temperature, as evidenced from their velocity autocorrelation functions. Like calcium and sodium, strontium ions only contribute to the lower frequency (around 100 cm^? 1) of the vibrational spectra the substitution has little effect on high frequency features and the general shape of the vibrational density of states. These results suggest that the increase of the dissolution rate in strontium containing glasses are mainly due to the increase of free volume and the non-local effect that weakens the silicon-oxygen network due to strontium ions.     

Theoretical study on the electronic structures and spectral properties of two series of osmium(II) complexes with different substituent groups

Two series of osmium(II) complexes with different substituent groups (-CF₃, -N(CH₃)₂) have been studied by using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, to investigate their electronic structures, absorption, and emission properties. The influence of different substituted groups on their photophysical properties has also been explored. The lowest energy absorption and emission wavelengths calculated are in good agreement with the available experimental values. Besides, ionization potential (IP), electron affinities (EA), and reorganization energy (λ) were calculated to evaluate the charge transfer and balance properties between hole and electron. It is expected that the theoretical studies could provide valuable information for the phosphorescent osmium(II) material for use in the organic light-emitting diodes.     

Growth and Optical Properties of Doped LiTaO3 Single Crystals

Single crystals of lithium tantalate (LiTaO₃) doped with Pr, Nd + Yb and Tm were grown by the Czochralski method. A thermal system with 50 mm diameter iridium crucible and two different afterheaters (active and passive) was checked with respect to temperature distribution in a pulling region. The obtained crystals were up to 20 mm in diameter and up 50 mm in length. Crystals were poled, and the Curie temperature was determined for specimens cut of from different parts of single crystals. The polarized absorption spectra, time resolved emission spectra and emission lifetime of Pr³⁺ doped LiTaO₃ crystals were measured. An intense emission from the ³P₀ level was observed. Optical properties of the Yb³⁺ ions excited by energy transfer from Nd3+ ions have been researched for LiTaO₃:Nd, Yb crystals.     

Possibility of Coexistence of Spin Density Wave and Superconductivity in Organic Conductor (TMTSF)2PF6

A theory for the coexistence problem of the spin density wave (SDW) and superconductivity (SC) in highly anisotropic materials is presented. On the basis of a simplified electronic band model a Hartree-Fock approximation is applied. It is concluded that SDW is precluded when SC developes at a higher temperature. When the SDW onset temperature is higher than that of SC, these long range orders generally coexist unless two orders interchange by a first order phase transition. Discussions on possible phase diagrams for (TMTSF)₂PF₆ under pressure are given.     

Optical properties and valence change of samarium ions in a sol–gel Al2O3–B2O3–SiO2 glass by femtosecond laser irradiation

10Al₂O₃–5B₂O₃–85SiO₂–xSm₂O₃ glasses were prepared by the sol–gel method. The emission spectra of the glasses indicate that the quench concentration of the Sm³⁺ ions is about 0.2 mol%. The emission spectra of the glasses after high-temperature treatment with H₂ gas exhibit the coexistence of the Sm³⁺ and Sm²⁺ ions. We observed the strong emission line of the Sm²⁺ ions and the emission band of the non-bridging oxygen hole center when the glasses were exposed to a femtosecond laser. It indicates that some Sm³⁺ ions were reduced to Sm²⁺ ions by femtosecond laser pulses and non-bridging oxygen hole centers were formed. The ⁵D₀–⁷F₀ emission line of the Sm²⁺ ions by femtosecond laser irradiation shows a red shift, compared with the emission of the Sm²⁺ ions by reduction with H₂ gas. The strong absorption band and weak, sharp absorption lines in the range from the UV to IR come from charge transfer and the transition from the ⁶H_5/2 state to the various excited states of the Sm³⁺ ions. The reduction mechanism of Sm³⁺ ions is discussed.     

Partial ordering of glass networks adjacent to simulated glass–crystal interfaces

Interfaces between sodium lithium borosilicate glasses and the (100) and (110) surfaces of MgO and CaO crystals were simulated using a melt-quench procedure, employing classical pair potentials and molecular dynamics. The density of network forming species within the glass at these interfaces was considered as a function of distance from the plane of the interface and the positions of network formers were calculated in relation to sites in the crystal surface.For each interface, a strong correlation was found between the position and orientation of borate and silicate coordination polyhedra within the glass and particular positions of ions in the crystal surface, indicating that different partial ordering of the glass had occurred. In addition, examination of oxygen density profiles revealed a sequence of consistently spaced layers of increased density extending into the glass for all interfacial systems.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

Structural analysis of Na2OGa2O3SiO2 glasses by the X-ray diffraction method

The structure of Na₂OGa₂O₃SiO₂ glasses of four different compositions containing up to 45 mol. % Ga₂O₃ has been determined by the X-ray diffraction method. The radial distribution function D(r) obtained indicates that Ga³⁺ ions are in fourfold coordination and GaO₄ tetrahedra are formed in these glasses. The reduced intensity function S·i(S) and radial distribution function D(r) which were calculated based on a structural model for A-25, 0.25(Ga₂O₃)·0.75(Na₂O·2SiO₂)/3 glass agreed well with the observed ones.     

Local and medium range structure of erbium containing glasses: A molecular dynamics study

Erbium containing glasses cover an important role in optical communications and laser technology. The local environment of the ions constituting the three dimensional network exerts a strong influence on the chemical and physical properties of the glasses. In particular the network modifier cations strongly influence the short and intermediate range order of the glass structure. The structural modifications induced by the alumina addition in the glass matrix, as a function of the erbium oxide concentration, have been studied using molecular dynamics (MD) simulations. While at low Er₂O₃ concentration the network former action of the Al³⁺ leads to a more polymerized glass network, at higher Er₂O₃ content the influence of the alumina on the erbium local structure is limited. Further investigation points out micro-heterogeneity in the Na⁺ and Ca²⁺ distribution as a function of the alumina concentration.     

New laser crystals of complex oxides doped with ions of d elements with variable valence and different structural localization. Review

Ions of d elements in complex oxides with olivine and phenakite structures exhibit diverse structural localization and may incorporate into crystals in various oxidation states. The complexity of the structure of these materials requires a complex approach in investigation of their spectroscopic characteristics with regard to the type of optical center and the growth conditions. The method of estimation of the distribution of an activator ion over crystallographically non-equivalent cationic positions on the basis of the symmetry analysis of the structure of these positions with regard to the spatial distribution of the activator-ion electron density is described. The results of experimental studies of the spectroscopic characteristics of vanadium and chromium ions in the crystals with the olivine (Mg₂SiO₄, Ca₂GeO₄, and CaMgSiO₄) and phenakite (LiGaSiO₄, LiAlGeO₄, and Zn₂SiO₄) structures performed by us (some experimental data are reported for the first time) are analyzed with regard to the growth conditions. Data in the literature are also analyzed. In order to estimate the structural and valence states of the d ions, a number of other experimental techniques were used, and the experimental data were compared with the calculated energy-level diagrams for activator ions in centers of various types. This approach made it possible to adequately describe the possibilities of complex oxides Mg₂SiO₄, Ca₂GeO₄, CaMgSiO₄, LiGaSiO₄, LiAlGeO₄, and Zn₂SiO₄ doped with d ions, regarded as new laser media.     

An Insight on the Polyacetylene Properties Through Resonant Raman Scattering Spectra Analysis

Resonant Raman Scattering spectra of trans polyace-tylene (CH)_X and (CD)_X are interpreted in terms of a model based on a bimodal distribution of conjugation length segments. Such a theory takes into account the electronic and lattice dynamics properties of different conjugation length segments, together with the electronic dipole moments from the transitions and the electron vibration interactions in the excited electronic states. This model accounts also for the Resonant Raman Scattering spectra taken during the isomerization process and from cis rich samples.     

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.     

Medium range structure and power law conductivity dispersion in superionic glasses

The relationship between the exponent of the power law behavior in the ac conductivity and the wave number of first sharp diffraction peak (FSDP) in superionic glasses has been studied. For the case of (AgI)_x(AgPO₃)_1?x glasses, it is shown that the exponent of the power law increases with the decrease of the FSDP wave number. Other superionic glasses such as (AgI)_0.2(0.6Ag₂O–0.4V₂O₅)_0.8 and (AgI)_0.75(Ag₂MoO₄)_0.25 follow this trend. The relationship found is explained by using the energy barrier profile for the mobile ions in a disordered structure. The local energy barrier for the ion movement is determined by the local structure and bonding, and the ions flow through the low energy barrier sites. According to the model, the exponent of the power law contains information about the degree of connectivity of these low energy sites that extend at the intermediate length scale. The result suggests also that the universal aspect of the power law reflects the universal pattern of the potential barrier at intermediate length scales.     

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni-Al alloys

Atomic and electronic structures of liquid and rapidly quenched Ni-Al (NiAl₃, NiAl, and Ni₃Al) alloys were studied by ab initio molecular dynamics simulation. Pair correlation function, structure factor, bond pair, electron population, and density of states were calculated. It is found that amorphous Ni-Al alloys can be prepared by rapid quenching of liquid alloys, with the former bearing similar structures to the latter, although amorphous alloys have a more ordered structure. Bond pair analysis indicates that both the liquid and amorphous Ni-Al alloys consist mainly of 1441, 1431, 1421, and 1422 pairs of tetrahedral local order. The positions of the first peaks of the Al-Ni pair correlation functions are lower than the sum of the metallic radii of Ni and Al, suggesting the occurrence of chemical bonding between Ni and Al in Ni-Al alloys. Electronic structure analysis further revealed that the interaction between the d-electrons of Ni and the p-electrons of Al is responsible for the bonding. The main peak positions of the total DOS of amorphous Ni-Al alloys become more and more positive when the content of Ni in the alloys increases.     

State of Fe3+ ion and Fe3+ −F− interaction in calcium fluorosilicate glasses

The state of Fe³⁺ ions and Fe³⁺ ?F^? interaction in calcium fluorosilicate glasses xCaF₂·(1- x)CaO·SiO₂) (0 ≤ x ≤ 0.3) containing a small amount of iron were investigated by ESR spectroscopy. Two resonances observed near g = 2.0 and g = 4.3 were assigned to dipole-dipole interacted Fe³⁺ ions and Fe³⁺ ions in a rhombic crystal field, respectively. The fraction of Fe³⁺ ions in a rhombic crystal field decreased and that of dipole-dipole interacted Fe³⁺ ions increased with increasing Fe₂O₃ content. It was found that the quantity of dipole-dipole interacted Fe³⁺ ions depends on the negative partial charge of fluorine ions and shows a maximum at 10 mol% CaF₂ (x = 0.2). The maximum is attributed to the largest difference between absolute values of the ionic potentials of ferric and fluorine ions which is caused by the smallest negative partial charge of flourine at 10 mol% CaF₂.