Masaru Aniya

Ionic conductivity of Agx(GeSe3)1−x (0≤x≤0.571) glasses

Abstract Electrical properties of Agx(GeSe3)1−x (0≤x≤0.571) glasses were investigated by impedance spectra and EMF measurements. As the concentration of silver is increased the total electrical conductivity steeply increases from 10−14 S/cm to 10−3 S/cm at about x=0.3. This rapid increase is chiefly due to an appearance of silver ion migration. Glass transition temperature showed a minimum at x=0.33 above which a first sharp diffraction-peak in powder X-ray diffraction diminishes. This unusual transition supports a percolation hypothesis on photo-dissolution of silver in these glasses.

A chemical approach for the microscopic mechanism of fast ion transport in solids

Abstract A microscopic mechanism that explains the various behaviors observed in superionic conductors is proposed. It is suggested that the local change of chemical bond which fluctuates in time is responsible for the fast ion transport in solids. Criteria which decide if a certain material can behave as superionic conductor or not are also given, and it is suggested that compounds based on cadmium and mercury chalcogenides are strong candidates to be superionics.

Pseudopotential Study of Bonding in the Superionic Material AgI

The bonding nature of the superionic material AgI is studied by using a pseudopotential theory. The result is compared with that for GaAs, which is a well-known semiconductor. It is found that the real space valence electron distribution between these compounds differs qualitatively, and that the bonding in AgI is determined mainly by the iodine. It is also suggested that the local electronic excitations from the bonding to the antibonding orbitals can trigger the migration of the mobile species, initiating in this way the superionic transport.

A model for the fragility of the melts

The concept of fragility has been used widely to characterize the temperature dependence of the viscosity of glass forming materials. However, the physical background that determines the degree of fragility is still not well understood. In the present study an expression for the fragility is derived based on a simple model of the melt. According to the model, the fragility is determined by the relaxation of structural units that form the melt, and is described in terms of the bond strength (E0), coordination number (Z0), and their fluctuations (ΔE and ΔZ). It is shown that a strong system is characterized by large value of total bond strength (Z0E0) and small value of its fluctuation ((ΔZ)2(ΔE)2). On the other hand, a fragile system is characterized by small value of total bond strength and large value of its fluctuation.

Average electronegativity, medium-range-order, and ionic conductivity in superionic glasses

The interrelations between ionic conductivity, network expansion, medium-range-order, and average electronegativity in superionic glasses are discussed. It is shown that salt doping in glassy matrix results in the decrease of the average electronegativity, increase of the network expansion, increase of the ionic conductivity, and decrease of the wave number of first sharp diffraction peak. A scaling behavior of ionic conductivity and network expansion when arranged against the average electronegativity is also found.

Diffusion of Mobile Ions and Bond Fluctuations in Superionic Conductor CuI from ab initio Molecular-Dynamics Simulations

The dynamic properties of mobile Cu ions in the superionic conductor CuI are studied by ab initio molecular-dynamics simulations. To investigate the bonding nature of the material, the gross charge of each ion and the overlap population between each Cu–I pair are calculated by the population analysis as a function of time. It is shown that the covalent bonding around the Cu ions weakens when they diffuse in the octahedron cage during the migration between neighboring tetrahedral sites. It is also shown that the ionicities of the Cu ions at the octahedral sites are larger than those of the Cu ions at the tetrahedral sites. These results clearly reveal that the time evolution of local bonding plays an important role in high ionic conductivity.

Anomalous cation-cation interactions in molten CuI: Ab initio molecular-dynamics simulations

The structural and electronic properties of molten CuI are studied by means of ab initio molecular-dynamics simulations. From the first-peak positions of the partial pair distribution functions, we confirm that the nearest-neighbor distance for Cu–Cu pair is almost the same as that for Cu–I pair in spite of the correlation between the same type of ions. It is shown that there exists a large fluctuation in the partial atomic concentration of Cu ions to retain the anomalously short Cu–Cu distance, while iodine ions distribute uniformly. It is clarified by the population analysis that there is a covalent-like bonding between neighboring Cu ions as well as between Cu and I ions.

Medium range structure and activation energy of ion transport in glasses

Most existing models of ionic conduction in glasses relate the ion transport to some specific structural features. However, there is a lack of investigation concerning the role of the medium range structure on the ionic conduction. In the present paper, with the objective to fill this gap and gain further insights about the microscopic mechanism of ion transport in glasses, the relationship between activation energy of ion transport and the wave number of First Sharp Diffraction Peak (FSDP) has been studied. It is found that the activation energy decreases with the decrease of FSDP wave number. The relationship has been interpreted by using the bond fluctuation model of superionic conductors.

Bond Strength—Coordination Number Fluctuation Model of Viscosity: An Alternative Model for the Vogel-Fulcher-Tammann Equation and an Application to Bulk Metallic Glass Forming Liquids

The Vogel-Fulcher-Tammann (VFT) equation has been used extensively in the analysis of the experimental data of temperature dependence of the viscosity or of the relaxation time in various types of supercooled liquids including metallic glass forming materials. In this article, it is shown that our model of viscosity, the Bond Strength—Coordination Number Fluctuation (BSCNF) model, can be used as an alternative model for the VFT equation. Using the BSCNF model, it was found that when the normalized bond strength and coordination number fluctuations of the structural units are equal, the viscosity behaviors described by both become identical. From this finding, an analytical expression that connects the parameters of the BSCNF model to the ideal glass transition temperature T0 of the VFT equation is obtained. The physical picture of the Kohlrausch-Williams-Watts relaxation function in the glass forming liquids is also discussed in terms of the cooperativity of the structural units that form the melt. An example of the application of the model is shown for metallic glass forming liquids.

A model for the photo-electro ionic phenomena in chalcogenide glasses

A model which explains in a unified way the photodoping, photosurface deposition and photoinduced chemical modification in chalcogenide glasses is presented. It is proposed that the local shift of the Fermi level induced by illumination triggers the flow of holes and the contraflow of metal ions, resulting in the observed photoinduced phenomena. Atomic processes of the ion dynamics has been presented also, in connection with the superionic transport.