Michisuke Kobayashi

Molecular Dynamics Studies of Yttria Stabilized Zirconia. I. Structure and Oxygen Diffusion

The structure and dynamical properties of oxygen conductor yttria stabilized zirconia, (ZrO 2 ) 1- x (Y 2 O 3 ) x , are investigated for three dopant concentrations of 4.85, 10.2 and 22.7 mol%Y 2 O 3 using a method of molecular dynamics simulation. It is shown that a Y–O nearest neighbor distance is longer than that for Zr–O, and an oxygen coordination number for Y ion is a little larger than that for Zr ion in all dopant concentrations. The self-diffusion constant of O ions, D , shows a maximum at 10.2 mol%Y 2 O 3 with increasing the dopant concentration. These results are in agreement with experimental measurements. It is shown that dopant Y ions play an important role in such notable behavior of oxygen diffusion.

Review on structural and dynamical properties of silver chalcogenides

Abstract Structural and dynamical properties of silver chalcogenides are reviewed. Since Rahlfs and Strock proposed that the four Ag ions in a unit cell of αAg 2 S are distributed with equal probability over the 42 crystallographic sites of 6(b), 12(d) and 24(h), many measurements have been performed. Recent precise measurements of neutron scattering, EXAFS, and X-ray diffraction have suggested that Ag ions in αAgI type superionic conductors are distributed over 12(d) sites with large asymmetric anharmonic thermal vibrations. The transport and electrochemical properties of the electronic and ionic conductivities and the self-diffusion coefficient and mobility of mobile ions have been investigated also extensively. These measurements have shown a remarkable deviation from the Einstein relation in Ag-chalcogenides. To interprete this behavior, Yokota presented a new diffusion theory, caterpillar mechanism. To understand the liquid-solid duality of an unusual state of materials in which some atoms have nearly liquid like properties while other atoms retain their regular crystalline arrangement, a continuum model has been presented. It is supposed that the lattice composed of anions is immersed in the cation liquid. The structure factors with which electrons are connected are expressed symmetrically in terms of the structure factors of ions in the long wavelength limit using the fluctuation-dissipation theorem and the Kramers-Kronig relation. The calculated conductivity satisfies the ƒ-sum rule and its ionic part has a broad peak at the optical-phonon frequency, which is the similar frequency dependence to that of αAgI. The frequency- and wave vector-dependent conductivity and the dispersion relations of collective modes are calculated. In particular the plasmon-LO phonon coupling is investigated in detail. A molecular dynamics method is applied for the further study of Ag-diffusion in αAg 2 Te for several temperatures. The density distribution of Ag ions suggests that a Ag ion locates at tetrahedral site for the most of time and then moves to its neighboring tetrahedral site through the vicinity of the octahedral site. The activation energy for an ionic diffusion also is obtained from the Arrhenius plotting of the self-diffusion coefficient of Ag ions. Its value of 0.17 eV is obtained, which is in good agreement with the experiment.

Molecular dynamics studies of molten AgI. I. Structure and dynamical properties

The structural and dynamical properties of molten AgI are investigated using molecular dynamics calculations and pair potentials originally given by Parrinello et al . and modified by authors. The total pair distribution functions, thus obtained, are in good agreement with experimental results by Takahashi et al . and with theoretical results by Stafford and Silbert. The wave-number-dependent static dielectric function e( k ) is also calculated with use of the charge correlation function in the both α- and molten phases. In the α-phase, e( k ) is negative for small wave numbers and approaches zero at special wave numbers, which correspond to the positions of the Debye lines in the reciprocal lattice space. e( k ) in the molten phase shows the forerunning phenomenon to the crystalization, which is a very similar behavior to that in the strongly coupled one component plasma.

Collective motion in superionic conductors

Abstract A theory of collective motion in superionic conductors is described by the use of a model of a crystalline cage immersed in a viscous liquid. The viscoelastic force and the interionic Coulomb force are considered as the cage—liquid interaction. The density-correlation functions and the frequency-dependent conductivity are calculated. The calculated conductivities for α-AgI are in good agreement with experiments. It is concluded that the structure in a.c. conductivity experimentally observed for α-AgI at frequencies below 10 cm -1 can be ascribed to acoustic phonons.

Difference of p–d hybridization in noble metal halides

Abstract The band calculations of noble metal halides are studied to reveal the high ionic conductivity of Ag + and Cu + in noble metal halides using the linear combination of atomic orbitals (LCAO) theory. It is found that the d states of Ag + are much more weakly coupled with the p states of halogen ions, while those of Cu + are much more strongly coupled with the p bands. The strength of p – d hybridization is discussed in relation to the activation energy for the ionic conduction. It is shown that the high ionic conductivity of AgX (X=halogen) primarily stems from the combination of the deformability of the d shell and the weakness of p – d hybridization.

MD Calculation Studies of Residence Site and Diffusion Path of Li Ions in Perovskite-Type (LaLi)TiO3

A computer simulation is performed by a molecular dynamics method to study the structural and dynamical properties in superionic conductor La 4/3- x Li\(_{3x}\square_{2/3-2x}\)Ti 2 O 6 (\(\square=\...

Magnetoplasma Oscillation of a Layered Electron Gas

The plasma dispersion of a layered electron gas in a magnetic field is calculated in the random phase approximation for the Voigt configuration. We found that the plasmon gradually changes its character from the two-dimensional to the three-dimensional ones as the spacing between the planes of the layer decreases.

p–d hybridization in superionic conductors

Abstract The band calculations of AgX (X=halogen), CuX (X=halogen) and β-Ag 3 SX (X=I, Br) are studied to reveal the high ionic conductivity of Ag + and Cu + in these superionic conductors using the linear combination of atomic orbitals (LCAO) theory. It is found that the d states of Ag + are much more weakly coupled with the p states of halogen ions, while those of Cu + are much more strongly coupled with the p bands. The strength of p – d hybridization is discussed to connect with the activation energy for the ionic conduction. It is shown that the high ionic conductivity of AgX (X=halogen) and β-Ag 3 SX (X=I, Br) primarily stems from the combination of the deformability of d shell and the weakness of p – d hybridization.

Ionic Thermoelectric Power of Superionic Conductors

The ionic thermoelectric power of a superionic conductor is calculated using a lattice gas model in one dimension for the case where the lattice sites are 50% occupied. The calculated thermoelectric power is inversely proportional to the absolute temperature and the activation energy for ionic conduction is equal to the heat of transport. The results explain experiments fairly good.

p–d Hybridization and Superionicity in Silver Ternary Compounds

The tight-binding electronic band calculations of β-Ag 3 SX (X=I, Br) are carried out in order to clarify the high ionic conductivity of silver ions. The d bands of Ag ions are much more weakly coupled with the p bands of halogen ions, than with the p bands of S ions. The strength of p – d hybridization is discussed to connect with the activation energy for the ionic conduction. It is shown that the high ionic conductivity of β-Ag 3 SX (X=I, Br) primarily stems from the weakness of the p – d hybridization.