Fumio Tachibana

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.

Design of New Material with High Ionic Conductivity

A computer simulation by a molecular dynamics method at constant volume has been performed to a model system which is composed of accumulating two different ionic conductors: ···AgI-Ag 2 S-AgI-Ag 2 S···. Along the c -axis ( z -axis), AgI-parts are compressed and Ag 2 S-parts are stretched. About 3% of Ag ions in Ag 2 S-parts have been transfered to AgI-parts. The self-diffusion coefficient of Ag ions in AgI-parts decreases by 40-50% compared with that in a single AgI-system, while the diffusion coefficient of Ag ions in Ag 2 S-parts increases by 30% compared with that in a single Ag 2 S-system. The ionic conductivity of the superlattice-system is larger than that of either material of the AgI-system or Ag 2 S-system. The calculation suggests the possibility of the existence of a new material which has a larger ionic conductivity.

Molecular dynamics study of ionic motion in α-Ag2Te

The migration of Ag ions in α-Ag2Te is investigated by the molecular dynamics technique using effective pair potentials. The anion fcc sublattice is divided into the tetrahedra and the octahedra and the motion of Ag ions is analyzed by deriving τO/τT, where τO and τT are the residence time of Ag ion in the octahedra and that in the tetrahedra, respectively. The temperature dependence of τO/τT is expressed as the Arrhenius formula with the activation energy 0.18 eV which is close to one obtained by the tracer diffusion experiment 0.14 eV. The density distribution of the Ag ions and its temperature dependence are also calculated, from which we can deduce the path of the Ag ion movement.

The Haven ratio of Ag ion in α-AgI by Monte Carlo method

Abstract The temperature dependence of the Haven ratio of Ag ion in α-AgI is computed by a Monte Carlo method under the following assumptions: (1) Ag ions travel on the non-Bravais lattice points formed by 12(d) sites of bcc by I ions. (2) The pair interaction between two Ag ions consists of a Coulomb term. (3) A caterpillar mechanism is introduced in a proper way. It is confirmed that there exist the correlation in successive jump of ions in diffusion process even for a disordered crystal such as α-AgI. The Haven ratio derived from the present calculation is in good agreement with the experimental one.

Correlation factor of silver ion in α-Ag2Te by molecular dynamics method

Abstract The correlation factor ƒ and the jump frequency Γ T of silver ions in αAg 2 Te are calculated by a molecular dynamics method introducing a polyhedron analysis. On a microscopic point of view, the migration of silver ion is discussed using factors ƒ and Γ T and it is found that the jump distance r is shorter than the distance between T-site and O-site of the fcc Te sublattice.

Monte carlo simulation for a caterpillar motion in α-Ag2X type superionic conductors

A Monte Carlo method was applied to the α-Ag 2 X (X=S, Se) type superionic conductors for studying the ion migration mechanism by calculating the Haven ratio of the mobile ion under the following assumptions: (1) Mobile ions migrate by hopping on the non-Bravais lattice formed by the 12(d) sites of the bcc lattice formed by the immobile anions. (2) The pair interaction between two Ag ions is described by the Coulomb term. (3) The caterpillar mechanism is introduced in a proper way. The Haven ratio derived by the present calculation is in good agreement with the experimental one.

A computer simulation of the haven ratio in alpha-AGI

Abstract A computer simulation by a Monte Carlo method has been carried out to derive the Haven ratio of Ag ion in α-AgI under the following assumptions: (1) The jumping mechanism was essential for the migration of Ag ion. (2) The mobile ion system was described by a lattice gas model. (3) A caterpillar mechanism was introduced in a proper way. The main results obtained by the present work were as follows: (1) The effective total displacement of charge for the cooperative jumps was equal to the displacement of the tracer. (2) The probability of the backward jump to the initial site was much larger than those to the other three directions. The Haven ratio estimated using the above results agreed very well with one derived from experimental values of the conductivity and the diffusion coefficient.

A cooperative motion of cations in α-Ag2Te

Abstract The existence of a cooperative jump of neigbouring cations have been found by analysing the molecular dynamics data of α-Ag 2 Te. We calculated the distribution of cosθ, where θ is the angle formed by the displacement vectors of two cations adjacent each other during a time interval τ. The main results obtained are: (1) When τ is short (several times of atomic vibration period), the probability distribution of cosθ, p (cosθ), is constant for any values of cosθ. (2) With increasing τ, p (cosθ= 1) increases while p (cosθ=−1) decreases. (3) At the time τ 0 = l / v th , p (1) has a maximum value and p (−1) has a minimum one, where l is the shortest distance between the tetrahedral site and the octahedral site in the fcc lattice and v th is the thermal velocity of mobile ions. (4) For τ>τ 0 , p (1) decreases and p (−1) increases with increasing τ.

Caterpillar motion in α-Ag2Te

Abstract The diffusion process of silver ions in α-Ag 2 Te is investigated. It is assumed that Ag ions occupy the tetrahedral sites and octahedral sites with different probability. To give an explanation of the Havens ratio obtained by a molecular dynamics calculation, a theory of caterpillar mechanism is used. It is shown that the ratio of the frequency of a single jump to that of cooperative jumps increases with increasing temperature.

Ionic Diffusion Phenomena in Superionic Conductors

A molecular dynamics method has been applied in a study of silver diffusion in superionic conductor Ag2Te for several temperatures with use of effective interionic potentials. The static and dynamical structures are calculated. The density distribution of silver ions also has been obtained and it has suggested that a Ag ion, located at a tetrahedral site for most of the time, moves to a neighboring tetrahedral site via the vicinity of an octahedral site. The activation energy for an ionic diffusion also has been obtained from the Arrhenius plotting of the self-diffusion coefficient of Ag. To give an explanation of the Haven’s ratio obtained by a molecular dynamics calculation, a theory of caterpillar mechanism proposed by Yokota has been used. It has been considered that Ag ions occupy the tetrahedral sites and octahedral sites with different probability. It has been shown that the ratio of the frequency of a single jump to that of cooperative jumps increases with increasing temperature.