Hideo Okazaki

Deviation from the Einstein Relation in Average Crystals Self-Diffusion of Ag+ Ions in α-Ag2S and α-Ag2Se

The mobility µ and the diffusion coefficient D of silver ions in Ag 2 S, Ag 2 Se and alloys Ag 2 S 1- x Se x have been measured by the aid of tracer technique using Ag 110m . The ratio µ/ D is found to deviate from the Einstein relation remarkably. Although such deviation is usually attributed to the existence of correlation between successive jumps of a diffusing ion, the present substances are so much disordered that the correlation effect theory cannot be applied. Recently Yokota has proposed a new model appropriate to our substances. The experimental results are compared with his theory with a minor modification. It is proved that the theory is applicable to our substances.

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.

Molecular Dynamics Studies of Yttria Stabilized Zirconia. II. Microscopic Mechanism of Oxygen Diffusion

The microscopic mechanism of oxygen diffusion in yttria stabilized zirconia (YSZ) is investigated by means of a polyhedron analysis. It is shown that the probability of finding O ion in a tetrahedron increases as Y ions at the corners of the tetrahedron increase, and this tendency becomes marked with increasing dopant concentration, which is consistent with the fact that the oxygen coordination number for Y ion is larger than that for Zr ion. By studying the oxygen migrations between tetrahedra in the [100] direction in detail, it is found that the migrations preferentially occur between tetrahedra having a common Zr–Zr edge, and Y ions restrict the diffusion paths of O ions. The restriction by the Y–Y edge causes the decrease of the oxygen diffusion with increasing dopant concentration. It is discussed that the fluorite structure of YSZ is guaranteed by the defective and disordered state of oxygen sublattice.

Deviation from the Einstein Relation in Average Crystals. II. Self-Diffusion of Ag+ Ions in α-Ag2Te

The self-diffusion coefficient D of silver ions in α-Ag 2 Te has been measured by the tracer technique using Ag 110m and the mobility µ of cations is estimated from the ionic conductivity σ. The ratio µ/ D is slightly deviated from the Einstein relation. This deviation is discussed in comparison with a theory appropriated to the average crystals proposed by Yokota.

X-Ray Diffraction Study on a Superionic Conductor: α-Ag2Se

An oscillatory background and Debye lines of α-Ag 2 Se which has so-called averaged structure were studied by X-ray diffraction experiments. The diffuse background was analyzed by assuming a liquid-like distribution of Ag atoms and the pair distribution function g Ag-Ag ( r ) was obtained. The calculated intensities of Debye lines by the liquid-like Ag distribution model agreed qualitatively with the observed intensities. Therefore, it is not necessary for Ag atoms to be confined to 42 sites which were introduced by Rahlfs. A mean potential U Ag ( r ) of Ag atoms was calculated from the mean density distribution of Ag atoms ρ Ag ( r ), and was related to an asymmetric thermal vibration of Ag atoms in α-Ag 2 Se.

First-Principles Molecular-Dynamics Simulation of Proton Diffusion in Sc-Doped SrTiO3.

The microscopic mechanism of proton diffusion in the protonic conductor Sc-doped SrTiO 3 is studied by a first-principles molecular-dynamics simulation. It is shown that the proton forms an O-H bond with a neighboring O ion, and the frequency of the stretching vibration of the O-H bond is about 2800 cm -1 , which agrees reasonably well with the result of experiment. Two types of diffusion paths are observed: one is diffusion around the O ion while retaining the O-H bond, and the other is diffusion between the two neighboring O ions while switching the O-H bond. During the former type of diffusion, the stretching vibration proceeds and the length of the O-H bond is almost unchanged. The latter type of diffusion occurs frequently and quickly.

Stable positions of a proton in Sc-doped SrTiO3 determined by the first-principles pseudopotential calculation

Stable positions of a proton in the protonic conductor Sc-doped SrTiO 3 are obtained by a steepest-descents optimization of the ionic configurations in the first-principles pseudopotential calculation. It is shown that, in a stable ionic configuration, a Ti (or Sc) ion moves away from the proton, while two O ions move towards the proton, from their positions in the ionic configuration of a perfect crystal SrTiO 3 , and that an O-H bond is formed, the length of which is about 1.03 A. The proton is located on the side opposite the Ti(Sc) ion with respect to a line connecting the two O ions, and the H-O-O angle is approximately 13°.

Specific heat of liquid In-Te alloys

The temperature and composition dependence of the specific heat in liquid In-Te alloys, measured with adiabatic calorimeter, are reported. The specific heat isotherms around the composition of In2Te3 are anomalously large above the liquidus temperature and they tend to be small at very high temperatures. The large specific heats and their temperature dependences are satisfactorily explained in terms of the partial dissociation of compound formation with increasing temperature. The required molar energy for the dissociation from semiconducting to metallic states in liquid Te is about 3.31 kcal mol-1 and that in liquid In2Te3 is 4.21 kcal mol-1.

Effects of doped acceptor ions on proton diffusion in perovskite oxides : a first-principles molecular-dynamics simulation

The effects of doped acceptor ions on proton diffusion in the protonic conductor Sc-doped are studied by means of a first-principles molecular-dynamics simulation. It is found that the proton forms an O-H bond with the neighbouring O ion, and that the frequency of the O-H stretching vibration depends on the position of the proton in the crystal. The frequencies obtained from our simulations are consistent with the experimental results obtained from the infrared-transmission spectra. It is shown that the position dependence of the O-H stretching vibration is caused by the electron density distribution in the Sc-doped . Near the Sc ion, electrons tend to localize around each ion causing higher frequencies, while, near the Ti ion, the electron density between the Ti and O ions is larger than that in the undoped crystal, giving lower frequencies.

Molecular Dynamics Study of Superlattice Superionic Conductor: Enhancement of Oxygen Ion Diffusion in YSZ

A constant-volume molecular dynamics calculation has been performed for a cubic yttria-stabilized zirconia (Y 2 O 3 ) c (ZrO 2 ) 1- c , where Y layers and Zr layers are alternately accumulated to form a superlattice. A considerable enhancement of the diffusion of oxygen ions was observed at the composition c =0.33, at which the diffusion coefficient in the superlattice is about three times larger than the case of a random distribution of Y ions.