Tomozo Tomoyose

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.

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 Conductivity and Thermoelectric Power of Superionic Conductors in the Lattice Gas Model

The ionic conductivity and the thermoelectric power in superionic conductors are expressed as functions of the concentration of ions and the interaction energy in the framework of the lattice gas model. Under appropriate conditions, the results obtained are in good agreement with those of experiments.

Ionic Plasma Model of Low-Energy Excitation in Cation Superionic Conductors

An ionic plasma model is presented to describe the low-energy excitation of cation superionic conductors. When taking account of an effective charge of cations, the model is supported by experiments except for β-Alumina type superionic conductors.

Effective Charges of AgI and CuX ( X=Cl, Br, I)

Effective charges of superionic conductors are calculated by using a microscopic dielectric function which is composed of valence electron and localized d electron terms. The d electron terms of the effective charge are evaluated by using the tight-binding approximation. It is found that the d electrons enhance the transverse effective charge.

Effective charges of γ-CuX and γ-AgI based on a lattice-distortion model

The Cu and Ag ion displacements in γ-CuX (X = Cl, Br, I) and γ-AgI are calculated by using the transverse effective charge based on a lattice-distortion model. These cation displacements are compared with the root-mean-square displacements of cations obtained from the anharmonic model for CuX and AgI. It is found that the lattice-distortion model is consistent with the anharmonic model.

Phonon-Assisted Ion Hopping in Superionic Conductors Based on the Lattice Gas Model

The jump diffusion of interacting mobile ions in superionic conductors (SIC) is studied by the phonon-assisted hopping theory. The interacting mobile ions are described by the lattice gas model. In order to study the correlated hopping of mobile ions, we calculate the average jump rate taking account of the average distribution of mobile ions at the nearest neighboring sites around the hopping ion after and before the hopping. We find that the average jump rate is thermally activated and that its activation energy is low at high concentration of mobile ions. This suggests that a low activation energy of SIC is due to the mutual interaction of mobile ions.

Analytical Model of Critical Ionicity for ANB8-N Binary Compounds

By using the pseudopotential method, the transverse effective charges Z T ZB and Z T RS of zincblende and rock-salt structures, respectively, are presented as a function of the Phillips ionicity f i . The critical ionicity F c is analytically determined from the critical point where Z T ZB is equal to Z T RS . Its average value for various tetrahedral compounds b a r F c is estimated to be \bar F c =0.78, which is very close to the Phillips critical ionicity F i =0.785. The relationship between F c and F i is discussed with respect to the structural instability caused by the large effective charge Z T .

Proton Diffusion in Perovskite-Type Oxides Based on Small Polaron Model

The ground state energy and the wavefunction of proton in the cubic perovskite-type oxides ABO 3 are studied by using the double-well potential constructed from the one-dimensional O–H–O bond. We h...

Dielectric Constants of Ge, Sn, and Isoelectronic Semiconductors

Static dielectric constants of Ge, Sn, and isoelectronic semiconductors are calculated by using the full f -sum rule including all possible transitions of d band electrons. The d band effect on the dielectric constant is expressed by using a newly defined D factor different from the empirical D factor introduced by Van Vechten. The new D factor is directly expressed in terms of the average band gap, the energy of the d band, and the coupling oscillator strength between valence and d bands so that it has a well defined meaning in contrast to the empirical D factor. The calculated D values well reproduce the observed dielectric constant and the effective number of valence electrons which represents the excess deviation from the usual valence electron number.