Kunio Wakamura

Correlation between high ionic conduction and anomaly of dielectric property

Abstract In cubic and near-cubic ionic and semiconducting compounds, we can find two correlationships among the optical dielectric constant ϵ∞, short-range and long-range forces. In the relationships, the chemical valence of high ionic conductors takes the value shifted from the formal value. The shift can be explained from the effect of d -electron energy level near the band gap. A linear relationship of ln (ϵ∞) to ln ( T c ) is found for fast-ionic conductors. From these relations, intimate correlation of the band gap energy to the fast-ionic conduction is suggested.

Vibrational Spectra and Effective Charges in Some Spinels

The infrared reflectivity spectra have been observed for sintered MCr2S4(M=Zn, Cd, Hg) near normal incidence in the range from 30 to 900cm-1. The effective charges of Cr and S ions are determined from the frequencies of transverse and longitudinal modes within small error. The Szigetieffective charges es* per effective valency Zeff for anions show the correlation with the low frequency electronic dielectric constant e∞ which has been established for selenide and oxide spinels and expressed as e∞-1=2.0/((es*/Zeffe)+(es*/Zeffe)2). By using this empirical relationship, the magnitude of effective charge of cation M is determined within the error in the charge of Cr ion. By the comparison among the obtained effective charges, dielectric constants e∞ and the lattice parameters, the increase of ionicity with the decrease of anion radius and a weak influence of chromium ion in comparison with that of cation M to the dielectric constant are shown. From these results, the similarity and the difference of dielectric properties between binary and ternary compounds are discussed.

Observation of characteristic phonon spectra for cage and mobile ions in the layered superionic conductor AgCrS2

Abstract The important role of interionic forces for the cage ions in superionic conductors has been theoretically demonstrated by molecular dynamics calculations and also by empirical correlations. To demonstrate this experimentally, we have measured the optical phonon spectra in the two-dimensional superionic conductor AgCrS 2 from 20 K to 770 K. A sharp change in the optical phonon spectra was observed at the superionic phase transition point, T c , i.e., among the six observed phonon bands, only bands due to the Cr ion show a decrease in phonon damping and an increase in frequency splitting between transverse and longitudinal optical phonons near and above T c . This behaviour is completely opposite to that of the phonon band for the mobile ions, and from this, we note the enhancement of long-range Coulomb forces among the cage ions above T c , and interpret the temperature dependence of the phonon band intensity in several superionic conductors.

Effect of magnetic ordering on phonon parameters for infrared active modes in ferromagnetic spinel CdCr2S4

Infrared reflectivity spectra of sintered crystal CdCr2S4 have been measured in the temperature range of 15–300 K. The frequencies of transverse and longitudinal phonon modes were determined from the spectra with the Kramers–Kronig relation. The temperature dependencies of those showed anomalously large shifts near the Curie temperature. The shifts for two modes on the higher‐energy side increase and those for other two modes decrease with temperature. These shifts are described well by the terms of spin‐correlation function and concluded to be originated from the spin‐dependent force constant. The mode dependence of shifts is explained qualitatively from the relation between the linkage of super‐exchange interaction and the direction of vibration of the ion. The effects of magnetic ordering were also observed on Szigeti effective charge, optical and static dielectric constants, and phonon damping constants. These are discussed from the viewpoint of the red shift of the absorption edge.

Infrared Reflectivity and Crystal Strusture of InS

Infrared reflection spectra of InS single crystals are observed in the wavenumber range between 50 and 900 cm -1 for each polarization in c -plane. The vibrational frequencies of longitudinal (ω L ) and transverse (ω T ) modes are determined by the Kramers-Kronig analyses as follows: ω L =309 and ω T =217 cm -1 for E // a , and ω L =110, 236, 322, and ω T =104, 196, 296 cm -1 for E // b polarizations. Almost the same frequencies are obtained from the fitting procedure by using the classical oscillator model. It is confirmed that the obtained dispersion parameters are more explicit values than those of previous works for twin crystals. The difference in crystal structures between InS and other layer semiconductors, e.g . GaS, GaSe, InSe, is explainable qualitatively in terms of effective charges or ionicities.

Correlation among the transition and melting temperatures and dielectric parameters in some fast ionic conductors

Abstract Three correlationships are established among the optical dielectric constant e∞, Szigeti effective charge e∗s, transition temperature Tc, melting temperature Tm, and optical phonon frequency for many cubic fast ionic conductors. These relations suggest an intense influence of electronic energy state near the band gap to the origin of fast ionic conduction, dominant contribution of short-range or long-range forces to the mobile ion below Tc or to the cage ion above Tc. A curious temperature dependence for e∞ and e∗s values is also interpreted by considering anharmonic dynamical effective charge.

Lattice Vibrations in Spinel Type Crystals CdxZn1-xCr2Se4

The lattice vibrations in Cd x Zn 1- x Cr 2 Se 4 crystals have been studied by measuring the reflection at normal incidence in the region of wave number above 30 cm -1 . Four reststrahlen bands were observed over all composition range, and the positions and the intensities of these bands vary with the composition. The behaviours of these variations show a characteristic of one-mode type. The transverse mode frequencies are calculated from the classical dispersion theory. The composition dependences of those transverse mode frequencies can be explained with a virtual crystal model which includes two kinds of units: a mass and force constants are varied monotonically with the fractional amount of Cd ion in the mixed crystal. The frequencies of the longitudinal modes are also calculated. The condition for the appearance of the one-mode or two-mode type is shortly discussed.

Thermal expansion and anharmonic phonon-phonon interaction effects on an effective charge in GaAs and KCl

Abstract The effects of anharmonic phonon-phonon interactions on an effective charge Z ∗ c ( T ) e in a typical semiconductor GaAs and an ionic compound KCl are investigated by dividing the frequency shifts of TO and LO phonons into two parts depending on the volume and the anhannonic phonon-phonon interaction. The Z ∗ c ( T ) value in GaAs is mainly decreased by the increasing volume but that of KCl is decreased dominantly by the phonon-phonon interaction, though the increase of volume enhances the charge. This difference between these compounds is understood by considering the contribution of crystal anharmonicity to the ionic charge. Such a consideration can also interpret curious temperature dependence of effective charge which has been observed in several semiconductors and a few superionic conductors.

Effects of electronic band on activation energy and of effective charge on lattice distortion in superionic conductors

Abstract A strong contribution of the electronic energy state near the band gap on superionic conduction is suggested by showing an empirical relationship between the optical dielectric constant and the activation energy, Eac. The transverse effective charge is connected with the transition temperature and Eac value. On the basis of this charge and the crystal symmetry, we can understand the structural phase transition in superionic conductors and the non-structural transition in high ionic conductors AgX (X = F, Cl and Br) and CuF.

Vibrational properties of one- and two-mode behaviour in spinel type mixed systems Zn1−xCdxCr2S4

Abstract The infrared reflectivity spectra of spinel type mixed crystals Zn 1− x Cd x Cr 2 S 4 have been measured from 40 to 900 cm −1 . Four or five bands were observed at both end members in the region of intermediate composition. The three bands in higher energy show one-mode behaviour and the other two bands show two-mode behaviour. The concentration dependences of these mode frequencies are interpreted by a model proposed already, in which the 2 basic units and the 6 force constants are contained. By comparing the force constants associated with the Zn-site in several spinels, it is suggested that the two-mode behaviour arises from the large difference in the force constants between the two end members. The E g (260 cm −1 ) and T 2g (160 cm −1 ) Raman modes for ZnCr 2 S 4 were also observed and used for the determination of the force constants.