Characteristic properties of dielectric and electronic structures in superionic conductors

Characteristics of dielectric parameters in superionic conductors (SICs) are clarified by proposing a relationship between the refractive index n and the band gap energy Eg, which is understood by applying Penn model. The relationship indicates that the average gap is diminished by the d-electron orbital near the band edge, suggesting high electronic polarization. A low electronic conductivity and a small Eg value in SICs, and also the decrease of the band gap energy with increasing temperature are interpreted from the tailing of band edge due to large ionic displacements of the anharmonic vibration. The decrease of optical dielectric constant with increasing temperature is understood from the dominant effect of the lattice expansion, based on the ionic character of frame ions. Close connection between the transverse effective charge and the ionic activation energy or the transition temperature is interpreted from the ionic or covalent character of interionic force in the frame or mobile ions.