Relationship between the interionic distance and the band gap in the normal and superionic phases in Ag3SI

To determine the band gap energy E _g of γ-Ag₃SI, we measured photoacoustic spectra at 100K. The relationship between the E _g and the interionic distance d _n is obtained as E _g ∝ d_n ² in the α-, β - and γ -phases. From this relation, the wide width of valence band in superionic conductors is suggested in comparison with that of ionic crystals.