Small band gap and high ionic conduction in Cu2S

Far-infrared reflectivity spectra of Cu2S have been measured from 20 to 700 cm−1 at several temperatures through the superionic transition temperature Tc. Many bands and two broad bands were observed at low and high temperatures, respectively. From the spectra, we determined optical dielectric constant ε∞, the angular frequencies of longitudinal (LO) and transverse (TO) optical phonons, and transverse effective charge. Based on these values, we suggest significant role of small band gap for high ionic conduction with small activation energy and the possibility of lattice distortion arising from an enhanced effective charge. Anomalous temperature dependence has been found for ε∞ and TO–LO splitting.