OBSERVATION OF ANOMALOUSLY INCREASING PHONON DAMPING CONSTANT IN THE BETA-PHASE OF THE FAST-IONIC CONDUCTOR AG3SI

The correlation of phonon frequency j and phonon damping j to the ionic conductivity has been observed from the temperature dependence of infrared-reflectivity spectra of a Ag3SI single crystal. Two reststrahlen bands and a rapid increase of reflectivity in low-frequency region were observed above Tc (157 K), the - phase transition temperature, and described by the dispersion relation due to the phonons and the diffusing ions. The j exhibits the drastic change at Tc and has an anomalously large magnitude above Tc. The shifts of j and j also showed appreciable changes at about 250 K. Furthermore, the j of the low-frequency mode in the phase increases with increasing temperature. We can qualitatively interpret these results from the disorder arrangement of the Ag ion above Tc and the coupling of mobile ions above 250 K with phonons near the Brillouin-zone boundary. © 1990 The American Physical Society.