The correlation of phonon frequency co, and phonon damping yj to the ionic conductivity cr 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 T, (=157 K), the P-y phase transition temperature, and described by the dispersion relation due to the phonons and the diffusing ions. The yj exhibits the drastic change at T, and has an anomalously large magnitude above T, . The shifts of y, and co, also showed appreciable changes at about 250 K. Furthermore, the toi of the low-frequency mode in the P phase increases with increasing temperature. We can qualitatively interpret these results from the disorder arrangement of the Ag ion above T, and the coupling of mobile ions above 250 K with phonons near the Brillouinzone boundary.
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