The temperature dependence of the phonon frequencies and effective cha
rges in the superionic material Ag3SI is reported. It was found that t
he effective change decreases with temperature T in the gamma-phase, j
umps at the gamma-beta phase transition point T=157 K, remains almost
insensitive to T until immediately above the beta-alpha phase transiti
on at T=519 K, and increases rapidly with T in the alpha-phase. The be
havior has been explained based on a model in which the basic process
is the local fluctuation of the chemical bond.