A Structural Phase Transition in Superionic Conductor Ag3SI

The phase transitions and the crystal structures of a superionic conductor Ag 3 SI were investigated by specific heat measurements and by X-ray as well as neutron diffraction methods. The transition entropies were measured to be 1.44 cal/mol·deg for the γ- β transition at 157 K and 2.71 cal/mol·deg for the β- α transition at 519 K. The least squares refinements of atomic positions and isotropic temperature factors were carried out. β-Ag 3 SI is cubic with space group Pm3m in which Ag is distributed into four positions apart 0.5 A from the face centered position toward [100], while in γ-Ag 3 SI below 157 K, Ag is located at one of these four positions obeying the symmetry of the rhombohedral space group R3. From these experimental results, it was found that the nature of the successive phase transitions in Ag 3 SI was not only of the order-disorder type but also of a structural phase transition. A comparison was made with the phase transition of CsPbCl 3 .