The Phase Transition and the Structures of Superionic Conductor Ag₃SBr

Abstract

The ordered phase concerning the Ag distribution of a superionic conductor Ag₃SBr was discovered below 128 K. The transition heat and the entropy change were measured to be 0.23 kcal/mol and 1.77 cal/mol·deg respectively. The transition is of the first order but is close to the second order one. The crystal structures of the newly found γ-phase and the room temperature β-phase were studied by a least-squares method using X-ray and neutron diffraction data. The structures of the γ-phase and the β-phase of Ag₃SBr were similar to those of Phase III and Phase I of CsPbCl₃ respectively. The space group of the γ-phase may be D_2h¹⁷-Cmcm. The lattice constants of the γ-phase were measured to be a(\simeqb)=9.423±0.008 A and c=9.711±0.009 A at 118 K. The nature of the phase transition of Ag₃SBr is discussed in connection with those of Ag₃SI and CsPbCl₃.