Positive Mixed Cation Effect in Ag/TlSO 4 Compositions and Phase Diagram for the Ag 2SO 4-Tl 2SO 4 System

• Published in: Journal of solid state chemistry 1995, Vol.114 (1), p.271-276
• Main Authors: Lu, Yanjia, Secco, E.A.
• Format: Article
• Language: English
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1006/jssc.1995.1039

The ac conductivity data for the binary system Ag 2SO 4-Tl 2SO 4 are presented and interpreted in terms of a positive mixed cation effect on cation conductivity that has features in common with the positive mixed alkali/cation effects observed for Na 2SO 4-Rb 2SO 4 and Ag 2SO 4-Rb 2SO 4 systems, respectively. The data point to other factors, such as ion-ion interactions, bonding, or "lattice loosening," that can promote ionic conductivity in isomorphous structure sulfates along with the larger isovalent guest-ion-free volume contribution. The relatively constant activation energy for ionic conductivity, Q c = 45 ± 5 kJ/mole, indicates a quasi-equal intersite electrostatic potential energy for the mobile cation in the hexagonal structure and indicates that the defect formation enthalpy term is zero or negligible. The ion transport mechanism is interpreted to be of the percolation type with structural connectivity being the critical parameter. The DSC calorimetry data show interesting solid phase transition behavior. The dependence of the solid phase transition enthalpy, Δ H t, on composition is presented, along with the complete phase diagram for the Ag 2SO 4-TlSO 4 system.