FAST ION CONDUCTIVITY AND PHASE-TRANSITIONS IN AGRBSO4 AND AGTLSO4

AgRbSO4 and AgTlSO4 undergo a solid phase transition to a high temperature high conductivity state. The results of ac electrical conductivity and DSC (calorimetry) measurements are reported for these compounds whose single-cation sulfates are isostructural in the hexagonal system for the high temperature phase. Distinct sigma-T behavior is observed in a common ionic framework with the common ion Ag+ hosting different guest ions, Rb+ and Tl+, of equal radius with dissimilar electron configurations and atomic masses. These parameters focus on the effects of lattice "free" volume, ion-ion interactions and vibrational amplitudes of neighboring ions. The number of activated mobile Ag+ ions n and their jump frequency nu, are directly linked to ion-ion interactions. The higher conductivity of AgTISO4 is attributed to larger n and higher nu along with the larger atomic mass Tl+ cation in the structure. Lattice "free" volume does not play a prominent role in the enhancement of conductivity. Both compounds show a constant activation energy for cation migration, viz. 0.47 +/- 0.03 eV, in the assumed hexagonal structure. The kinetic and structural interpretations of these observations along with activation energy, thermal hysteresis and enthalpy/entropy of transition are discussed.