TI+ ION CONDUCTIVITY IN RBXTI(1-X)I FOR 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.10, COEXISTENCE OF MIXED PHASES, AND PHASE STABILIZATION

The ac electrical conductivity versus temperature dependence of RbxTl(1-x)I for 0 ≤ x ≤ 0.10 is reported. The solid-solid phase transition (yellow) β-TlI ⇄ (red) α-TlI occurs at 172°C with transition enthalpy ΔH1 = 865 J/mole and is accompanied by a sharp jump in conductivity, ca. two orders of magnitude. The reverse α → β transition exhibits a distinct hysteresis dependent on pressure and rate of cooling. The high conductivity α-phase can be obtained at room temperature (i) by application of pressure to the β-solid, (ii) by rapid quenching of the melt, and (iii) by incorporating 5-9 mole% Rb1 in TlI lattice. The conductivity data and Qc values, while sensitive to sample history and pretreatment, show reproducible behavior. The observed Qc (eV) values are 0.39 ± 0.04 for the cool mode of compressed α- and β-TlI and the heat mode of Rb0.05Tl0.95I, 0.50-0.62 for all α-TlI samples and Rb0.10Tl0.90I, and 0.64-0.97 for all β-TlI samples prior to the β-α transition. No solid-solid phase transition is observed for RbI and its conductivity is lower than that of TlI by a factor of 105 at ∼ 175°C. The mechanism and structure interpretations of these results are presented. © 1993 Academic Press, Inc.