PRESSURE-TEMPERATURE STUDIES ON CONDUCTIVITY OF TLX (X=CL, BR, I) COMPOUNDS - I - PHASE-DIAGRAM - II - IONIC MOBILITY

We have measured the conductivity sigma of TlX(X = Cl, Br, I) compound s up to 5.3 GPa and between 300-823 K. The sigma-T dependence for all compounds can be divided into three distinct regions: (i) low temperat ure (LT), < 400 K, with unusual negative sigma-T dependence, (ii) inte rmediate temperature (IT), 400 < 650 K, with positive sigma-T dependen ce and (iii) high temperature (HT), T > 650 K, with positive sigma-T d ependence. The sigma-T isobars were used to construct the T-P solid ph ase diagram for each compound. The LT region data indicate a new meta- stable phase in the 1.0-3.5 GPa range. The LT --> IT transition is cha racterized by an inverse sigma-T dependence followed by normal Arrheni us behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and I-IT regions above 3 GP a for each compound in the P-T plot yields a value close to its respec tive normal (1 atm) T-melt suggesting a solid order-disorder transitio n type paralleling alpha-AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5-4.1 GPa of all compounds is at var iance with the Arrhenius behavior observed for unperturbed ion migrati on implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius sigma-T dependence prevails in both IT and HT regions with Q(c) values of 85-100 kJ mol(-1) and 50 -75 kJ mol(-1), respectively. The higher conductivities at 0.4 GPa for TlBr and TlI relative to their 1 atm data and the increasing sigma wi th P are in strong contrast to the normal sigma-P behavior of TlCl. Th e dependence of activation volume Delta V double dagger on T for TlCl, i.e. Delta V double dagger > 0, shows abnormally high values with a m aximum at 500 K for P < 3.0 GPa but reasonable Delta V double dagger v alues appear above 3.0 GPa. The Delta V double dagger-T dependence for both TlBr and TLI with Delta V double dagger < 0 is incompatible with an ion transport mechanism suggesting an electronic conduction proces s and implying an ionic-metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in te rms of electronegativity differences and bonding character rather than structure. (C) 1998 Elsevier Science B.V. All rights reserved.