Pressure–temperature studies on conductivity of TlX(X=Cl,Br,I) compounds:: I: phase diagram. II: ionic mobility

Abstract

We have measured the conductivity σ of TlX(X=Cl, Br, I) compounds up to 5.3 GPa and between 300–823 K. The σ– T dependence for all compounds can be divided into three distinct regions: (i) low temperature (LT), <400 K, with unusual negative σ– T dependence, (ii) intermediate temperature (IT), 400<650 K, with positive σ– T dependence and (iii) high temperature (HT), T>650 K, with positive σ– T dependence. The σ– T isobars were used to construct the T– P solid phase 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 characterized by an inverse σ– T dependence followed by normal Arrhenius behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and HT regions above 3 GPa for each compound in the P– T plot yields a value close to its respective normal (1 atm) T melt suggesting a solid order–disorder transition type paralleling α-AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5–4.1 GPa of all compounds is at variance with the Arrhenius behavior observed for unperturbed ion migration implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius σ– 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 σ with P are in strong contrast to the normal σ- P behavior of TlCl. The dependence of activation volume Δ V ‡ on T for TlCl, i.e. Δ V ‡>0, shows abnormally high values with a maximum at 500 K for P<3.0 GPa but reasonable Δ V ‡ values appear above 3.0 GPa. The Δ V ‡– T dependence for both TlBr and TlI with Δ V ‡<0 is incompatible with an ion transport mechanism suggesting an electronic conduction process and implying an ionic–metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in terms of electronegativity differences and bonding character rather than structure.