High pressure and temperature studies of the ionic conductivity and crystal structure of these materials revealed discontinuous and reproducible increases in the conductivity upon heating them at constant pressure. However, there was no evidence of structural changes in high-pressure and high-temperature X-ray diffraction data. For each material, the pressure-dependent solid/solid boundary between the low-temperature and high-temperature conductivity regions extrapolated to a 1atm temperature which coincided with the 1atm melting point to within 15K. This was explained in terms of solid-state partial disorder caused by a sudden increase in defects, and was related to the highly polarizable Tl+ site. Simultaneous increases in charge-carrier concentration and mobility, and decreases in activation energy without structural transformation, demonstrated the importance of non-structural features in fast-ion conduction.

Defects and Ionic Conductivity in TlCl, TlBr, and TlI at High Pressure and Temperature. R.A.Secco, E.A.Secco, Q.Chen: Journal of Solid State Chemistry, 1998, 141[2], 462-5