Defects and ionic conductivity in TlCl, TlBr, and TlI at high pressure and temperature

High-pressure and temperature experiments of ionic conductivity and crystal structure of TlCl, TlBr, and TlI reveal discontinuous and reproducible increases in conductivity on heating at constant pressure but no evidence for structural change from high-pressure and -temperature X-ray diffraction data. For each compound, the pressure-dependent solid-solid boundary between the low- and the high-temperature conductivity regions extrapolates to a l-atm temperature value, which coincides with its l-atm melting temperature to within 15 K, This behavior is discussed in terms of solid state partial disorder, caused by a sudden increase in defects, and is linked to the highly polarizable Tl+ site. Concomitant increases in charge carrier concentration and mobility and decreases in activation energy without structural transformation emphasize the importance of nonstructural factors in fast-ion conduction. (C) 1998 Academic Press.