Structure and dynamics of superionic conductors at high temperatures and high pressures

Superionic conductors are materials with high ionic conductivities in the solid state approaching the typical value of that found in a molten ionic solid. The copper and silver monohalides have aroused much interest, because a number of these simple compounds display superionic behaviour at room pressure and high temperatures. Many studies of these compounds have been undertaken using varied techniques from simple alternating current conductivity measurements and differential thermal analysis studies, investigating their dynamical properties, through to X-ray and neutron diffraction, investigating their structural nature. High pressure measurements have indicated that these compounds have complicated pressure-temperature phase diagrams, although relatively few studies have been undertaken using the aforementioned techniques at high pressure. AgI at 1.1 GPa has the rocksalt structure. As the temperature is increased, our neutron diffraction measurements have observed a lattice parameter anomaly, a peak in the linear expansivity and a rise in interstitial occupancy of 8(c) sites at (1/4, 1/4, 1/4) which all occur at the same temperature. These measurements indicate that rocksalt AgI passes through a gradual transition into a superionic state, similar to the behaviour of Type II superionic conductors such as #beta#-PbF_2. CuI at pressures below 1.7 GPa exists in a zincblende structure that has a face centred cubic unit cell. At 1.30(8) GPa and increased temperature, our neutron diffraction measurements indicate that high pressure CuI adopts several highly disordered structures. Evidence is reported of a disordered rhombohedral structure and of the first measurements of a body centred cubic structured phase isostructural to #alpha#-AgI, with lattice parameter of 4.7983(4) A at 966 (6) K and 1.30(8) GPa. At high temperatures (> 473 K) Ag_3SI exists as a disordered #alpha#-phase similar to #alpha#-AgI, with the anions distributed at random amongst the body centred cubic sites. On gradual cooling Ag_3SI passes through two phase transitions into #beta#-Ag_3SI and then #gamma#-Ag_3SI, whereas rapid cooling enables the #alpha#-phase to be stabilised at temperatures below room temperature. We measured both the ionic conductivity and the neutron diffraction pattern in these phases at temperatures ranging from #approx# 10 K to 573 K and determined the structures of these phases and the nature of the gradual #beta# #-># #gamma# phase transition. The ionic conductivity measurements give dynamical measurements of the same batch of samples as used in the neutron diffraction experiments. (author)SIGLEAvailable from British Library Document Supply Centre- DSC:D210251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo