Evidence for and against the paddle-wheel mechanism of ion transport in superionic sulphate phases

Conductivity of alkali perchlorates at high temperatures

“…The relative free-volume increases with the cation size, promoting the reorientation of perchlorate anions. In turn, the reorientation of large perchlorate anion in the lattice assists the cation hops by the paddle-wheel mechanism [9]. In terms of this model, the elementary acts of migration and reorientation are correlated and are determined by the probability of simultaneous proceeding of both processes.…”

Infrared spectroscopy of superionic conductor LiNaSO4: Vibrational modes and thermodynamics

“…Among different models, the ''paddle-wheel'' mechanism mainly attributes the higher conductivity to the strong coupling between the rotational motions of SO 4 anions and cations and considers the radii of the cations of little importance for their diffusion rates [20 -22,42]. According to this model, the rotating sulphate anions may push the cations and/or possibly lower potential thresholds making it easier for a vibrating ion to jump to an empty adjacent position [20]. In contrast, the percolation mechanism [43,44] emphasizes the role of cation radii and mass on the diffusion coefficients and it favours the idea that the diffusion of cation is enhanced by the lattice expansion and/or incorporation of ion vacancies by guest ion presence and the structural transition.…”

Lithium Ion Conduction in Oxides

“…This necessarily involves movement of the oxide anions around the sulfur centre and it is now generally agreed that in the high temperature phase the sulfate anions are capable of rotation. Additional evidence for this has come from both experimental calorimetry measurements [33] and computer simulation. [34] Calorimetric measurements from a number of groups have shown a consistently high value for the heat of transition of ca 25 kJ mol À1 compared with the enthalpy of melting of 9 kJ mol À1 .…”