It was noted that a few high-temperature sulfate phases were both plastic crystals and solid electrolytes. In the latter case, hindered rotational motion of the sulfate ions enhanced the cation mobility via a so-called paddle-wheel mechanism. It was considered to be obvious that cation migration became a much more complicated process in a plastic ionic crystal than in a crystal with a stiff, time-independent, structure. There were strongly enhanced contributions which arose from conventional migration mechanisms, such as jumping from well-defined lattice sites. However, it was evident that there were also contributions which were specific to the paddle-wheel mechanism. By using molecular dynamics methods, it had become possible to identify separately the contributions which arose from center-of-mass displacements and from rotation of the sulfate group. The so-called paddle-wheel enhanced not only bulk migration but also migration along interfaces and surfaces. The mobility could also be increased in the case of monovalent anions. It was pointed out that there were other types of mobility enhancement which were also due to the libration or rotation of polyatomic anions.

A.Lundén: Zeitschrift für Naturforschung, 1995, 50a[11], 1067-76