On the Paddle -Wheel Mechanism for Cation Conduction in Lithium Sulphate

A few high-temperature sulphate phases are both plastic crystals and solid electrolytes, the latter because the hindered rotational motion of the sulphate ions enhances the mobility of the cations. This interpretation has been called the paddle-wheel model, and it is obvious that cation migration becomes a much more complicated process in a plastic ionic crystal than in a crystal with a stiff, time-independent structure. Thus, there are strongly enhanced contributions from conventional migration mechanisms, such as jumping from well-defined lattice sites, but it is evident that there also are contributions which are specific for the paddle-wheel mechanism. By the molecular dynamics study by Ferrario, Klein and McDonald it has become possible to identify separately the contributions from centre-of-mass displacements and rotations of the sulphate group. Information in this direction has also been obtained recently by Karlsson and McGreevy in a neutron powder diffraction study where the reverse Monte Carlo method is used for modelling the data. The latter authors have modified the terminology slightly, which causes confusion regarding the meaning of the term "paddle-wheel mechanism". The "paddle-wheel" enhances not only bulk migration but also migration along interfaces and surfaces. The mobility can also be increased for monovalent anions present. Some examples are given of other types of mobility enhancements which also are due to libration or rotation of polyatomic anions.