ON THE PADDLE-WHEEL MECHANISM FOR CATION CONDUCTION IN LITHIUM-SULFATE

A few high-temperature sulphate phases are both plastic crystals and s olid electrolytes, the latter because the hindered rotational motion o f the sulphate ions enhances the mobility of the cations. This interpr etation 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 struct ure. 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 specif ic for the paddle-wheel mechanism. By the molecular dynamics study by Ferrario, Klein and McDonald it has become possible to identify separa tely the contributions from centre-of-mass displacements and rotations of the sulphate group. Information in this direction has also been ob tained recently by Karlsson and McGreevy in a neutron powder diffracti on study where the reverse Monte Carlo method is used for modelling th e data. The latter authors have modified the terminology slightly, whi ch causes confusion regarding the meaning of the term ''paddle-wheel m echanism''. 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 o ther types of mobility enhancements which also are due to libration or rotation of polyatomic anions.