PADDLE-WHEEL VERSUS PERCOLATION MODEL, REVISITED

Secco has proposed a percolation model for the large cation mobility in some inorganic rotor phases. He originally based this model on a couple of measurements where it was overlooked that the recorded impedances actually were some 10 to 100 times larger than the actual resistances of the thin and wide pellet samples (thickness 1-2 mm, cross-section about 1 cm2). This explains why the results reported by Secco et al. deviate from those of all other investigations of the conductivity of these high-temperature rotor phases. Several other properties of these phases are in conflict with Secco's percolation model, but in accordance with the so-called paddle-wheel mechanism. The activation energies of cation and anion diffusion will be taken as an example. In addition, some comments will be made on the fact that Secco makes no distinction between pure stoichiometric compounds, solid solutions and phase mixtures in his argumentation for his ''percolation type'' of transport mechanism. What Secco calls ''new positive mixed alkali and mixed anion effects'' have previously been considered by a number of authors as composite electrolytes. Furthermore, the conductivity is strongly enhanced in some isovalent solid solutions due to the presence of a large number of cation vacancies, e.g. in hexagonal Na2SO4 (Li).