It was recalled that conductivity data for some compositions of this type had been analyzed in the context of a percolation-type mechanism. However, no distinction had been made between stoichiometric compounds, solid solutions and 2-phase regions. Also, correlations between conductivity and so-called free volume had been made without presenting any facts concerning the latter concept. It was concluded that the presentation of the underlying experimental data had been questionable, and that the approach offered no advantage when compared with previous concepts. It was suggested that the conductivity enhancements which were characteristic of aliovalent solid solutions were due to generally accepted vacancy mechanisms.

A.Lundén: Solid State Ionics, 1995, 76[3-4], 249-52