A model which explained the high mobility of polyvalent metals in complex compounds was analyzed in terms of the covalency, high energy and easy polarizability of metal-O bonds. The transfer of V, Nb, Mo and W was considered to involve a crawling behavior, due to rearrangement of the latter bonds. The transfer was facilitated by the presence of pyramidal defect centers of the type, [WO3VO]0, and was promoted by their potential capability for shape-inversion and conformational change. In the case of polycrystals, an important role was played by MoO3-, WO3- and V2O5-based mobile adsorbed phases at the grain boundaries.

A.J.Neiman: Solid State Ionics, 1996, 83[3-4], 263-73