Why Pb(B,B ')O-3 perovskites disorder at lower temperatures than Ba(B,B ')O-3 perovskites

Plane-wave pseudopotential calculations of total energies were performed fo r three ordered perovskite related supercells in each of the eight stoichio metries of A(B1/3B2/3')O-3; A = Pb, Ba, B = Zn, Mg and, B' = Nb, Ta; and th e eight stoichiometries of A(B1/2B1/2')O-3; B = Sc,In. A striking differenc e between the Pb and Ba(B1/3B2/3')O-3 systems is that the differences in to tal energies for Pb systems span ranges that are roughly an order of magnit ude smaller than those for the corresponding Ba systems. This indicates muc h lower energetic barriers to disordering in the Pb systems, consistent wit h experiments. We explain this trend as a consequence of enhanced Pb-O bond ing to underbonded oxygens in B2+-O-B2+ environments. [S0163-1829(99)51642- 1].