Why Pb(B1/3B′2/3)O3 perovskites disorder more easily than Ba(B1/3B′2/3)O3 perovskites and the thermodynamics of 1:1-type short-range order in PMN

Fully relaxed, planewave pseudopotential calculations of formation energies (Delta E) were performed for ordered supercells in the perovskite based system (1 - X). PbNbO3 - (X). PbMgO3, including six different supercells at the X = 1/3 composition [Pb(Mg1/3Nb2/3)O-3; PMN]. Some of the corresponding supercell calculations were also performed for the eight different stoichiometries of A(B1/3B'(2/3))O-3 perovskite in which A = Pb, Ba and B = Zn, Mg and B' = Nb, Ta. A striking difference between the Pb(B1/3B'(2/3))O-3 systems and the Ba(B1/3B'(2/3))O-3 systems is that the differences in formation energies for any Pb(B1/3B'(2/3))O-3 system are roughly an order of magnitude smaller than those for the corresponding Ba(B1/3B'(2/3))O-3 system, which indicates that the energetic barriers to disordering are much lower in the Pb(B1/3B'(2/3))O-3 systems. The range of energies spanned by various PMN-stoichiometry supercells is particularly narrow, suggesting that cation disorder should persist to lower temperatures in PMN than in any of the others. Ising type thermodynamic models, that are roughly consistent with the Delta E results, were used to simulate the finite temperature behavior of a prototype A(B1/3B'(2/3))O-3 perovskite that exhibits the transition sequence 1 : 2 --> 1 : 1 --> DIS. This model reproduces the characteristic microstructure of PMN, and explains it in terms of equilibrium 1:1 short-range order in the disordered phase. (C) 1999 Elsevier Science Ltd. All rights reserved.