Pb(B-1/2'B-1/2'')O-3-type perovskites .1. Pair-correlation theory of order-disorder phase transition

The limitation of the long-range order parameter and the necessity of the short-range order parameter for the thermodynamic description of Pb(B-1/2'B-1/2'')O-3-type perovskites are discussed. Based on the discussion, a statistical thermodynamic model that takes into account the configuration of the neighboring B-site ions (B' and B '' cations) was developed. A pair-correlation approximation was used in the calculation of the configurational entropy and the long-range coulombic interaction energy between the nearest B-site ions. The theoretical calculations using Pb(Sc1/2Ta1/2)O-3 (PST) and Pb(Sc1/2Nb1/2)O-3 (PSN) systems indicate that the short-range order parameter persists over a wide range of temperatures examined (0-1800 K) and that there possibly occur consecutive long-range order-disorder transitions in the configuration of B-site cations. The possibility of the existence of short-range ordering above the long-range order-disorder transition temperature was also examined using the annealed PSN specimen as a typical example of Pb(B-1/2'B-1/2'')O-3-type perovskites.