Cation order-disorder in Pb(B-II,B-V)O-3-related relaxors: The random-layer model investigated by Monte Carlo simulation

The charge-balanced random-layer model for ordered lead-based perovskites Pb((B1/3B2/3V)-B-II)O-3 was investigated by using the standard Metropolis Monte Carlo method on a rigid lattice with simple ionic model. Our results show that in the structure formula Pb[B '](1/2)[B ''](1/2)O-3, where all B '' sites are occupied by B-V cations, chemical order of B-II and B-V cations does exist in B-sites and the ordered structure has an hexagonal symmetry. An order-disorder transition as a function of temperature is evidenced by an abrupt variation of both the heat capacity and a long-range order parameter. Finally, the evolution of the short-range order parameter versus temperature shows that a local order remains in B-sites contrary to the charge-balanced random-layer model that suggests that B-sites are randomly occupie