Cation order-disorder in Pb(BII,BV)O3-related relaxors: The random-layer model investigated by Monte Carlo simulation

The charge-balanced random-layer model for ordered lead-based perovskites Pb(B1/3IIB2/3V)O3 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″]1/2O3, where all B″-sites are occupied by BV cations, chemical order of BII and BV 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 occupied. This local order could be helpful to clarify some experimental results.