Order-disorder transitions: structural and magnetic ordering in double perovskites

Using an effective lattice gas-like model for B-B' site ordering we study order-disorder transitions in double perovskite materials A(2)BB'O-6. We motivate this effective model from a microscopic hamiltonian. Using this model, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B' species. We also study ordering in the 'ternary' compounds A(2)BB'(1-y)BaEuro(y)(3)O(6), using a variant of the Blume-Emery-Griffiths model. Several issues related to structural and magnetic ordering are discussed.