The atomic ordering in complex perovskite alloys is investigated by the cluster variation method. For the 1/3{111}-type ordered structure, the order-disorder phase transition is the first order, and the order parameter of the 1:2 complex perovskite reaches its maximum near $x=\mathrm{0.25.}\mathrm{}$ For the 1/2{111}-type ordered structure, the ordering transition is the second order. Phase diagrams for both ordered structures are obtained. The order-disorder line obeys the linear law.

• Received 7 April 1999

DOI:https://doi.org/10.1103/PhysRevB.61.11918