CVM studies on the atomic ordering in complex perovskite alloys
  The atomic ordering in complex perovskite alloys is investigated by the cluster variation method (CVM). 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=0.25. 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. 
Zhi-Rong Liu, Jian-She Liu, Bing-Lin Gu, and Xiao-Wen Zhang