A.model of cation ordering in A(B'_xB''_1-x)O_3 relaxors
  We have shown that the lattice-gas model with four repulsive pair interaction constants (corresponding to the four nearest coordination shells) on a simple cubic lattice is a sufficient model to describe the main types of cation ordering in relaxors. The phase diagram, obtained by the cluster variation method, shows the sequences of transitions between the phases 1:2 - 1:1 - disordered and 1:2 - disordered. 
S. Lapinskas, E. E. Tornau, A. Rosengren