  The B-site disorder in the d5 - d3 system of perovskites has been analyzed
with molecular field theory and Monte Carlo method. The model is applicable to
RFe1-pCrpO3 at any p value. When the saturation magnetization MS and phase
transition temperature TP are known, a model can be built to calculate the
order or disorder distribution of lattice B-sites. We analyze the case that the
Fe-Cr superexchange is antiferromagnetic and ferromagnetic coupling
respectively. The simulation result shows that the theoretical calculation
formula is suitable for the calculation of different B-site distribution.
Through the simulation, we find that when the x and y are large, the system
will appear obvious long-range order. The DM interaction has a certain
influence on the saturation magnetization. Via calculation, we found that the
distribution states of Fe and Cr do not always conform to the uniform
distribution but rather exhibit an effect that reduces the Fe-Fe clustering.
The establishment of this model offers an explanation for several previously
contentious issues, e.g., what is the phase transition temperature range of
double perovskite, and why the different phase transition temperatures with the
same doping proportion. It provides theoretical guidance for the design of
functional materials with an arbitrary phase transition temperature.
