Generalized Bragg–Williams model for the size-dependent order–disorder transition of bimetallic nanoparticles

Considering the different effects of exterior atoms (face, edge and corner atoms), the Bragg–Williams model is generalized to account for the size, shape and composition-dependent order–disorder transition of bimetallic nanoparticles (NPs) with B2, L10 and L12 ordered structures. The results show that the order–disorder temperatures T C,p are different for different shapes even in the identical particle size. The order of order–disorder temperatures of different shapes varies for different sizes. The long-range order parameter decreases with the increase in temperature in all size ranges and decreases smoothly in large sizes, but drops dramatically in small sizes. Moreover, it is also found that the order–disorder temperature of bimetallic NPs rises with increasing particle sizes and decreases with a deviation from the ideal compositions. The present predictions are consistent with the available literature results, indicating its capability in predicting other order–disorder transition phenomena of bimetallic NPs.