First-principles study of binary bcc alloys using special quasirandom structures

We present three 16-atom special quasirandom structures (SQS's) for A _1-xB_x bcc substitutional alloys at compositions x=0.25, 0.50 and 0.75, respectively. The structures possess local pair and multisite correlation functions that mimic those of the corresponding random bcc alloy. The introduction of these SQS's allows for the possibility of first-principles calculations of bcc solid solutions, even those with significant size-mismatch or atomic relaxation. We have tested our SQS's via first-principles calculations in the Mo-Nb, Ta-W and Cr-Fe systems, in which the bcc solid solution is observed to be stable over the whole composition range. Our first-principles SQS results provide formation enthalpies, equilibrium lattice parameters and magnetic moments of these bcc alloys which agree satisfactorily with most existing experimental data in the literature. In an effort to understand the atomic relaxation behavior in bcc solid solutions, we have also investigated the nearest neighbor bond length distributions in the random bcc alloys. The proposed bcc SQS's are quite general and can be applied to other binary bcc alloys.