Figure 2.

Identification of structures with chemical order. Calculated energy difference between M′_2–xM′′_xAlB₂ and corresponding ternaries M′₂AlB₂ and M′′₂AlB₂ at 0 K as a function of x for chemically ordered (×) and disordered (solid line) candidate structures, assuming (a and b) Cmmm and (c and d) P6̅m2 symmetry of M′_2–xM′′_xAlB₂ for M′ + M′′ being (a and c) V + Cr and (b and d) Sc + Mo. For chemical disorder, the entropy contribution to the Gibbs free energy at 2000 K has also been approximated (dashed line). Comparison of different in-plane chemically ordered candidate structures and a schematic illustration of four stacking sequences identified for Mo_4/3Sc_2/3AlB₂ with (e) P6̅2m, (f) P6̅2c, (g) R3̅m, and (h) C2 symmetry along three zone axes. The unit cells are marked by black lines.