A First-Principles Study of Energetics of As_xP_1-xalloys with A7 and A17 Symmetries

The most stable allotropes of Phosphorus (P) and Arsenic (As) are Black phosphorous (space group of A17 or -P) and Gray arsenic (space group of A7 or -As) respectively. The fact that, the conversion between and the phase can be possible, by alloying them with different compositions can lead to a structural transition followed by a phase coexistence between these two systems. Recently, we performed a first-principles calculation and systematically investigated the optimum geometry, electronic structure, and the effect of alloying in the structural transition of As_xP_1-x alloys. A random substitution of P with As in -P results in black As_xP_1-x alloys. Such systems were optimized within a full relaxation process allowing all atoms fully relaxed without any restriction on the lattice symmetry and the volume. Following the same procedure, As_xP_x-1 alloys with A7 symmetry were also obtained by substituting As with P in -As. The calculated formation energy of As_xP_1-x-alloys shows a cross over point at x = 0.25 and a co-existence in the range of x = 0.25 - 0.5, independent of the size of supercells chosen in the calculations. The low formation energy in As rich area implies that the As_xP_1-x alloys with A7 symmetry are energetically more stable than As_xP_1-x alloys with A17 symmetry.