First-principles prediction of stabilities and instabilities of compounds and alloys in the ternary B-As-P system

摘要

We examine the thermodynamic stability of compounds and alloys in the ternary B- As-P system theoretically using first-principles calculations. We demonstrate that the icosahedral B_(12)As_2 is the only stable compound in the binary B-As system, while the zinc-blende BAs is thermodynamically unstable with respect to B_(12)As_2 and the pure arsenic phase at 0 K, and increasingly so at higher temperature, suggesting that BAs may merely exist as a metastable phase. On the contrary, in the binary B-P system, both zinc-blende BP and icosahedral B_(12)P_2 are predicted to be stable. As for the binary As-P system, As_(1−x)P_x disordered alloys are predicted at elevated temperature-for example, a disordered solid solution of up to ～75 at.% As in black phosphorus as well as a small solubility of ～1 at.% P in gray arsenic at T = 750 K, together with the presence ofmiscibility gaps. The calculated large solubility of As in black phosphorus explains the experimental syntheses of black- phosphorus-type As_(1−x)P_x alloys with tunable compositions, recently reported in the literature. We investigate the phase stabilities in the ternary B-As-P system and demonstrate a high tendency for a formation of alloys in the icosahedral B_(12)(As_(1−x)P_x )_2 structure by intermixing of As and P atoms at the diatomic chain sites. The phase diagram displays noticeable mutual solubility of the icosahedral subpnictides in each other even at room temperature as well as a closure of a pseudobinary miscibility gap around 900 K. As for pseudobinary BAs_(1−x)P_x alloys, only a tiny amount of BAs is predicted to be able to dissolve in BP to form the BAs_(1−x)P_x disordered alloys at elevated temperature. For example, less than 5% of BAs can dissolve in BP at T = 1000 K. The small solubility limit of BAs in BP is attributed to the thermodynamic instability of BAs with respect to B_(12)As_2 and As.