First-principles investigation of mechanical properties, elastic anisotropy, and ultralow lattice thermal conductivities of ductile Mg–Bi alloys

Abstract

The crystal structures, mechanical properties, anisotropic properties, and lattice thermal conductivities of Mg–Bi alloys have been investigated by particle swarm optimization structure prediction method and first-principles calculations. Apart from the well-known stoichiometry of Mg3Bi2, the novel stable stoichiometry of Mg4Bi, as well as several metastable stoichiometries (i.e., Mg3Bi, Mg2Bi, and MgBi) are predicted. Meanwhile, the calculated phonon dispersion curves and elastic constants show that the above five Mg–Bi alloys are dynamically and mechanically stable, respectively. Moreover, the calculated mechanical properties indicate that the studied Mg–Bi alloys with various stoichiometries behave ductility. Simultaneously, they exhibit obvious anisotropic characters and the degree of anisotropy follows the order of MgBi > Mg2Bi > Mg3Bi > Mg4Bi > Mg3Bi2. More importantly, the minimum lattice thermal conductivities of MgBi and Mg2Bi are smaller than that of the well-known Mg3Bi2. The adsorption of O atom on the Mg3Bi (001) surface is more sensitive compared with other four Mg–Bi (001) surfaces.