Physical properties of osmium dinitride with fluorite, pyrite, marcasite, and monoclinic structures under high pressure: First-principles study

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The structure, phase transition, elastic, and thermodynamic properties of OsN2 have been studied via first-principles calculations. It is shown that the CoSb2 structure is more stable than other structures. By the calculated H-P relations at 0 K, we found that the phase transition of OsN2 from CoSb2 structure to marcasite structure (ε → δ) occurs at 16.8 GPa, while the phase transition pressure between pyrite structure and fluorite structure (γ → α) is 80 GPa. The results of obtained phase transitions are also confirmed by bond length, sound velocity, and thermal expansion coefficient under different pressures. The pressure dependences of the elastic constants, mechanical stability, and mechanical anisotropy of four structures of OsN2 have been investigated by finding that the fluorite (pyrite and marcasite) structure OsN2 is mechanically stable under hydrostatic pressure (up to 60 GPa). However, the monoclinic structure is mechanically unstable under pressure from 0 to 60 Gpa. The calculated elastic anisotropic factors show that OsN2 possesses high elastic anisotropy under pressure. Moreover, the calculations on total density of states show that OsN2 of different structures has a metallic character, in agreement with previous theoretical results. The thermodynamic properties and sound velocity under diverse pressures of OsN2 of the four structures have been also investigated successfully.