First-principles study of magnetic disordering and alloying effects on phase stability and elastic constants of Co₂Cr<i>Z</i> (<i>Z</i> = Ga, Si, Ge) alloys

Abstract

Using the exact Muffin-Tin orbital method combined with the coherent potential approximation, the effects of magnetic disordering and alloying effects on the phase stability of L2₁- and D0₂₂-Co₂Cr<i>Z</i> (<i>Z</i> = Ga, Si, Ge) alloys are systematically investigated at 0 K in the present work. It is shown that at 0 K, the lattice parameter, bulk modulus, magnetic moments, and elastic constants of the studied L2₁ alloys are in line with the available theoretical and experimental data. In the ferromagnetic state, these alloys possess L2₁ structure; with the magnetic disordering degree (<i>y</i>) increasing, the energy of the phase increases relatively and finally turns from lower than D0₂₂ phase to higher than D0₂₂ phase. As a result, when <i>y</i> ≥ 0.1 (0.2), then <i>Z</i> = Si and Ge (<i>Z</i> = Ga) alloys are stabilized by the D0₂₂ phase. With <i>y</i> increasing, the tetragonal shear elastic modulus (<i>C</i><i>'</i> = (<i>C</i>₁₁–<i>C</i>₁₂)/2) also turns soft, indicating that the magnetic disorderingis conducive to the lattice tetragonal deformation in the three alloys from both the energetic view and the mechanical view. The electronic origination of the magnetic disordering effect on the stabilities of the L2₁ and D0₂₂ phases can be ascribed to the Jahn-Teller instability effect. In the FM L2₁-Co₂CrGa_1–<i>x</i>Si_<i>x</i> and L2₁-Co₂CrGa_1–<i>x</i>Ge_<i>x</i> quaternary alloys, with <i>x</i> increasing, the total magnetic moment increases monotonically according to the Slater-Pauling rule, and <i>C</i><i>'</i> also stiffens, reflecting that the adding of Si and Ge can promote the mechanical stability of L2₁-Co₂CrGa alloy, thereby depressing the lattice tetragonal deformation.