Co过量影响Co_(2)NiGa合金原子占位及相稳定性的第一性原理计算

First-principles calculations of Co excess effects on atomic occupation and phase stability of Co_(2)NiGa alloys

掌握Co_(2)NiGa磁形状记忆合金的原子占位、结构相变和磁性质是设计高性能Co-Ni-Ga磁形状记忆合金的关键。采用精确的Muffin-Tin轨道结合相干势近似方法,系统计算了0 K下Co含量对Co_(2+x)Ni_(1-x)Ga和Co_(2+x)NiGa_(1-x)(x=0,0.05和0.10)合金立方相的晶格结构、原子占位、晶格参数、磁矩以及四方晶格变化的影响规律和物理机理。研究结果表明,2种非化学计量比合金在立方相均具有X A稳定结构,且过量的Co原子分别占据在不足的Ni和Ga原子位置上;Co取代Ni时不利于Co_(2)NiGa合金发生马氏体相变,而Co取代Ga时有利于Co 2NiGa合金发生马氏体相变;2种合金的总磁矩主要来源于Co原子,且随四方晶格的增加均发生非单调变化,四方相的总磁矩相比立方相略低0.2μB左右。上述理论结果有望为Co_(2)NiGa合金性能的进一步优化提供理论参考。

Knowledge of the atomic occupation,structural phase transitions and magnetic properties of Co-Ni-Ga magnetic shape memory alloys is the key to designing high performance Co_(2)NiGa magnetic shape memory alloys.In this paper,the effect of Co content on the lattice structure,atomic occupation,lattice parameters,magnetic moments and tetragonal lattice changes of Co_(2+x)Ni_(1-x)Ga and Co_(2+x)NiGa_(1-x)(x=0,0.05 and 0.10)alloys in the cubic phase at 0 K was studied systematically by using the exact Muffin-Tin orbitals combined with the coherent potential approximation method.The results show that both non-stoichiometric alloys have X A stable structures in the cubic phase,and the excess Co atoms occupy the sites of the deficient Ni and Ga atoms,respectively.When Co substitutes for Ni,it is not conducive to the martensitic transformation of Co_(2)NiGa alloy,however,and when Co substitutes for Ga,it is conducive to the martensitic transformation of Co_(2)NiGa alloy.The total magnetic moments of the two alloys are mainly derived from the Co atoms.The total magnetic moments of the two alloys are mainly derived from the Co atoms,and a non-monotonic trend occurs with the increase of the tetragonal lattice in both alloys.The total magnetic moment of the tetragonal phase is slightly lower compared to the cubic phase by about 0.2μB or so.The above theoretical results are expected to provide a theoretical reference for further optimization of the properties of Co_(2)NiGa alloy.