Ga2基Heusler合金Ga2XCr(X = Mn,Fe, Co,Ni, Cu)的四方畸变、电子结构、磁性及声子谱的第一性原理计算

运用基于密度泛函理论的第一性原理的方法, 对Ga₂基Heusler合金Ga₂XCr (X = Mn, Fe, Co, Ni, Cu)的四方畸变、电子结构、磁性及声子谱特性进行了系统的研究. 结果表明, 在保持体积不变的四方畸变中, 五种合金的磁矩主要由Cr元素提供; Ga₂FeCr, Ga₂CoCr和Ga₂CuCr保持稳定的立方相, 而在Ga₂MnCr和Ga₂NiCr 中观察到能量更低的四方相, 且其能量最低点对应的c/a分别位于1.28和1.11处, 而对应的能量差ΔE 分别为-8.26 meV和-6.14 meV. 电子结构显示, Ga₂MnCr和Ga₂ NiCr的费米能级附近存在尖锐的电子态密度峰, 导致3d电子能级杂化向宽能量范围扩展, 以消除体系的高能量不稳态, 这个过程导致结构转变的发生. 基于适度的畸变度和能量差, 本文认为Ga₂MnCr有存在铁磁马氏体相变的可能, 其声学支虚频的出现, 也进一步表明体系有声子模软化的行为.

First-principles investigations on tetragonal distortion,electronic structure,magnetism, and phonon dispersion of Ga2XCr (X = Mn,Fe, Co,Ni, Cu) Heusler alloys

In Ga₂-based Heusler alloys Ga₂XCr (X = Mn, Fe, Co, Ni, Cu) the tetragonal distortion, electronic structure, magnetism and phonon dispersion have been studied by first-principles calculations based on the density functional theory. The volume-conserving tetragonal distortions of the cubic Ga₂XCr show that Cr atom makes the greatest contribution to the total magnetic moment. No martensitic transformation has been found in Ga₂FeCr, Ga₂CoCr and Ga₂CuCr. For both Ga₂MnCr and Ga₂NiCr, the tetragonal phase is lower in energy as compared with the cubic phase. Ga₂MnCr and Ga₂NiCr have the lowest total energy at c/a = 1.28 and 1.11, respectively. Correspondingly, the energy difference ΔE between the cubic and the tetragonal phase is -8.26 meV in Ga₂MnCr and -6.14 meV in Ga₂NiCr. For Ga₂MnCr and Ga₂NiCr, calculations of electronic structure and phonon dispersion reveal that a sharp peak near the Fermi level will lead to a structural instability by increasing the energy of the system, which can result in a broadening in the energy range due to hybridizations between 3d electrons as well as the potential structural transformation. With proper c/a and ΔE a potential tetragonal martensitic transformation can be expected in Ga₂MnCr, the phonon dispersion of which further shows that the acoustic modes tend to be softened.