摘要:
运用基于密度泛函理论的投影缀加波方法研究了Heusler合金Mn2NiGa的四方变形, 对立方和四方结构的磁矩、电子结构、弹性常数及声子谱进行了计算和分析. Mn原子是Mn2NiGa总磁矩的主要贡献者, 但Mn（A）、Mn（B）原子磁矩的值不等且呈反平行耦合, 因而Mn2NiGa合金在两种状态下均表现为亚铁磁结构. 四方变形中, Mn2NiGa在c/a=0.94和c/a=1.27处出现总能的局域极小值和局域最小值, 分别对应一个稳定的马氏体. 弹性常数的计算结果显示, Mn2NiGa的立方结构不满足立方相稳定性判据, 四方结构（c/a=0.94和c/a=1.27）的弹性常数满足相应的稳定性判据. 立方结构声子谱中存在虚频, 而四方结构（c/a=0.94和c/a=1.27）则不存在虚频, 验证了Mn2NiGa四方结构比立方结构稳定. c/a=1.27的四方结构Mn2NiGa转变为c/a=1.0的立方结构的相变温度在315 K左右.
Abstract:
Tetragonal distortion of Mn2NiGa Heusler alloy is calculated by first-principles based on density functional theory with projector augmented wave pseudopotential, and the magnetism, electronic structure, elastic constants and phonon frequencies are also calculated and analyzed. The contribution of the spin magnetic moments of Mn atom to the total moment is largest for Mn2NiGa, and the Mn2NiGa alloy shows ferrimagnetism in these two cases, owning to the antiparallel but unbalanced magnetic moments of Mn (A) atom and Mn (B) atom. Analysis of tetragonal distortion shows that there is a local minimum total energy at c/a=0.94 and c/a=1.27, which corresponds to a stable martensitic phase. Elastic constants of Mn2NiGa reveal that cubic structure does not satisfy stability conditions, but tetragonal structure (c/a=0.94 and c/a=1.27) does. The imaginary values of phonon frequencies in cubic structures validate that tetragonal structure (c/a=0.94 and c/a=1.27) of Mn2NiGa is more stable than cubic structure. The phase transition temperature of c/a=1.27 tetragonal structure converting to cubic structure is about 315 K.
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