等价电子数组元Heusler合金Fe_2RuSi中的原子占位

对等价电子数组元Heusler合金Fe_2RuSi的原子占位、电子结构与磁性进行了理论与实验研究.第一性原理计算表明,虽然Fe_2RuSi中Fe,Ru均有8个价电子,但是Ru仍表现出强烈的占据A,C晶位倾向.基态总能最低的是Fe与Ru分别占据A,C晶位的XA结构,次低的是Fe,Ru在A,C位混乱占位的L2_1B结构,且两者能量差很小.这说明决定Heusler合金中过渡族原子占位的因素除价电子数以外还可能有原子半径和共价杂化作用等.态密度和差分电荷密度计算表明Heusler合金中主族元素与最近邻过渡族元素之间的p-d共价杂化对Heusler合金的占位有明显影响,在XA结构中Ru与Si和Fe(B)之间都存在明显的杂化作用,而在高能的L2_1结构中,Si与最近邻的Fe杂化作用相当弱.XRD测试表明在室温Fe_2RuSi存在A,C位之间的Fe-Ru反占位,形成了能量次高的L21B结构,这主要来自于混合熵对自由能的贡献及其引起的原子自发混乱占位.在5 K下Fe_2RuSi的饱和磁矩为4.87μB/f.u.,与计算值符合得相当好.

Site preference in isoelectronic Heusler alloy Fe2RuSi

The site preference, electronic structure, and magnetism of Heusler alloy Fe₂RuSi are investigated theoretically and experimentally. The magnetic and electronic properties of Heusler alloys are strongly related to the atomic ordering and site preference in them. Usually, the site preference of the transition metal elements is determined by the number of their valence electrons. However, the recent results suggest that some new possible factors such as atomic radius should also be considered. Here we compare the phase stabilities of several different atomic orderings like XA, L2₁, DO3, L2₁B in Fe₂RuSi, in which Fe and Ru atom have 8 valence electrons each, thus the influence of “number of their valence electrons” can be omitted. First-principles calculations suggest that Ru atom prefers entering sites A and C in the lattice. In ground state, the most stable structure is of XA type, in which Fe and Ru atoms occupy A and C sites, respectively and the second stable structure is L2₁B type, in which Fe and Ru atoms occupy A and C sites randomly. With Ru atom entering into the B site, the total energy increases rapidly. Thus there is still a strongly preferable occupation of Ru though Fe and Ru atom are isoelectronic. This confirms that the “valence electrons rule” may be not enough to determine the site preference of the transition metal element in Heusler alloy. The preferable occupation of Ru atom in Fe₂RuSi can be explained from the electronic structure. It is found that in the XA DOS, there is strong hybridization between the electrons of the nearest Ru and Si or Fe (B) atom. However, in the high energy L2₁ structure the hybridization between Ru and the nearest Fe (A, C) is weak, which reduces its phase stability. This is confirmed further by the charge density difference calculation. Single phase Fe₂RuSi with a lattice parameter of 5.79 Å is synthesized successfully. Comparing the superlattice reflections (111) and (200) in the experimental XRD pattern with those in the simulated patterns for different structures, we find that Fe₂RuSi crystallizes in L2₁B structure rather than the most stable XA one at room temperature, which mainly originates from the contribution of mixed entropy to the free energy, and its caused atomic disorder at high temperatures. This disorder can be retained during the cooling procedure, while it does not influence the conclusion that Ru atom prefers the (A, C) sites in Fe₂RuSi strongly. Finally, the saturation magnetization M_s at 5 K is 4.87 μ_B/f.u., which agrees well with the theoretical result. The large total magnetic moment mainly comes from the contributions of Fe, especially Fe magnetic moments on B sites.