An application of the "coloring problem": structure-composition-bonding relationships in the magnetocaloric materials LaFe13-xSix.
An application of the "coloring problem": structure-composition-bonding relationships in the magnetocaloric materials LaFe13-xSix.
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DOI：
10.1021/ic701311b
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发表时间：
2008-01
影响因子：
4.6
通讯作者：
Mi-Kyung Han;G. Miller
中科院分区：
文献类型：
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作者：
Mi-Kyung Han;G. Miller
对LaFe(13)-(x)Si(x)（1.0 <或= x <或= 5.0）系列进行了实验和理论研究，以获得对这些化合物中与磁热效应相关的几何结构，化学组成，磁性行为和物理性质之间关系的可能理解。随着Si浓度的增加，由于Fe和Si原子的优先排序，LaFe(13)-(x)Si(x)呈现出从立方NaZn（13）结构类型到四方导数的结构转变。在室温下，LaFe(13)-(x)Si(x)在1 < or = x < or = 2.6范围内结晶为立方结构，在3.2 < or = x < or = 5范围内结晶为四方结构。在2.6 < or = x < or = 3.2范围内，表示为两相混合物。在相应的居里温度附近对室温立方相进行了温度相关的单晶x射线衍射实验，以研究大等温磁熵变化的来源。对数据进行全面的统计和结构分析表明，非中心对称的F43c空间群比中心对称的Fm3c空间群提供了更充分的原子排列。这种空间群的变化导致了特定的Fe-Fe距离在居里温度以下的发散，这是由以铁为中心的[Fe(12)-(x)Si(x)]二十面体的倾斜引起的。非中心对称的空间群也符合缺乏局部逆对称的二十面体星团的优势。通过对各种模型结构的扩展h<s:1> ckel和紧密结合线性松饼锡轨道（tblmto）电子结构的计算，F43c模型比Fm3c模型在能量上更有利。还对各种二十面体[Fe(12)-(n)Si(n)] （n = 1-5）簇进行了扩展h<s:1> ckel计算，并对“LaFe（13）”、“LaFe(11)Si(2)”和“LaFe(9)Si(4)”进行了pb - lmto计算，以研究主族元素（Si）对稳定立方NaZn（13）型结构的影响，影响立方对称性和四方对称性之间的转变，并研究了它们的化学键和磁性之间的关系。
The LaFe(13)-(x)Si(x) (1.0 < or = x < or = 5.0) series is studied experimentally and theoretically to gain possible understanding for the relationships among geometrical structure, chemical composition, magnetic behavior, and physical properties as related to the magnetocaloric effect in these compounds. As the Si concentration increases, LaFe(13)-(x)Si(x) exhibits a structural transformation from the cubic NaZn(13) structure type to a tetragonal derivative due primarily to preferential ordering of Fe and Si atoms. At room temperature, LaFe(13)-(x)Si(x) crystallize in the cubic structure for the range 1 < or = x < or = 2.6 and in the tetragonal for 3.2 < or = x < or = 5. In the range 2.6 < or = x < or = 3.2, it shows a two-phase mixture. Temperature-dependent single-crystal X-ray diffraction experiments near the corresponding Curie temperatures were performed on the room-temperature cubic phases to examine the origin of the large isothermal magnetic entropy changes. A thorough statistical and structural analysis of the data indicates that the noncentrosymmetric F43c space group provides a more adequate atomic arrangement than the centrosymmetric Fm3c space group. This change in space group leads to divergence for specific sets of Fe-Fe distances below the Curie temperature that arises from tilting of Fe-centered [Fe(12)-(x)Si(x)] icosahedra. The noncentrosymmetric space group also agrees with the predominance of icosahedral clusters lacking local inversion symmetry. From extended Hückel and tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations on various model structures, the F43c model is more energetically favorable than the Fm3c model. Extended Hückel calculations on various icosahedral [Fe(12)-(n)Si(n)] (n = 1-5) clusters and TB-LMTO calculations on "LaFe(13)," LaFe(11)Si(2), and LaFe(9)Si(4) have also been carried out to study the effects of a main group element (Si) on stabilizing the cubic NaZn(13)-type structure, influencing the transformation between cubic and tetragonal symmetries, and to study relationships among their chemical bonding and magnetic properties.