Cation-Poor Complex Metallic Alloys in Ba(Eu)–Au–Al(Ga) Systems: Identifying the Keys that Control Structural Arrangements and Atom Distributions at the Atomic Level

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2015-01-01
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Steinberg, Simon
Mudryk, Yaroslav
Pecharsky, Vitalij
Miller, Gordon
Mudring, Anja
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Miller, Gordon
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Mudring, Anja
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Materials Science and Engineering
Materials engineers create new materials and improve existing materials. Everything is limited by the materials that are used to produce it. Materials engineers understand the relationship between the properties of a material and its internal structure — from the macro level down to the atomic level. The better the materials, the better the end result — it’s as simple as that.
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Ames National LaboratoryMaterials Science and EngineeringChemistry
Abstract

Four complex intermetallic compounds BaAuxGay (x = 1, y = 0.9) (I), BaAuxAly (x = 0.9, y = 0.6) (II), EuAu6.2Ga5.8 (III), and EuAu6.1Al5.9 (IV) have been synthesized, and their structures and homogeneity ranges have been determined by single crystal and powder X-ray diffraction. Whereas I and II originate from the NaZn13-type structure (cF104–112, Fmc), III (tP52, P4/nbm) is derived from the tetragonal Ce2Ni17Si9-type, and IV (oP104, Pbcm) crystallizes in a new orthorhombic structure type. Both I and II feature formally anionic networks with completely mixed site occupation by Au and triel (Tr = Al, Ga) atoms, while a successive decrease of local symmetry from the parental structures of I and II to III and, ultimately, to IV correlates with increasing separation of Au and Tr on individual crystallographic sites. Density functional theory-based calculations were employed to determine the crystallographic site preferences of Au and the respective triel element to elucidate reasons for the atom distribution (“coloring scheme”). Chemical bonding analyses for two different “EuAu6Tr6” models reveal maximization of the number of heteroatomic Au–Tr bonds as the driving force for atom organization. The Fermi levels fall in broad pseudogaps for both models allowing some electronic flexibility. Spin-polarized band structure calculations on the “EuAu6Tr6” models hint to singlet ground states for europium and long-range magnetic coupling for both EuAu6.2Ga5.8 (III) and EuAu6.1Al5.9 (IV). This is substantiated by experimental evidence because both compounds show nearly identical magnetic behavior with ferromagnetic transitions at TC = 6 K and net magnetic moments of 7.35 μB/f.u. at 2 K. The effective moments of 8.3 μB/f.u., determined from Curie–Weiss fits, point to divalent oxidation states for europium in both III and IV.

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Reprinted (adapted) with permission from Inorg. Chem., 2015, 54 (21), pp 10296–10308. Copyright 2015 American Chemical Society.

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Thu Jan 01 00:00:00 UTC 2015
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