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

Abstract

Four complex intermetallic compounds BaAu(6±x)Ga(6±y) (x = 1, y = 0.9) (I), BaAu(6±x)Al(6±y) (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, Fm3̅c), 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.

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

“…Because of the complexity of the crystal structures showing defects such as disorders of vacancies, some components of this group can also be assigned to the broad family of complex metallic alloys [108,109]. For instance, the crystal structures of EuAu 6.1 Al 5.9 and EuAu 6.2 Ga 5.8 are both derived from the NaZn 13 -type and include icosahedra, tetrahedral stars, and europium-centered snub cubes formed by the gold and post-transition-metal atoms ( Figure 5) [99]. An examination of the COHP curves and ICOHP values for models approximating the real crystal structures of these intermetallics demonstrates that the driving force stabilizing these materials stems from the maximization of the amounts of the heteroatomic Au−post-transition-metal contacts.…”

Revealing the Nature of Chemical Bonding in an ALn2Ag3Te5-Type Alkaline-Metal (A) Lanthanide (Ln) Silver Telluride

“…To provide an insight into the bonding situation in RbPr2Ag3Te5, we followed up with an examination of the crystal orbital Hamilton population curves (COHP; Figure 2) and their respective integrated values (Table 3). In this context, the cumulative -ICOHP/cell values, i.e., the sums of the negative ICOHP/bond values of all nearest neighboring contacts within one unit cell, were projected as percentages of the net bonding capabilities for each sort of interaction to identify roles of the diverse interactions in overall bonding-a procedure that has been largely employed elsewhere [21,22,[33][34][35][36]. (Figure 2) reveals that the occupied states close to the Fermi level, E F , originate to a large extent from the Ag-d and Te-p atomic orbitals beside minor contributions from the Pr-d states.…”

“…The following orbitals were employed as basis sets in the computations (downfolded [63] orbitals in parentheses): Rb-5s/(-5p)/(-4d)/(-4f ); Pr-6s/(-6p)/-5d; Ag-5s/-5p/-4d/(-4f ); Te-5s/-5p/(-5d)/(-4f ) with the corresponding WS radii (Å): Rb, 4.73; Pr, 3.58; Ag, 2.86-2.89; Te, 3.24-3.35. The praseodymium 4f states were treated as core-like states-a procedure that has been widely used elsewhere [34,35,64,65]. More specifically, the aforementioned handling of the rare-earth-metal 4f states has been substantiated by previous explorations [34,35,[65][66][67] on the electronic structures of rare-earth-containing compounds.…”

“…The praseodymium 4f states were treated as core-like states-a procedure that has been widely used elsewhere [34,35,64,65]. More specifically, the aforementioned handling of the rare-earth-metal 4f states has been substantiated by previous explorations [34,35,[65][66][67] on the electronic structures of rare-earth-containing compounds. Namely, the bands corresponding to the rare-earth-metal 4f atomic orbitals are typically related to modest dispersions and, hence, are rather localized such that these states scarcely participate in overall bonding.…”

On Ternary Intermetallic Aurides: CaAu2Al2, SrAu2–xAl2+x and Ba3Au5+xAl6–x

“…These are e.g. CaAg 2.2 Al 2.8 and SrAg 2.5 Al 2.5 (CaCu 5 type), CaAu 3 Al 7 (ScRh 3 Si 7 type), the NaZn 13 type representatives SrAg 7.36 Al 5.64 , SrAg 5.5 Al 7.5 , BaAg 5.59 Al 7.41 , and BaAu 6.86 Al 6.14 , or Ba 2 Au 6 Zn 3 type representatives Sr 2 Au 6 Al 3 and Sr 2 Au 6.18 Al 2.82 . Also the ThCr 2 Si 2 type representatives CaAg 0.7 Al 3.3 , CaAu 0.8 Al 3.2 , SrAg 0.8 Al 3.2 , SrAuAl 3 , and CaAu 0.9 Al 3.1 or the CaBe 2 Ge 2 type SrAu 2 Al 2 as well as the modulated SrPt 2 Al 2 (also CaBe 2 Ge 2 type) have been reported.…”