Metallic alloys at the edge of complexity: structural aspects, chemical bonding and physical properties*

Abstract

Complex metallic alloys belong to the vast family of intermetallic compounds and are hallmarked by extremely large unit cells and, in many cases, extensive crystallographic disorder. Early studies of complex intermetallics were focusing on the elucidation of their crystal structures and classification of the underlying building principles. More recently, ab initio computational analysis and detailed examination of the physical properties have become feasible and opened new perspectives for these materials. The present review paper provides a summary of the literature data on the reported compositions with exceptional structural complexity and their properties, and highlights the factors leading to the emergence of their crystal structures and the methods of characterization and systematization of these compounds.

Binary Intermetallics in the 70 atom % R Region of Two R–Pd Systems (R = Tb and Er): Hidden, Obscured, or Nonexistent?

“…The so-called Dy 5 Pd 2 is a representative of complex intermetallics crystallizing in the cubic crystal system with a = 13.529(5) Å . The authors realized that the compound has a composition close to 5:2 but not exactly.…”

Variants of the X-phase in the Mn–Co–Ge system

“…Polyhedra for M3 are always in pairs (while others alternate), sharing one of the Ge atoms between them. Considering that Ge is the most electronegative atom in the present case [ Ge = 2.01, Co = 1.88 and Mn = 1.55, according to the Pauling scale (Pauling, 1932)] and that the M3-Ge distances [2.3543 (3), 2.3942 (3) and 2.3948 (2) Å ] are the shortest in the structure [for comparison, the M2-M2 distance is 2.3826 (4) Å ], this might be a reason why the position of M3 becomes unstable with the introduction of Mn (r Mn = 1.27 Å ; Pearson, 1972). The Co-Ge distances around M3 are in the range 2.3911 (4)-2.4564 (4) Å , which is similar to what is seen for M2, but the shortest Mn-Ge distance of 2.7233 (5) Å in the same area exhibits a sizeable difference [Mn-M2 = 2.6607 (4) Å ].…”

“…Topologically close packed (TCP) phases or Frank-Kasper phases as they are also known encompass a large number of intermetallic compounds (Dshemuchadse & Steurer, 2015;Ovchinnikov et al, 2020). As a result, TCP phases often appear in various widely used alloys such as steels.…”

“…When atoms of slightly different sizes pack together, TCPs can arise as they achieve a better packing by forming small non-uniform tetrahedral interstices. The non-uniform nature of the tetrahedral interstices allow for coordination numbers of 12, 14, 15 and 16 (Frank & Kasper, 1958;Wang & Mar, 2001;Ovchinnikov et al, 2020). The Valence Electron Concentration (VEC) also plays a role in determining the structure and stability of the TCP phase, so much so that maps based on the VEC and divergence from the average atomic size can be used to predict the various phases seen (Seiser et al, 2011;Hammerschmidt et al, 2013).…”

Spotlight on Alkali Metals: The Structural Chemistry of Alkali Metal Thallides

“…16. Thallides very rich in alkali metal are only reported for lithium and sodium and in general, a high content of alkali metal increases the complexity of the compounds [70,71]. Lithium compounds crystallize in known intermetallic structure types, interestingly they are isotypic to lithium tetrelides, The corresponding range of distances to the adjacent atoms as well as the Wyckoff letters are given in Table 16.…”