Interpenetrating Polar and Nonpolar Sublattices in Intermetallics: The NaCd₂Structure

Abstract

In the 1960s, Samson solved the structures of some of the most complicated intermetallic phases known, including those of NaCd(2), Mg(2)Al(3), and Cu(3)Cd(4) (each with over 1000 atoms per unit cell). Following remarkable earlier constructions by Samson and by Andersson, we use quantum-mechanical calculations as a guide to describing and understanding these structures. Our electronic Aufbau begins with the relatively simple Mg(17)Al(12) structure and works up to Samson's NaCd(2) structure. In both structures, a division of the sites into electron-rich and electron-poor (with respect to an average electron count) reveals MgCu(2)-type fragments. Between the interiors and exteriors of these fragments, a change in bonding character takes place-the interiors are more polar, the interfaces relatively nonpolar. This electronic situation is traced to the geometry of the interface sites; they lie simultaneously on electron-rich and electron-poor networks. The resulting polar and nonpolar sites in NaCd(2) are separated by a minimal surface, the D surface. The driving force for assuming this structure is electronic: NaCd(2) features an interpenetration of polar and nonpolar bonding regions. This sort of thinking can be applied to other structures.

Synthesis, Crystal Structure, and Magnetic Properties of Giant Unit Cell Intermetallics R117Co52+δSn112+γ (R = Y, La, Pr, Nd, Ho)

“…The great complexity of these crystal structures stems from the ability of constituent atoms to adopt a range of coordination numbers and a variety of coordination environments, which lead to rather poorly defined rules for the assembly of such structures [1]. Complex intermetallics are also of current interest for potential applications as thermoelectric materials, heterogeneous catalysts, and models for understanding self-organized nanoscale architectures [2,3].…”

Synergistic Geometrical and Electronic Features in the Intermetallic Phases Ca2AgM2, Ca2MgM2, and Ca2GaM2 (M = Pd, Pt)

“…Therefore, as our understanding of some phenomenon improves, its "complexity" may lessen and even turn out to be rather simple; meaning that complexity is a rather subjective issue. [1] For example, intermetallic phases with at least 1000 atoms per unit cell are referred to as complex metal alloys (CMA), but they can often be formulated simply as in the typical examples of NaCd 2 or β-Mg 2 Al 3 . [2,3] On the other hand, understanding the structuredirecting forces of even simple intermetallic structures like the orthorhombic W 2 CoB 2 type (space group Immm) [4] can be very challenging, and may eventually reveal astonishing complexity and/or diversity in bonding features and stability principles.…”

“…[1] For example, intermetallic phases with at least 1000 atoms per unit cell are referred to as complex metal alloys (CMA), but they can often be formulated simply as in the typical examples of NaCd 2 or β-Mg 2 Al 3 . [2,3] On the other hand, understanding the structuredirecting forces of even simple intermetallic structures like the orthorhombic W 2 CoB 2 type (space group Immm) [4] can be very challenging, and may eventually reveal astonishing complexity and/or diversity in bonding features and stability principles. Thus, more than 60 representatives of the W 2 CoB 2 -type family are reported, and are formed by very different element types and combinations, showing strikingly different bonding pictures with direct impact on their physical properties.…”

PRINCEPS: A Computer-Based Approach to the Structural Description and Recognition of Trends within Structural Databases, and Its Application to the Ce-Ni-Si System

“…However, a successful structural description is not always easy to achieve: while for certain materials, such as oxides and metal-organic frameworks, our chemical understanding is deep enough that their structural units can be easily identified, for other materials it can be far more challenging [4,5]. Among crystalline phases, the structures of intermetallic compounds are particularly difficult to analyze: they exhibit a vast and intriguing structural diversity [6,7], but the lack of a clear bonding picture for most phases creates ambiguities as one tries to make sense of this diversity through structural descriptions [8,9]. There exist many ways to connect the seemingly irregular array of atoms together, but chemical or physical insights are necessary for choosing among these paths to achieve a description.…”