Frontiers in Intermetallics

Abstract

Intermetallic compounds provide a rich and diverse resource to study the relationships among chemical composition, atomic structure, electronic structure and physical properties – all issues of importance of chemists. Since the majority of chemical elements are “metals,” then, even with thermodynamic constraints, the numbers, types, and complexity of intermetallic compounds are immense. What are intermetallic compounds? At first glance, the answer seems obvious: intermetallic compounds are chemical substances formed when two or more metallic elements combine with a definite composition. They typically have small heats of formation (exothermic by less than ca. 50 kJ/mole) when compared to salts and other valence solids, but frequently show finite ranges in composition for a single phase, called homogeneity widths. Some melt congruently without decomposition into different phases, while many decompose peritectically into a liquid phase and another solid with a different composition. Concerning their structures, the various metal constituents are usually ordered in different sub-lattices, each with its own distinct population of atoms. In this way, elements of differing sizes or electronegativities can combine to give new compounds. Often, the corresponding structures are distinct from those of the component elements, but this is not necessary. Among inorganic solids, intermetallics are the least understood concerning relationships among their chemical compositions, bonding, and properties. Unlike ionic and covalent compounds, it is virtually impossible to apply simple heuristic concepts to reliably deduce compositions, structural features, or the nature of the bonding from the elements involved – there are just no guiding rules, at present, in the search for new intermetallic compounds. This is mainly because some features of the metal–metal bond, such as the degrees of valence electron transfer and localization, vary strongly and almost continuously with composition and nature of the elements involved. Also, the complexities of intermetallic structures and bonding patterns often pose great challenges for establishing structure-property relationships that would immensely facilitate the design and discovery of advanced energy materials. In this special issue of the Zeitschrift für Anorganische und Allgemeine Chemie, we have assembled a diverse collection of emphasizing intermetallic compounds. The contributions range from novel synthetic strategies to obtain new compounds as well as to grow crystals suitable for specific measurements of physical properties, through complex structural problems solved by combinations of diffraction and electronic structure theory, to summarizing some important and novel applications for these materials. We hope you enjoy both the intrinsic beauty of these structures, especially regarding their symmetry, as well as their capacity to offer extraordinary insights into chemical bonding. Thomas Fässler Technische Universität München Gordon J. Miller Iowa State University