Abstract:Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the Crystal Orbital Hamilton Population (COHP) method has been established as an efficient and reliable tool to extract the chemical-bonding information based on electronic-structure calculations of various quantum-chemical types. In this review, we present a brief introduction into the theoretical background of the COHP method and illustrate the latter by diverse applications, in particular by looking at representatives of the class of (polar) intermetallic compounds, usually considered as "black sheep" in the light of valence-electron counting schemes.

A robust tool to extract Mulliken and Löwdin charges for (extended) solids from plane waves has been developed and applied.

Understanding chemical bonding is of significant interest since it allows us to comprehend and tailor certain material properties, [1,2] which could be utilized, e.g., to optimize phase-change materials (PCMs) [3-7] or thermoelectrics. [8,9] The first steps to understand the nature of the chemical bond were already taken almost a century ago by Linus Pauling [10] and others. [11,12] In the meantime, enormous developments have taken place in both, quantum-mechanical and experimental techniques, [13-15] which help us to explore chemical bonding with unprecedented detail. Recently, these advances have also led to the concept of metavalent bonding (MVB), describing a bonding mechanism in between electron delocalization (i.e., metallic bonding) and electron localization at the ion cores (i.e., ionic bonding) as well as within the interatomic region (i.e., covalent bonding). [16-18] Metavalent bonding has been categorized by combining both quantummechanical and experimentally accessible bonding descriptors. [16-18] The Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O), a combination of property-, bond-breaking-, and quantummechanical bonding descriptors are applied. The outcome of the explorations reveals an electron-transfer-driven transition from metavalent bonding in PbX (X = Te, Se, S) to iono-covalent bonding in β-PbO. Metavalent bonding is characterized by adjacent atoms being held together by sharing about a single electron (ES ≈ 1) and small electron transfer (ET). The transition from metavalent to iono-covalent bonding manifests itself in clear changes in these quantum-mechanical descriptors (ES and ET), as well as in property-based descriptors (i.e., Born effective charge (Z*), dielectric function ε(ω), effective coordination number (ECoN), and mode-specific Grüneisen parameter (γ TO)), and in bond-breaking descriptors. Metavalent bonding collapses if significant charge localization occurs at the ion cores (ET) and/or in the interatomic region (ES). Predominantly changing the degree of electron transfer opens possibilities to tailor material properties such as the chemical bond (Z*) and electronic (ε ∞) polarizability, optical bandgap, and optical interband transitions characterized by ε 2 (ω). Hence, the insights gained from this study highlight the technological relevance of the concept of metavalent bonding and its potential for materials design.

A hexagonal phase in the ternary Ge-Se-Te system with an approximate composition of GeSe Te has been known since the 1960s but its structure has remained unknown. We have succeeded in growing single crystals by chemical transport as a prerequisite to solve and refine the Ge Se Te structure. It consists of layers that are held together by van der Waals type weak chalcogenide-chalcogenide interactions but also display unexpected Ge-Ge contacts, as confirmed by electron microscopy analysis. The nature of the electronic structure of Ge Se Te was characterized by chemical bonding analysis, in particular by the newly introduced density of energy (DOE) function. The Ge-Ge bonding interactions serve to hold electrons that would otherwise go into antibonding Ge-Te contacts.

The combination of laser‐heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high‐temperature high‐pressure chemical reactions of iron and iron nitride Fe2N with nitrogen. At pressures between 10 and 45 GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300 K. Subsequent in situ X‐ray diffraction reveals a new modification of FeN with NiAs‐type crystal structure, as also rationalized by first‐principles total‐energy and chemical‐bonding studies.