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.
We present an update on recently developed methodology and functionality in the computer program Local Orbital Basis Suite Toward Electronic‐Structure Reconstruction (LOBSTER) for chemical‐bonding ...analysis in periodic systems. LOBSTER is based on an analytic projection from projector‐augmented wave (PAW) density‐functional theory (DFT) computations (Maintz et al., J. Comput. Chem. 2013, 34, 2557), reconstructing chemical information in terms of local, auxiliary atomic orbitals and thereby opening the output of PAW‐based DFT codes to chemical interpretation. We demonstrate how LOBSTER has been improved by taking into account time‐reversal symmetry, thereby speeding up the DFT and LOBSTER calculations by a factor of 2. Over the recent years, the functionalities have also been continually expanded, including accurate projected densities of states (DOSs), crystal orbital Hamilton population (COHP) analysis, atomic and orbital charges, gross populations, and the recently introduced k‐dependent COHP. The software is offered free‐of‐charge for non‐commercial research.
The computer program LOBSTER for chemical‐bonding analysis in periodic systems has been updated. LOBSTER extracts local chemical information from PAW‐based DFT codes and now takes into account time‐reversal symmetry, thereby speeding up both DFT and LOBSTER calculations by a factor of 2. Besides accurate local DOS and COHP analysis, it also delivers atomic and orbital charges directly and rapidly from the wavefunction, in addition to the recently introduced k‐dependent COHP.
The crystal orbital bond index (COBI) is a new and intuitive method for quantifying covalent bonding in solid-state materials. COBI is based on the bond index by Wiberg and Mayer and extends their ...ideas to the case of translationally invariant objects, that is, crystalline matter. COBI’s qualitative interpretation resembles the well-established crystal orbital overlap population and crystal orbital Hamilton population methods but should be more familiar to chemists since it directly relates to the classical bond order. In contrast to the aforementioned descriptors, COBI also allows for examining multicenter interactions within a local-orbital framework. As an additional bonding indicator, we refer to the Ewald sum for electrostatic lattice potentials, thereby enabling the calculation of electrostatic lattice energies as well as site potentials from quantum-mechanical charges as directly derived from the wave function, not from the density.
Chemically understanding the electronic structure of a given material provides valuable information about its chemical as well as physical nature and, hence, is the key to designing materials with ...desired properties. For example, to rationalize the structures of solid-state materials in terms of the valence-electron distribution, highly schematic, essentially non-quantum-mechanical electron-partitioning models such as the Zintl-Klemm concept have been introduced by assuming idealized ionic charges. To go beyond the limits of the aforementioned concept, a Mulliken and Löwdin population analytical tool has been developed to accurately calculate the charges in solid-state materials solely from first-principles plane-wave-based computations. This population analysis tool, which has been implemented into the LOBSTER code, has been applied to diverse solid-state materials including polar intermetallics to prove its capability, including quick access to Madelung energies. In addition, a former weakness of the population analysis (namely, the basis-set dependency) no longer exists for the present approach which therefore represents a comparatively fast and accurate wave-function-based alternative for plane-wave calculations for which density-based charge approaches (
e.g.
, Bader like) have been very popular.
A robust tool to extract Mulliken and Löwdin charges for (extended) solids from plane waves has been developed and applied.
Simple, yet predictive bonding models are essential achievements of chemistry. In the solid state, in particular, they often appear in the form of visual bonding indicators. Because the latter ...require the crystal orbitals to be constructed from local basis sets, the application of the most popular density-functional theory codes (namely, those based on plane waves and pseudopotentials) appears as being ill-fitted to retrieve the chemical bonding information. In this paper, we describe a way to re-extract Hamilton-weighted populations from plane-wave electronic-structure calculations to develop a tool analogous to the familiar crystal orbital Hamilton population (COHP) method. We derive the new technique, dubbed “projected COHP” (pCOHP), and demonstrate its viability using examples of covalent, ionic, and metallic crystals (diamond, GaAs, CsCl, and Na). For the first time, this chemical bonding information is directly extracted from the results of plane-wave calculations.
Laser‐assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant ...differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser‐assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.
Bond breaking is characterized by laser‐assisted field evaporation in atom probe tomography. This reveals different bond breaking mechanisms, where large numbers of fragments are only observed for crystalline phase change materials (PCMs). This finding is related to the recently conjectured mechanism of metavalent bonding. Instead, amorphous PCMs, other covalently bonded compounds, and metals form much fewer fragments.
Nonvolatile phase-change memory has been successfully commercialized, but further density scaling below 10 nanometers requires compositionally and structurally homogeneous materials for both the ...memory cell and the associated vertically stacked two-terminal access switch. The selector switches are mostly amorphous-chalcogenide Ovonic threshold switches (OTSs), operating with a nonlinear current response above a threshold voltage in the amorphous state. However, they currently suffer from the chemical complexity introduced by the quaternary or even more diverse chalcogenide compositions used. We present a single-element tellurium (Te) volatile switch with a large (≥11 megaamperes per square centimeter) drive current density, ~10
ON/OFF current ratio, and faster than 20 nanosecond switching speed. The low OFF current arises from the existence of a ~0.95–electron volt Schottky barrier at the Te–electrode interface, whereas a transient, voltage pulse–induced crystal-liquid melting transition of the pure Te leads to a high ON current. Our discovery of a single-element electrical switch may help realize denser memory chips.
Phase‐change materials (PCMs) are widely used for data storage and in other functional devices. Despite their seemingly simple compositions, these materials exhibit intriguing microscopic complexity ...and a portfolio of interesting properties. In this Feature Article, it is shown that structural and electronic peculiarities on the atomic scale are key determinants for the technological success of PCMs. Particular emphasis is put on the interplay of different experimental and theoretical methods, on the bonding nature of crystalline and amorphous PCMs, and on the role of surfaces and nanostructures. Then, unconventional transport properties of the crystalline phases are highlighted, both with regard to electrical and heat conduction. Finally, perspectives and future directions are drawn: for finding new PCMs based on microscopic understanding, and also for new applications of these materials in emerging fields.
The property contrast of phase‐change materials (PCMs), used to encode “ones” and “zeroes” in digital memories, originates on the atomic scale. This Feature Article reviews unconventional structural, bonding, and transport properties of seemingly simple PCMs. This intriguing microscopic complexity can be exploited for new applications in data storage and beyond.
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