A Classification of Covalent, Ionic, and Metallic Solids Based on the Electron Density
Paula Mori-Sa´nchez,†,‡ A. Martı´n Penda´s,‡ and Vı´ctor Luan˜a*,‡
Contribution from the Department of Chemistry, Duke UniVersity, Box 90354,
Durham, North Carolina 27708-0354, and UniVersidad de OViedo,
Departamento de Quı´mica Fı´sica y Analı´tica, E-33006-OViedo, Spain
Abstract:
The electron density of crystals contains all of the information required to complete a classification of their bonding types. We propose here a set of three different indexes, flatness, charge transfer, and molecularity, easily obtained from the experimental or theoretical electron density, which give rise to a classification in close resemblance to the classical van Arkel-Ketelaar diagrams.
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A detailed analysis of the electron density of many crystals is presented here to show that the classification of bonding properties, with resemblances to the classical van ArkelKetelaar diagram (VK), can be directly recovered from the electron density, with no explicit recourse to empirical or otherwise defined electronegativity scales.
The classification of compounds according to their chemical bonding type lies at the core of the chemistry language. About 50 years ago, van Arkel and Ketelaar1,2 proposed an outstanding classification of binary crystals along an ionic-covalentmetallic triangular diagram. In a modern description, the triangle is based on the relative electronegativity of their atomic components: the covalency-ionicity of the AB compound is provided by the |øA - øB| difference in electronegativity, whereas the average (øA + øB)/2 measures the metallicitycovalency character. The classical VK diagram has been revisited many times,3-6 analyzing the role of different electronegativity scales, including nonbinary compounds, or even adding a fourth vertex to the triangle to represent van der Waals bonding and other forms of molecular interactions. Most remarkably, Sproul5 has shown, on the basis of their statistical significance, that the many electronegativity scales proposed over the years provide different measurements of one and the same physical property.
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