A Quantum‐Mechanical Map for Bonding and Properties in Solids

Abstract

solids; a "semiconductor" has a narrow bandgap across which electrons can be excited by light; these classifications are therefore based on measurable, macroscopic properties. A fingerprint of five coexisting identifiers was recently used to define a concept termed "metavalent" bonding (MVB): [4,5] metavalent solids show i) moderate electronic conductivity (≈10 2 -10 4 S cm −1 ); ii) increased coordination numbers incompatible with the (8-N) rule for semiconductors; iii) large optical dielectric constants, ε ∞ ; iv) large bond polarizability, as measured by Born effective charges, Z*; and v) large lattice anharmonicity, as measured by the Grüneisen parameter, |γ TO |. In terms of conductivity and coordination numbers, metavalent solids are therefore located between the covalent and metallic regimes-but they are distinctly different from both because they show anomalously large response properties [5] and a unique bond-breaking mechanism [4] not observed in either covalent or metallic solids. This definition based on a set of observable properties directly led to a revision of the "resonant bonding" model (which had previously been widely used to describe the bonding in PCMs [6] ) by showing that the response properties of PCMs are fundamentally different from those of resonantly bonded benzene and graphite. [5] A 2D map is created for solid-state materials based on a quantum-mechanical description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary compounds; furthermore, it highlights a distinct region for a mechanism recently termed "metavalent" bonding. Then, it is shown how this materials map can be extended in the third dimension by including physical properties of application interest. Finally, it is shown how the map coordinates yield new insight into the nature of the Peierls distortion in phase-change materials and thermoelectrics. These findings and conceptual approaches provide a novel avenue to tailor material properties. Materials DesignThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

“…The numerical values for the different entries are collected in Table S1 (Supporting Information) and in ref. .…”

“…[63] Hence, the depopulation of antibonding states is caused by cation vacancy formation, which creates holes in the valence band and hence p-type conductivity, Table S1 (Supporting Information) and in ref. [50].…”

“…They also possess a property combination which is rather exceptional and is found in none of the materials employing covalent, ionic, or metallic bonding. This unconventional property combination has recently been attributed to a special bonding mechanism, coined metavalent bonding . Yet, for the present purpose it is not relevant if the bonding in chalcogenides is a novel bonding mechanism, or a strange mixture of known bonding mechanisms.…”

“…This unconventional property combination has recently been attributed to a special bonding mechanism, coined metavalent bonding. [49,50] Yet, for the present purpose it is not relevant if the bonding in chalcogenides is a novel bonding mechanism, or a strange mixture of known bonding mechanisms. Instead, two questions are crucial to identify novel thermoelectrics with superior performance based on chalcogenides.…”

Expression and interactions of stereochemically active lone pairs and their relation to structural distortions and thermal conductivity

“…44 Similarly, it was recently shown that increased lone pair expression along the Peierls distortion in GeTe is associated with more covalent bonding and a more rigid electronic structure evidenced by a decrease in response properties such as the Born effective charge. 45 The effect of atomic displacement on the electronic structure may give rise to anharmonic vibrations, e.g.…”