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.

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and β-As 2 Te 3 ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi 2 Te 3 , Bi 2 Se 3 , Sb 2 Te 3 , and β-As 2 Te 3 ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201904316.Chalcogenides are attracting considerable attention due to their striking properties. These characteristics enable a wide range of applications ranging from phase-change materials (PCMs) [1][2][3] to thermoelectrics [4] and topological insulators [5,6] for the heavier chalcogenides. The remarkable application potential has been attributed to an unconventional property portfolio. [2,[7][8][9] Adv. Mater. 2019, 31, 1904316

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.