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.
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
InSe has been known for over 100 years and recently attracted interest as a promising candidate for a variety of applications, such as solar cells, photodiodes, and phase-change memories. Despite the broad concern for possible uses, its polymorphism and structure are poorly characterized. By combining X-ray diffraction, transmission electron microscopy, and quantum-chemical calculations, we present here the crystal structures of two layered room-temperature polytypes: 3R and 2H InSe. Both polymorphs are stacking variants of the same Se-In-Se-In-Se layers comprising two coordination environments for the In atoms, one tetrahedral and one octahedral. By using chemical-bonding analysis, we look at the different In positions in α-InSe and compare them to those in the metastable β-phase.
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.
Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge4Se3Te, GaSe, and Sb2Te3 are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.
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