The interplay between Peierls distortions and metavalent bonding in IV–VI compounds: comparing GeTe with related monochalcogenides

In this article, we revisit bonding in crystalline GeTe, a simple binary alloy that is also a popular phase change material, and use an ab initio approach that goes beyond the usual one electron description obtained with density functional theory. By considering the electron pair density, we obtain a measure of the number of pairs of electrons that are shared between neighbors. Employing the charge transfer between adjacent atoms as the second quantifier of chemical bonding, we obtain a map which separates ionic, covalent and metallic bonding. Interestingly, GeTe is not located in any of these regions, but instead is located in a region where materials with a peculiar set of properties prevails. The corresponding materials have been coined incipient metals and their bonding 'metavalent bonding' (MVB). They often possess a Peierls distortion, which stabilizes the rhombohedral crystal structure by breaking the cubic symmetry. For these materials, the electron population of longer and shorter bonds is close to one-half, and charge transfer between adjacent atoms is quasi-independent of the degree of distortion. The energy gained by the Peierls distortion is much smaller than the energy gained by creating the cubic structure, delocalizing one electron over two bonds. Such Peierls distortions are not observed for aromatic compounds which utilize resonant bonding and have properties which differ significantly from the property portfolio of metavalently bonded materials. This stresses the difference between metavalent bonding and the resonant valence bond view of aromatic compounds and molecules. MVB is also responsible for the anomalies in dielectric properties and the anharmonicity of the solids. The comparison between PbTe, GeTe and GeS is particularly instructive, showing that bonding in these materials shows interesting differences, where metavalent bonds govern the behavior of PbTe and GeTe, while GeS is dominated by the Peierls distortion. (paper)