Searching for the Interlayer Band and Unravelling the Bonding in β-ThSi2 and α-ThSi2 with NMTO Wannier-like Functions

Metavalent bonding in chalcogenides: DFT-chemical pressure approach

Understanding the chemical bond nature has attracted considerable attention as it is crucial to analyze and comprehend the different physical and chemical properties of materials. This work is considered a complementary part of our previous work in studying the nature of different types of bonding interactions in a wide variety of molecules and materials using the DFT Chemical Pressure (CP) approach. Recently, a new type of chemical bond, the metavalent bond (MVB), has been defined. We show how the CP formalism can be used to analyze and study the establishment of MVB in two chalcogenides, GeSe and PbSe, in a similar fashion as the electron localization function (ELF) profiles. This is accomplished by analyzing the CP maps of these two chalcogenides at different pressures (up to 40 GPa for GeSe and 10 GPa for PbSe). The CP maps show distinctive features related to the MVB, providing insights into the existence of such chemical interaction in the crystal structure of the two compounds. Similar to ELF profiles, CP maps can visualize and track the strength of the MVB in GeSe and PbSe under pressure.

Chemical Pressure Maps of Molecules and Materials: Merging the Visual and Physical in Bonding Analysis

The characterization of bonding interactions in molecules and materials is one of the major applications of quantum mechanical calculations. Numerous schemes have been devised to identify and visualize chemical bonds, including the electron localization function, quantum theory of atoms in molecules, and natural bond orbital analysis, whereas the energetics of bond formation are generally analyzed in qualitative terms through various forms of energy partitioning schemes. In this Article, we illustrate how the chemical pressure (CP) approach recently developed for analyzing atomic size effects in solid state compounds provides a basis for merging these two approaches, in which bonds are revealed through the forces of attraction and repulsion acting between the atoms. Using a series of model systems that include simple molecules (H₂, CO₂, and S₈), extended structures (graphene and diamond), and systems exhibiting intermolecular interactions (ice and graphite), as well as simple representatives of metallic and ionic bonding (Na and NaH, respectively), we show how CP maps can differentiate a range of bonding phenomena. The approach also allows for the partitioning of the potential and kinetic contributions to the interatomic interactions, yielding schemes that capture the physical model for the chemical bond offered by Ruedenberg and co-workers.

Theoretical predictions of novel potassium chloride phases under pressure

Above 350 GPa KCl assumes an hcp lattice that is reminiscent of the isoelectronic noble gas Ar.

Compressed cesium polyhydrides: Cs+ sublattices and H3(-) three-connected nets

The cesium polyhydrides (CsH(n), n > 1) are predicted to become stable, with respect to decomposition into CsH and H2, at pressures as low as 2 GPa. The CsH3 stoichiometry is found to have the lowest enthalpy of formation from CsH and H2 between 30 and 200 GPa. Evolutionary algorithms predict five distinct, mechanically stable, nearly isoenthalpic CsH3 phases consisting of H3(–) molecules and Cs+ atoms. The H3(–) sublattices in two of these adopt a hexagonal three-connected net; in the other three the net is twisted, like the silicon sublattice in the α-ThSi2 structure. The former emerge as being metallic below 100 GPa in our screened hybrid density functional theory calculations, whereas the latter remain insulating up to pressures greater than 250 GPa. The Cs+ cations in the most-stable I4(1)/amd CsH3 phase adopt the positions of the Cs atoms in Cs-IV, and the H3(–) molecules are found in the (interstitial) regions which display a maximum in the electron density.

Rubidium polyhydrides under pressure: emergence of the linear H3(-) species

The structures of compressed rubidium polyhydrides, RbH(n) with n>1, and their evolution under pressure are studied using density functional theory calculations. These phases, which start to stabilize at only P = 2 GPa, consist of Rb(+) cations and one or more of the following species: H(-) anions, H(2) molecules, and H(3)(-) molecules. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH(5) and RbH(3) , which metallize above 200 GPa. At the highest pressures studied, our evolutionary searches find an RbH(6) phase which contains polymeric (H(3)(-))(∞) chains that show signs of one-dimensional liquid-like behavior.