Theoretical Study on the Electronic Structure of Heavy Alkali-Metal Suboxides

Abstract

On the metal-rich side of the phase diagrams of the Rb–O, Cs–O, and Rb–Cs–O systems, one can find a variety of stoichiometries: for example, Rb9O2, Rb6O, Cs4O, Cs7O, Cs11O3, RbCs11O3, and Rb7Cs11O3. They may be termed heavy alkali-metal suboxides. The application of the standard electron-counting scheme to these compounds suggests the presence of surplus electrons. This motivated us to carry out a theoretical study using the first-principles density functional theory (DFT) method. The structures of these compounds are based on either a formally cationic Rb9O2 or Cs11O3 cluster. The analyses of the partial charge density just below the Fermi level and the electron localization function (ELF) have revealed that there exist surplus electrons in interstitial regions of all the investigated suboxides so that the excess positive charge of the cluster can be compensated. Density of states (DOS) calculations suggest that all of the compounds are metallic. Therefore, the suboxides listed above may be regarded as a new family of metallic electrides, where coreless electrons reside in interstitial spaces and provide a conduction channel. Except for the phases of Rb9O2 and Cs11O3, the suboxide structures include both the cationic clusters and alkali-metal matrix. Several charge analyses indicate that the interstitial surplus-electron density can be assigned to the alkali-metal atoms in the metal matrix, leading to the possibility of the presence of negatively charged alkali-metal atoms, namely Rb– (rubidide) and Cs– (caeside) ions, a.k.a. alkalides. In Rb6O, Rb–, Rb0, and Rb+ are found to coexist in the same crystal structure. Similarly, in Cs7O, one can find the three types of Cs atoms. However, in Cs4O, no Cs0 state is identified. In the Rb–Cs–O ternary suboxides, Rb takes a negatively charged anion state or neutral state, while all of the Cs atoms are found to be cationic because they get involved in the Cs11O3 cluster and all the Rb atoms exist in interstitial sites. Orbital interactions between the clusters are analyzed to understand how the condensation of the clusters into the solid happens and how the electride nature ensues. These clusters are found to have some superatomic character.

Competition between Hydrogen Bonding and Dispersion Force in Water Adsorption and Epoxy Adhesion to Boron Nitride: From the Flat to the Curved

“…One could use a distance histogram for the same purpose as what is described above. Generally, these two lead us to the same conclusion . In the following, we will use the distance histogram to explore the interaction due to dispersion forces between the BNNT and the epoxy fragment.…”

“…Taking a look at the electron localization function (ELF) is a good way to probe the increase in the localization of the lone pair on the N atom of the N­(BH 2 ) 3 molecule. − One should notice that ELF plots can highlight cores, bonds, and lone pairs Figure shows how the lone pair sticks out directly above the N atom as the structure is distorted from the plane, where the isovalue of ELF is set to 0.9 so that the presence of localized electrons can be highlighted.…”

“…Generally, these two lead us to the same conclusion. 57 In the following, we will use the distance histogram to explore the interaction due to dispersion forces between the BNNT and the epoxy fragment. Figure 10b shows the frequency histogram of distances between the atoms in the epoxy fragment and those in each of the (3, 0), (6, 0), and (18, 0) BNNT surfaces.…”

Toward Computational Screening of Bimetallic Alloys for Methane Activation: A Case Study of MgPt Alloy

“…However, the peak width of the Pt DOS gets narrower when moving from the metallic to the alloyed, suggesting that the alloying process combines the delocalized metallic electronic structure with the localized one characteristic of ionic compounds. 79 The DOS peak for the Pt atom in MgPt shares almost the same energy range with that for the Mg atom in MgPt. The intensity of the peaks is higher for Pt than for Mg, suggesting that the electrons are somewhat localized toward the Pt side.…”

“…On the other hand, the center of the Pt 6s-5d bands remains almost unchanged, located at −2.0 eV in the pure metal state while at −2.1 eV in the alloyed. However, the peak width of the Pt DOS gets narrower when moving from the metallic to the alloyed, suggesting that the alloying process combines the delocalized metallic electronic structure with the localized one characteristic of ionic compounds . The DOS peak for the Pt atom in MgPt shares almost the same energy range with that for the Mg atom in MgPt.…”