Ionic clusters isolated in a metal matrix: The structure and vibrational frequencies of Rb9O2 and Cs11O3 clusters in alkali metal suboxides

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

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.