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.
