Density functional analysis of the electronic structure of Cs9InO4: Evidence for the presence of a Cs anion

Compounds of the type Cs9MO4 (M = In, Fe, Sc..) [1] are excellent electric conductors, their resistivity approaching that of metallic cesium. The complex oxoindate Cs9InO4 can be described in terms of an ion/electron counting scheme (Cs)9[(In)(O)4]+4e. The surplus of electrons is associated with the cesium atoms, so the structure can be divided into an ionic part with localized electrons in the oxoindate anion and a metallic part with delocalized electrons in the cesium sublattice, see Figure 1a). However, the cesium atoms in Cs9InO4 exhibit quite different environments so that the valence electron distribution within the metallic part of the structure is expected to be highly non-uniform. In Cs9InO4 the shortest Cs–O distances involving the Cs(1), Cs(2) and Cs(3) atoms (i.e. 281, 292 and 312 pm, respectively) are similar to those in the structure of salt-like Cs2O (i.e. 292 pm), indicating that these Cs atoms behave as Cs ions towards oxygen. In contrast, the Cs(4) atom is located at the center of a Cs16 cage made up of eight Cs(1), four Cs(2), four Cs(3) and two additional Cs(4) atoms from an adjacent cage (Figure 1b) with Cs–Cs distances ranging from 531 to 645 pm, similar to what is found for elemental bcc cesium. This analysis of the Cs–Cs distances corroborates our view of the metal-metal bonding between the cesium atoms resulting in a delocalized electronic system and the ionic bonding to the InO4 entities according to (Cs)9[InO4]+4e. However, the excess cannot be uniformly distributed among the cesium atoms because they have very different environments.