Electronic Structure of the A8Tr11 (A = K, Rb, Cs; Tr = Ga, In, Tl) Zintl Phases: Possible Chemical Reasons Behind Their Activated versus Non Activated Conductivity
journal contributionposted on 19.10.2009, 00:00 by Manuel Cobián, Pere Alemany, Alberto García, Enric Canadell
The electronic structure of the A8Tr11 (A = K, Cs; Tr = Ga, In, Tl) Zintl phases has been studied by means of first-principles density functional theory (DFT) calculations. It is shown that the hypoelectronic Tr11 cluster in these phases must be considered as Tr118− even if it would require just a 7− charge to maximize its bonding, filling all its bonding and nonbonding levels. Our calculations show that the lowest empty orbital of the isolated Tr117− clusters is an a1-type orbital. However, a degenerate e-type set of orbitals is higher but quite close in the case of the Ga117− clusters. Thus, for the isolated Tr118− clusters, the extra electron occupies always an a1-type antibonding orbital that contains, however, some Tr−Tr bonding component thus leading to a weak global antibonding character. In the solid, cluster−alkali-metal bonding interactions occur and spread the cluster levels into bands, but the extra electron still fills the a1-type cluster level for most of the A8Tr11 phases. The cluster−alkali-metal interactions have a minor role in stabilizing this orbital but they provide the necessary delocalization to lead to the metallic character of these phases. In contrast, the e-type antibonding levels of the Ga117− isolated cluster are those which become filled by the extra electron in the Cs8Ga11 solid. This phase should be metallic, but occupation of this degenerate pair of cluster levels would lead to a structural instability that may be avoided by reducing the interactions of the alkali-metal atoms with the cluster levels. In that way the occupation appropriate for the isolated cluster is restored (i.e., one electron fills the a1 cluster orbital), but the extra electron now remains localized on the cluster, thus leading to the unexpected activated conductivity observed for the Cs8Ga11 phase.