On the electronic structure of electrides

Abstract

We explore a mechanism for the remarkable charge isolation of the localized, or trapped, electrons found in the crystalline electrides Cs(18C6)2 and Cs(15C5)2. 133Cs NMR measurements show only ≊ 0.05% atomic character of the spin density at the Cs nucleus, consistent with many features of the structure and measured properties which indicate that the localized electron distribution is centered at the anion vacancies. The optical absorption data suggest that the localized electrons, which give rise to the Curie-law spin susceptibility, must penetrate appreciably into the crown ethers, (18C6) and (15C5), which encapsulate the Cs. We suggest that the large reduction of the spin density at the Cs nucleus is due to a Coulomb barrier resulting from negative charge on the oxygens. A crude model, one electron moving in two spherical charged shells surrounding the Cs core, illustrates the mechanism and accounts accurately for the ratio of spin densities at the Cs nucleus found in the 18C6 and the 15C5 electrides. Hartree–Fock calculations for an idealized model of an isolated Cs(18C6)2 molecule, namely Li(9C3)2, tend to support the mechanism.