THEORETICAL DETERMINATION THAT ELECTRONS ACT AS ANIONS IN THE ELECTRIDE CS+(15-CROWN-5)2.E-

ELECTRIDES are crystalline salts formed from complexed alkali-metal cations. There has been some dispute as to whether the valence electron from the alkali ion becomes a trapped interstitial anion1,2 or resides at or near the alkali-metal nucleus3. If the former description holds, electrides would represent stoichiometric counterparts of ionic insulators containing 'F-centre' electronic defects. Experiments1,2 have so far failed to resolve the question. Here we present ab initio self-consistent density-functional calculations4 of the electron distribution in the electride Cs+(15-crown-5)2.e-. We find that a spatially localized electron is located at the anion site, in accord with the F-centre model. Although the potential is in fact repulsive in this region, the electron is apparently forced to reside here by the need to lower its kinetic energy. We suggest that this picture may hold for other electrides as well.