Structure of K+(cryptand[2.2.2J) electride and evidence for trapped electron pairs

Abstract

Electrides are crystalline salts in which stoichiometric amounts of trapped or itinerant electrons serve as the anions1–8. The cations are alkali metal cations complexed by cyclic or bicyclic polyethers of the crown ether9 or cryptand10 classes. Optical spectra, powder conductivities and magnetic susceptibilities show that, in most electrides, individual electron localization occurs, presumably centred at the anionic sites, with trap depths of 0.5–1.0 eV. An exception is K+(cryptand[2.2.2]) ˙e-, which has a plasma-like optical absorption spectrum2, high microwave conductivity, a low activation energy for direct current powder conductivity (∼0.02 eV), and a weak, temperature-dependent electronic paramagnetic susceptibility. The crystal structure of this electride shows the presence of large (4×6×12A) vacancies of complex shape, interconnected in two directions by zigzag channels, but blocked in the third direction. The structure and properties suggest the presence of weakly bound electron pairs.