Electrides: From 1D Heisenberg Chains to 2D Pseudo-Metals
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Abstract
Electrides are ionic compounds in which the cations are complexed by cryptands or crown ethers and the "anions" are trapped electrons. The crystal structures of five electrides are known and are similar to the corresponding alkalides (in which the anions are alkali metal anions) except that the anionic sites are "empty". Theory and experiment strongly support a model in which the "excess" electrons are trapped in these anionic cavities and interact with each other through connecting channels, whose geometries vary significantly from one electride to another. Measurements of optical, alkali metal NMR, and EPR spectra, magnetic susceptibilities, and conductivities provide many data that can be correlated with the structures. Three electrides have essentially 1D chains of cavities connected by channels through which the electrons communicate, as indicated by magnetic susceptibilities that are well described by a 1D Heisenberg model. The electride, K+(cryptand[2.2.2])e- has a 2D array of cavities and channels. It appears that defects, probably missing electrons (holes), are responsible for its near-metallic conductivity. The fifth electride of known structure contains Cs+ complexed by a mixed sandwich of 15-crown-5 and 18-crown-6 and has a complex cavity−channel geometry, dominated by rings of six cavities. The arguments in favor of the proposed electride model, nearly-free electrons confined as a "lattice gas" in a complex array of cavities and channels, are presented in this paper.
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