Structure and Properties of a New Electride, Rb⁺(cryptand[2.2.2])e

This is the sixth electride whose crystal structure has been determined and the fourth to show polymorphism. Crystals of the title electride prepared from mixed solvents have a structure similar to that of Li⁺(cryptand[2.1.1])e^-. Electrons occupy cavities that are connected by "ladder-like" channels. The static and spin magnetic susceptibilities of polycrystalline samples that contain this polymorph (called phase α) show Heisenberg 1D antiferromagnetic behavior with -J/k_B = 30 K. Similar to other electrides with "localized" electrons, this electride is a poor conductor (σ < 10^{- 4} ohm^-1cm^-1). Thin films prepared by high vacuum co-deposition of Rb metal and cryptand[2.2.2] have optical spectra and near-metallic electrical conductivity nearly identical with those of K⁺(cryptand[2.2.2])e^-. These properties would not be expected if the film structure were the same as that obtained for crystals. Rather, they suggest that the films consist of microcrystals whose structure is similar to that of K⁺(cryptand[2.2.2])e^-. Polycrystalline samples prepared by slow evaporation of methylamine from stoichiometric solutions at -78 °C (called phase β) have properties similar to those of K⁺(cryptand[2.2.2])e^-. The conductivity of samples that contain phase β is more than an order of magnitude larger than those with phase α. Magnetic and spin susceptibilities show that phase β samples have much larger electron-electron interactions. As with K⁺(cryptand[2.2.2])e^-, the magnetic susceptibility of phase β is compatible with alternating linear chain Heisenberg antiferromagnetism, with -J/k_B ≈ 300 K and -J'/k_B ≈ 240 K. Thin vapor co-deposited films show abrupt changes in the conductivity and optical spectrum at -12 °C that suggest a transition that may be conversion of phase β to phase α.