250- and 9.5-GHz EPR studies of an electride and two alkalides

The EPR spectra of polycrystalline samples of Cs⁺(18-crown-6)₂X^-, in which X^- = e^-, Na^-, or Cs^-, were studied at both X-band (9 GHz) and at 250 GHz. The high frequency affords much better g-factor resolution and reveals at least two types of asymmetric electron sites in each of these compounds. The defect electrons detected in the alkalides Cs⁺(18C6)₂Na^- and Cs⁺(18C6)₂Cs^- appear to be located at isolated centers in the crystal lattice with the strongest signal originating from electrons trapped at anion vacancies. These species exhibit broadening due to unresolved superhyperfine interactions at the periphery of the trapping site. In contrast, the X-band spectra of the two-electron sites in the electride appear to be exchange-narrowed, indicating that the electron spins have some degree of mobility within the crystal lattice. Comparison of the 9- and 250-GHz spectra of the electride places an upper bound of about 1 × 10⁷ s^-1 on the rate of spin exchange. The temperature and saturation behavior of the electride at X-band are consistent with the "F-center" model, in which electrons serve to balance the charge of the complexed cations, with the center of charge of each electron at an anionic site. The strongest signal in Cs⁺(18C6)₂e^- has nearly the same g-anisotropy as one of the signals from the isostructural sodide, Cs⁺(18C6)₂Na^-, indicating that at least one of the electron-trapping sites detected in the two crystal lattices is the same. The temperature dependence of the two EPR signals from Cs⁺(18C6)₂Cs^- at X-band suggests an activated process that populates two types of unpaired electron sites from a common spin-paired precursor.