EPR study of europium ions in type A zeolite. The general classification of the EPR spectra of S‐state rare‐earth ions in disordered polycrystalline or glassy matrices

A study of polycrystalline A zeolite exchanged with Eu(OH)₂ solution has been performed using EPR spectroscopy. Detailed computer simulation techniques have been used to simulate EPR spectra from first principles, with the explicit incorporation of random distributions in the crystal field interaction parameters. This method is equally well applicable to the analysis of the spectra of glasses. Two components in the spectrum of the dehydrated material have been distinguished. The first corresponds to three‐ coordinate Eu²⁺ ions at regular six‐ring sites in the zeolite, giving rise to an axial EPR spectrum with a large (b⁰₂)_avg value of 0.32 cm⁻¹. The second corresponds to an orthorhombic EPR spectrum with a (b⁰₂)_avg value of similar magnitude, and a rhombic component b²₂≂0.34 (b⁰₂)_avg. This latter feature is attributed to three‐coordinate Eu²⁺ ions located at six‐ring sites, where one of the oxide ions in the ring has been protonated. The magnitudes of the axial components of these spin Hamiltonian crystal field terms are the largest reported to date for Eu²⁺ or Gd³⁺. A general classification into four types has been made for the S‐state rare‐earth ion EPR spectra in zeolites. The same classification is valid for other glassy or disordered polycrystalline hosts. The spectra types are distinguished by the average magnitude of the b⁰₂ component in the spin Hamiltonian, and the dominant symmetry as defined by the ratio λ′=‖ b²₂/b⁰₂ ‖. The use of the superposition model theory and the literature of structural information permit the correlation of each type of spectrum with a model of the local coordination of the cations in the host material.