Mössbauer effect studies of coordination and oxidation-reduction properties of Eu ions in zeolites

Zeolites exchanged with Eu³⁺ ions were prepared based on three kinds of zeolite hosts: A zeolite, X zeolite, and Y zeolite. The ¹⁵²Eu Mössbauer effect was used to study the coordination properties of Eu³⁺ ions in these zeolites. The results showed that EU³⁺ ions occupy two sites where the electric-field gradient tensors are axially symmetric. The isomer shift and quadrupole splitting of Mössbauer spectra of Eu depend on the electronegativity of the zeolite hosts. With moderate conditions (less than 720°C), the Eu³⁺ ions in the three kinds of zeolites were reproduced to Eu²⁺ ions. The coordination property and the stability of the Eu²⁺ were also studied with ¹⁵¹Eu Mössbauer effect spectroscopy. The results indicated that the symmetry of the electric-field gradient at the Eu site is lowered.