Abstract

The local environment of Er³⁺ ions in microporous titanosilicate ETS-10 and in synthetic narsarsukite and glassy materials obtained by calcination of ETS-10 has been investigated by EXAFS, Raman and photoluminescence spectroscopies. Er L_III-edge EXAFS studies of Er³⁺-doped ETS-10 support the view that the exchanged Er³⁺ ions reside close to the (negatively charged) TiO₆ octahedra. In ETS-10, Er³⁺ is partially bonded to framework oxygen atoms and hydration water molecules. The Er⋯Ti distance (3.3 Å) is similar to the Na⋯Ti distances (3.15–3.20 Å) reported previously for Na–ETS-10. Although the exact location of the ErO₆ units within the host structure of Er³⁺-doped synthetic narsarsukite is still an open question, it is most likely that Er³⁺ substitutes Ti⁴⁺ rather than Na⁺ ions. EXAFS spectroscopy indicates that no significant clustering of erbium atoms occurs in the titanosilicate samples studied. Evidence for the insertion of Er³⁺ ions in the framework of narsarsukite has been obtained by Raman spectroscopy. This is indicated by the increasing full-width at half-maximum (FWHM) of the 775 cm⁻¹ peak and the increasing intensity of the anatase peaks as the erbium content increases. In addition, as the narsarsukite Er³⁺ content increases a band at ca. 515 cm⁻¹ firstly broadens and subsequently a new peak appears at ca. 507 cm⁻¹. Er³⁺-doped narsarsukite exhibits a characteristic local vibrational frequency, ħωca. 330 cm⁻¹, with an electron–phonon coupling, gca. 0.2, which constitutes additional evidence for framework Er³⁺ insertion. The number of lines in the infrared emission spectrum of synthetic narsarsukite indicates the presence of two optically-active erbium centres with very similar local environments and an average ⁴I_13/2 lifetime of 7.8 ± 0.2 ms.