Photoluminescence study of layered niobates intercalated with Eu 3+ ions

Abstract

In the present work the lamellar phases of composition K 4Nb 6O 17, KNb 3O 8 and KCa 2Nb 3O 10 were used as precursors to prepare new compounds through the potassium exchange reaction with Eu 3+ ion via a soft chemistry route. The precursors show absorption bands in the UV range and KNb 3O 8 exhibits blue emission at room and liquid nitrogen temperature, while K 4Nb 6O 17 presents emission spectra only at low temperature. The aim of this work is to investigate the photoluminescence properties of Eu 3+-exchanged layered oxides through the analysis of their excitation and emission spectra. The compounds show the general formulae Eu x/3 K 4− x Nb 6O 17· zH 2O (1), Eu x/3 K 1− x Nb 3O 8· zH 2O (2) and Eu x/3 K 1− x Ca 2Nb 3O 10. zH 2O (3) and were characterized by X-ray diffraction and europium and potassium analyses. The emission spectra of the samples recorded at 298 and 77 K temperatures showed transitions between the 5D 0 and 7F J (J=0–4) levels that indicate the presence of Eu 3+ ions in C nv site symmetry. It was observed for systems (2) and (3) that the 5D 0→ 7F 0 transition does not split, which indicates that the Eu 3+ ion is found only in one site symmetry. On the other hand, the system (1) spectra indicated the presence of the rare earth ions in two different site symmetries. Since the precursor K 4Nb 6O 17 has two crystallographically distinct interlayer regions, we suggest that the K + ion is replaced by the Eu 3+ ion in the two interlamellar regions of the material (1). In the case of the system (2), two types of emission spectra were observed at room temperature: blue luminescence due to matrix excitation and red emission due to the direct excitation of the Eu 3+ ion. The values of the Ω λ (λ=2, 4) experimental intensity parameters for these three lamellar compounds suggest that the short distance effects are not dominant.