Photoluminescence study of layered niobates intercalated with Eu3+ ions

In the present work the lamellar phases of composition K4Nb6O17, KNb3O8 and KCa2Nb3O10 were used as precursors to prepare new compounds through the potassium exchange reaction with Eu3+ ion via a soft chemistry route. The precursors show absorption bands in the UV range and KNb3O8 exhibits blue emission at room and liquid nitrogen temperature, while K4Nb6O17 presents emission spectra only at low temperature. The aim of this work is to investigate the photoluminescence properties of Eu3+-exchanged layered oxides through the analysis of their excitation and emission spectra. The compounds show the general formulae Eux/3K4-xNb6O17. zH(2)O (1), Eux/3K1-xNb3O8. zH(2)O (2) and Eux/3K1-xCa2Nb3O10.zH(2)O (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 D-5(0) and F-7(J) (J=0-4) levels that indicate the presence of Eu3+ ions in C-nv site symmetry. It was observed for systems (2) and (3) that the D-5(0)-->F-7(0) transition does not split, which indicates that the Eu3+ 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 K4Nb6O17 has two crystallographically distinct interlayer regions, we suggest that the K+ ion is replaced by the Eu3+ 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 Eu3+ ion. The values of the Omega(lambda) (lambda = 2, 4) experimental intensity parameters for these three lamellar compounds suggest that the short distance effects are not dominant. (C) 1998 Elsevier Science S.A. All rights reserved.