PHOTOLUMINESCENCE STUDY OF LAYERED NIOBATES INTERCALATED WITH EU3+ IONS

In the present work the lamellar phases of composition K4Nb6O17, KNb3O 8 and KCa2Nb3O10 were used as precursors to prepare new compounds thro ugh the potassium exchange reaction with Eu3+ ion via a soft chemistry route. The precursors show absorption bands in the UV range and KNb3O 8 exhibits blue emission at room and liquid nitrogen temperature, whil e 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 an d emission spectra. The compounds show the general formulae Eux/3K4-xN b6O17. zH(2)O (1), Eux/3K1-xNb3O8. zH(2)O (2) and Eux/3K1-xCa2Nb3O10.z H(2)O (3) and were characterized by X-ray diffraction and europium and potassium analyses. The emission spectra of the samples recorded at 2 98 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 sit e 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) sp ectra indicated the presence of the rare earth ions in two different s ite symmetries. Since the precursor K4Nb6O17 has two crystallographica lly distinct interlayer regions, we suggest that the K+ ion is replace d 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 ob served at room temperature: blue luminescence due to matrix excitation and red emission due to the direct excitation of the Eu3+ ion. The va lues of the Omega(lambda) (lambda = 2, 4) experimental intensity param eters for these three lamellar compounds suggest that the short distan ce effects are not dominant. (C) 1998 Elsevier Science S.A. All rights reserved.