Luminescence and energy transfer of the europium (III) tungstate obtained via the Pechini method

This work presents the preparation of the europium tungstate compound, Eu-2(WO4)(3), through the use of a polymeric precursor according to the Pechini method. In the preparation of this precursor ammonium tungstate, europium nitrate, edta, citric acid and ethylene glycol were mixed and heated at 700degreesC for 4h, leading to a luminescent compound. X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and diffuse reflectance spectroscopy were used to characterize the luminescent material. The photoluminescent properties of this compound were studied based on the f(6)-intraconfigurational transitions in the spectral range from 250 to 720 nm. When the photoexcitation is monitored into the O-->W (260 nm) and O-->Eu3+ (310 nm) ligand-to-metal charge-transfer states (LMCT) and F-7(0)-->L-5(6) transition of the Eu3+ ion (393 nm) the emission spectra present transitions from the 5 Do emitting level at 77 and 298 K. The emission spectra at room temperature, with excitation at the O-->W and O-->Eu3+ LMCT states, do not present the emission bands from the D-5(3), D-5(1) and D-5(1) emitting levels. On the other hand, the emission spectrum under excitation at the F-7(0)-->L-5(6) transition of the Eu3+ ion shows narrow emission bands arising from the D-5(J) levels (J = 1, 2 and 3). One of the main points of the present study is that this phenomenon is interpreted on the basis of a resonance crossover between the LMCT states and the D-5(J) levels (J = 1, 2 and 3). The emission quantum efficiency of the Eu3+ ion in this system is also discussed. (C) 2002 Published by Elsevier Science B.V.