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

This work presents the preparation of the europium tungstate compound, Eu2(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 700°C for 4 h, 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 f6-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 7F0→5L6 transition of the Eu3+ ion (393 nm) the emission spectra present transitions from the 5D0 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 5D3, 5D2 and 5D1 emitting levels. On the other hand, the emission spectrum under excitation at the 7F0→5L6 transition of the Eu3+ ion shows narrow emission bands arising from the 5DJ 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 5DJ levels (J=1, 2 and 3). The emission quantum efficiency of the Eu3+ ion in this system is also discussed.