Metastable scheelite CdWO4:Eu3+ nanophosphors: Solvothermal synthesis, phase transitions and their polymorph-dependent luminescence properties

This work explores the phase transition and its impact on photoluminescent properties of metastable scheelite-type, with and without Eu 3+ doped, CdWO 4 nanophosphors. A series of scheelite structure CdWO 4 :xEu 3+ (x = 0, 0.025, 0.05, 0.075, 0.10) samples were synthesized using a simple solvothermal method. The X-ray diffraction results indicate that the prepared samples were single phase in a tetragonal scheelite-type structure, and completely transformed into pure monoclinic wolframite structure after sintering at 450 °C for 2 h. It was revealed by Rietveld analysis that Eu 3+ doping induced a lattice contraction of both scheelite and wolframite CdWO 4 structures. The impact of Eu 3+ doping on the crystal structure and the band structure were further probed by FT-IR, Raman and UV–Vis spectroscopy. SEM images show that the morphology of the prepared samples also changed in the phase transition and the Eu 3+ doping processes. All samples showed a broad multi-color luminescence emission in the range from 400 to 640 nm under 280 nm excitation. The relative intensities of the blue-green emission originated from CdWO 4 matrix and the red emission ascribed to Eu 3+ can be regulated by both the crystal structure and Eu 3+ doping levels. The energy transfer process between WO 4 groups in scheelite structure CdWO 4 and WO 6 groups in wolframite structure CdWO 4 and Eu 3+ was discussed in detail based on the photoluminescent excitation and emission spectra. The synthetic approach reported here allows the control over CdWO 4 polymorphs and this kind of luminescent material has wide applications in white-LED, chemosensors and bioimaging .