Raman and luminescence spectroscopy study of europium doped zirconia
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Robert Withnall and Jack Silver Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom (Received 10 September 2007; accepted 14 January 2008)
A Raman spectrometer was used to probe the structure and luminescence of a range of europium-doped zirconia phosphors prepared by different routes. We have demonstrated that the synthesis method and precursor type have a strong influence on the structure and luminescence of the final phosphor product. Raman spectroscopy has also demonstrated the presence of local order around the dopant ions that is not apparent in x-ray diffraction (XRD) and corresponds with changes in luminescence. As europium concentration is increased from 1 mol% to 20 mol%, the long range structure (from XRD) changes from tetragonal to cubic. Raman spectroscopy, however, shows that the 1 mol% material has a localized structure similar to the monoclinic undoped zirconia. This localized symmetry can explain the differences observed previously in emission spectra.
I. INTRODUCTION
Europium is a widely used dopant in the phosphor industry, producing a blue emission in its 2+ state and a red emission in its 3+ state.1 Eu3+ has been used as the dopant of choice in a variety of lattices to produce the red color in television screens since the 1960s.2 Phosphor materials generally exhibit their luminescent properties when they are highly crystalline and for this reason are often produced as calcined ceramic powders such as Y2O3:Eu3+, where the Eu3+ dopant has a concentration of around 4–5 mol%.3 Zirconia is a particularly useful ceramic and has found applications in many areas from oxygen ion conduction when doped with yttria to structural uses as a material for knives. Zirconia can exist in a number of polymorphs; however, only the monoclinic form is stable at room temperature. The other polymorphs can be obtained by heating the zirconia: the tetragonal phase forms above 1175 °C, and the cubic phase forms above 2370 °C, but both revert to monoclinic upon cooling.4–6 These phase transformations limit the use of pure zirconia due to the associated large volume changes; however, zirconia can be stabilized in the higher temperature polymorphs by doping with small amounts of other materials such as a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0231 1854
J. Mater. Res., Vol. 23, No. 7, Jul 2008
yttria, thus eliminating the phase changes during heating.7–9 Recently, zirconia has attracted significant research attention as a waveguide material and as a potential host lattice for inorganic phosphors in photonics due to its high refractive index, optical transparency, and excellent thermal, chemical, and mechanical properties.4 Rare-earth-doped zirconia thin films were prepared by Reisfeld et al. in 2000, using europium, terbium, and samarium, and their photoluminescence (PL) excitation and emission spectra were studied.10 They found that the zirconia host materials manifested PL brightness for europium-doped films su
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