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n Lina) Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China (Received 17 December 2004; accepted 9 May 2005)

R2MoO6:Eu3+ (R ⳱ Gd, Y, La) phosphors were prepared by the Pechini sol-gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), reflectance spectra, photoluminescence (PL) spectra, and lifetimes were used to characterize the resulting phosphors. The results of XRD indicate that all of the R1.96Eu0.04MoO6 (R ⳱ Gd, Y, La) phosphors crystallized completely at 800 °C. Y1.96Eu0.04MoO6 and Gd1.96Eu0.04MoO6 are of isomorphous monoclinic (␣) structure, while La1.96Eu0.04MoO6 preferentially adopts the tetragonal (␥) form. FE-SEM study reveals that the samples mainly consist of aggregated particles with an average grain size ranging from 100 to 250 nm. The luminescent properties of R2MoO6:Eu3+ (R ⳱ Gd, Y, La) phosphors are largely dependent on their structure, grain size, and powder morphology. The isomorphous Y2MoO6:Eu3+ and Gd2MoO6:Eu3+ phosphors show very similar luminescence properties, which differ greatly from that of the La2MoO6:Eu3+ phosphor.

I. INTRODUCTION

White light emitting diodes (LEDs) have attracted much attention in recent years.1–3 The currently used phosphor in white LEDs is the Y3Al5O12:Ce3+ (yellow emission) using the 430 nm blue emission from GaN as the excitation source. To increase the efficiency of white LEDs, people are exploring the tricolor (red, green and blue) phosphors suitable for excitation between 350 and 400 nm ultraviolet (UV). Molybdates are interesting materials to meet these demands to some extent based on the intense oxygen-to-metal (O-Mo) charge transfer bands in the near UV region, which can absorb the emission from a GaN-based LED very efficiently. When doping Eu3+ in the molybdates, the UV excitation energy can be transferred to the activator ion Eu3+ non-radiatively, resulting in the red emission of Eu3+. As a good example, Neeraj and coworker investigated NaM(WO4)2−x(MoO4)x:Eu3+ (M ⳱ Gd, Y, Bi) as novel red phosphors for solid-state lighting using 400 nm UV emission from GaN as excitation source.4

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0356 2676

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 10, Oct 2005 Downloaded: 29 Nov 2014

Rare-earth elements can form a series of molybdates with general formula R2MoO6 (R ⳱ rare earths or Bi). Depending on the radii of R and synthesis conditions, R2MoO6 compounds may crystallize in three polymorphs with monoclinic (␣), cubic (␤), or tetragonal (␥) structures.5–8 Up to now, several papers have been published on the investigation of the crystal structures of the R2MoO6 compounds with little attention on their photoluminescence properties.5–8 Among the previously published results, nearly all of the R2MoO6 compounds were prepared by the solid-state reaction method.5–8 This technique generally needs high temperature an