Nano-to millisecond lifetime luminescence properties in Ln2(WO4)3 (Ln = La, Ho, Tm and Eu) microcrystalline powders with different crystal structures

Abstract

Ln2(WO4)3 crystals (Ln = La, Ho, Tm, Eu) have been synthesized using the conventional solid-state ceramic technique. X-ray diffraction was employed to identify the crystal structure of the compounds. Depending on the ionic radii of RE3+, two different structures are found, α-Eu2(WO4)3 (modulated scheelite-type) or γ-Sc2(WO4)3 (phase with negative thermal expansion coefficient). The optical characterization is based on absorption, photoluminescence, excitation and kinetic decay studies. Under UV excitation at the (WO4)2− absorption band, two different behaviors are recorded. La2(WO4)3 compounds show a broad visible emission band with a nanosecond range decay rate, and no energy transfer (ET) from the (WO4)2− moiety to the lanthanide ion is observed. However, evidences of ET are observed for Ln = Tm, Ho and Eu. In particular, the (WO4)2− → Eu3+ ET is so high that the fluorescence of the (WO4)2− moiety is almost completely quenched. Consequently, upon UV excitation Eu2(WO4)3 provides an intense visible emission with a 120 μs day lifetime. Although (WO4)2− → Ln3+ ET also occurs for Ln = Ho and Tm, the relatively small energy gap between energy levels of these ions avoids any detectable luminescence, which is quenched by multiphonon processes. Moreover, the effect of a structural phase transition in Ho2(WO4)3 crystals on their luminescence properties is analyzed. The broad range of PL response of Ln2(WO4)3 under UV excitation both in the spectral position of the emission bands (from UV to visible) as well as in the decay range (from ns to hundreds of μs) shows the tunability of wolframates for scintillator applications.