Effect of Alkali-Metal Ions on the Local Structure and Luminescence for Double Tungstate Compounds AEu(WO4)2 (A = Li, Na, K)

The effect of alkali-metal ions on the local structure and luminescence properties for alkali-metal europium double tungstate compounds AEu(WO4)2 (A = Li, Na, K) has been investigated by a dual-space structural technique, atomic pair distribution function (PDF) analysis, and the Rietveld method of powder X-ray diffraction. The compounds AEu(WO4)2 (A = Li, Na) crystallize in the isostructure with the tetragonal space group I41/a (No. 88) and show the same luminescence properties in spite of the different doped alkali metals. However, KEu(WO4)2 crystallizes in monoclinic symmetry with the space group C2/c (No. 15). Compared with the two other counterparts, KEu(WO4)2 exhibits a more effective charge-transfer excitation, a larger Stokes shift, and a broader 612 nm emission band. This phenomenon is ascribed to the lower crystal symmetry in KEu(WO4)2, which influences bond distances and the coordination number of Eu3+. Two complementary methods, the Rietveld method and PDF analysis, reveal that both LiEu(WO4)2 and NaEu(WO4)2 afford the same local surroundings of Eu3+. The local structure determined by the Rietveld and PDF methods well account for the observed luminescent properties.