Extendible synthesis of NaLnW2O8∙2H2O (Ln=Sm–Lu, and Y) via one-step hydrothermal reaction: Phase, morphology and luminescence

Abstract

The Hydrous lanthanide (Ln) double tungstates of NaLnW₂O₈·2H₂O in orthorhombic structure have been directly produced by hydrothermal reaction of Na₂WO₄ and Ln(NO₃)₃ at pH=8 for 24 h under 200 °C for the small ions of Ln = Tm–Lu, under 150 °C for the slightly larger lanthanides of Ln = Ho, Y and Er, and under 100 °C for the even larger lanthanides of Sm–Dy. Moreover, NaLnW₂O₈·2H₂O for Ln = Eu–Dy can also be synthesized at a slightly higher reaction pH of 9 at 150 °C, while it was failed for the other members in the lanthanide series of La–Nd even though various efforts were attempted in hydrothermal conditions. On the basis of the crystal structure of NaLnW₂O₈·2H₂O and lanthanide contraction, this outcome conformed well to Pauling rules. Taking Ln = Lu for example, the phase evolution and morphology were studied by the combined techniques of XRD, FE-SEM, IR and TG/DSC. The photoluminescence properties of NaEuW₂O₈∙2H₂O and NaTbW₂O₈∙2H₂O, including excitation/emission, CIE chromaticity, quantum yields, and fluorescence decay were characterized.