Cation-Induced Variation of Micromorphology and Luminescence Properties of Tungstate Phosphors by a Hydrothermal Method.

Eu3+-doped MWO4 (M = Zn, Cd, Ca, Sr, or Ba) nanorods and rodlike, spherical, dumbbell-like, and double-tapter-like grains have been obtained via a hydrothermal method. The distinct differences in cationic radius lead to a special morphology, which is attributed to the symmetry of the crystal structure and the differences in the growth rates of various crystals, and it further leads to the variation of luminescence. It was found that the charge transfer band of MWO4:0.04Eu3+ exhibits a blue shift with an increasing cationic radius, and the shift is ascribed to less covalency being caused by an increase in the cationic radius. The emission intensity obviously increases with cationic radius, increasing for the samples with a monoclinic phase; however, it is the opposite for the samples with a tetragonal phase, and CaWO4:0.04Eu3+ exhibits an optimal emission intensity. In addition, the possible reasons for the decay lifetime are also discussed in detail. Our results indicate that cations can effectively control the crystal structure, micromorphology, and luminescence in tungstate phosphors, and thus, our approach is effective for obtaining materials with the desired morphology and crystal structure.