Luminescence properties of Tm3+/Yb3+, Er3+/Yb3+ and Ho3+/Yb3+ activated calcium tungstate

CaWO4 phosphor activated by the Tm3+/Yb3+, Er3+/Yb3+ and Ho3+/Yb3+ ions were synthesized by a traditional high-temperature solid-state method. The crystal structures and morphologies of the products were characterized by X-ray powders diffraction method (XRD), infrared spectra (FT-IR) and scanning electron microscopy (SEM). The samples were found to show up-conversion luminescence properties. CaWO4 doped with Tm3+/Yb3+ showed blue luminescence characteristic of Tm(III) ion in the range of 460–485 nm, corresponding to the 1G4→3H6 electronic transition. CaWO4 doped with Er3+/Yb3+ showed strong green luminescence at 510–565 nm (2H11/2, 4S3/2→4I15/2) and weak red luminescence at 640–685 nm (4F9/2→4I15/2) of Er(III) ion. CaWO4 doped with Ho3+/Yb3+ phosphor emitted green luminescence at 525–560 nm (5S2, 5F4→5I8) and red luminescence at 630–670 nm (5F5→5I8) and at 730–770 nm (5S2, 5F4→5I7), which is the characteristic of Ho(III) ion.