Spectroscopic investigations of Cd0.25Gd0.50□0.25WO4:Eu3+ - A new promising red phosphor

Novel red-emitting phosphors scheelite-like tungstates of the formula Cd0.25Eu0.50 square 0.25WO4 and Cd0.25Gd0.50 square 0.25WO4:Eu3+ doped phases with a wide concentration range of the Eu3+ ion (0.5, 1, 5, 10 mol%) were prepared. Their photoluminescence properties under ultraviolet-visible (UV) as well as under vacuum-ultraviolet-visible (VUV) excitation were studied in detail. The excitation into the ligand-to-metal charge-transfer (LMCT) states (O -> W at 275 nm and O -> Eu3+ at 315 nm) led to blue-green emission. All samples could be efficiently excited by the UV light (315 as well as 395 nm) resulting in strong red emission. The emission color of prepared phosphor depended on the active ions concentration. The optical properties suggest that Cd0.25Gd0.50 square 0.25WO4:Eu3+ could be an efficient red-emitting phosphor for whitelight-emitting diodes (WLEDs) applications. The shape of the EPR spectra suggested a local low symmetry environment of Gd3+ sites in the investigated samples. Only samples with enough large concentration of Gd3+ ions revealed strong antiferromagnetic interactions with the Curie-Weiss temperature parameter, e.g., circle minus = -25.6 K (10 mol% Eu3+). A change in the magnetic interaction type with a change in the nominal Eu3+ concentration indicates possible limits of the concentration quenching phenomenon observed in the same range of the Eu3+ concentration in optical spectra. (C) 2010 Elsevier B.V. All rights reserved.