New reddish-orange and greenish-yellow light emitting phosphors: Eu ³⁺ and Tb³⁺/Eu³⁺ in sodium germanate glass

A spectroscopic analysis of sodium germanate glasses activated with Eu ³⁺, Tb³⁺ and Eu³⁺/Tb³⁺ is performed from their photoluminescence spectra and decay times. In the Eu ³⁺-singly doped glass reddish-orange light emission, with x=0.64 and y=0.35 CIE1931 chromaticity coordinates, is obtained upon Eu³⁺ excitation at 393 nm. Such chromaticity coordinates are close to those (0.67, 0.33) proposed by the National Television Standard Committee for the red phosphor. When the sodium germanate glass is co-doped with Tb³⁺ and Eu³⁺ greenish-yellow light emission, with (0.41, 0.46) CIE1931 chromaticity coordinates, is obtained upon Tb³⁺ excitation at 344 nm. Such greenish-yellow luminescence is due mainly to the terbium ⁵D₄→⁷F_6,5 and europium ⁵D₀→⁷F_1,2 emissions, Eu ³⁺ being sensitized by Tb³⁺ through a non-radiative energy transfer. The non-radiative nature of this energy transfer is inferred from the increase in the decay rate of the Tb³⁺ emission when the glass is co-doped with Eu³⁺. From an analysis of the Tb³⁺ emission decay time curves it is inferred that such energy transfer might take place between Tb³⁺ and Eu³⁺ clusters through a short-range interaction mechanism.