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

A spectroscopic analysis of sodium germanate glasses activated with Eu3+, Tb3+ and Eu3+/Tb3+ is performed from their photoluminescence spectra and decay times. In the Eu3+-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→7F6,5 and europium 5D0→7F1,2 emissions, Eu3+ 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.