Color tuning of Bi2+ luminescence in barium borates

The luminescence of Bi2+ has its origin in p-p transitions between different spin-orbit states in the 6s26p1 electronic configuration. The divalent state is highly unusual for Bi and because of the low stability of Bi2+, research on the intraconfigurational p-p transitions of Bi2+ is limited. Narrow-band emission has been observed in the orange/red spectral region, which makes Bi2+ luminescence interesting for application in white light LEDs. In this paper we investigate the luminescence of Bi2+ in a variety of borate host lattices with the aim to tune the emission wavelength and to investigate the influence of the host on the luminescence properties. Bi2+ was doped into Sr1−xBaxB4O7, α-BaB2O4, α-BaB4O7, Ba2B10O17 and BaB8O13. Luminescence was observed for Bi2+ in all hosts, except α-BaB2O4, indicating that Bi2+ cannot be stabilized in this host. In the other borates the emission wavelength varies between 586 nm (in BaB8O13) and 671 nm (in α-BaB4O7). The shift in emission wavelength is explained by a variation in crystal field splitting and spin-orbit coupling. The luminescence lifetime of the p-p emission is in the μs range, varying between 6 and 13μs, reflecting the parity forbidden character of the p-p transition. Narrow-band red emission at 612 nm (FWHM=35 nm) is observed for Ba2B10O17:Bi2+. These luminescence characteristics of Bi2+ are favorable for application in w-LEDs, but an important drawback is that only low concentrations (in the ppm range) can be stabilized in the divalent state.