Anomalous bluish-green light emitting features of europium doped barium aluminate (BaAl2O4:Eu2+) phosphor

Abstract

Eu doped barium aluminate is an important material for light emitting devices. Here we report structural, optical absorption and emission features of 1–5 % Eu doped barium aluminate phosphor and explore its ‘anomalous’ emission feature. The structural analysis suggests that the undoped sample has a mixture of BaAl2O4 and BaAl12O19. On the other hand, the samples doped with Eu have a mixture of BaAl2O4, Ba(Al2O4) and BaAl12O19 phases. Among these, the major phase in all the samples was BaAl2O4. The undoped sample shows a high absorption peak at around 209 nm followed by three weak intensity peaks apparent at around 250, 288 and 338 nm. In contrast, all the doped samples show three maxima at around 209, 266 and 340 nm. The position and intensity of these absorption maxima are found to vary with the Eu concentration. The optical bandgap of the undoped sample is estimated to be 5.05 eV and it is found to reduce to 4.68 eV after doping of Eu at 5 %. All the samples produce common PL emission peaks centred at ∼405, 428 and 456 nm due to potential oxygen vacancies. The doped samples show additional peaks at around 485, 518 and 657 nm due to Eu2+ and Eu3+ ions. In particular, the sample doped with 2 % of Eu produces a strong anomalous emission band of Eu2+ ion in the bluish-green region. Our study reveals that the broadband bluish-green emission of Eu2+ ions can be achieved when the barium aluminate crystal is doped by 2 % of Eu. When the host is doped by Eu at >2 % or <2 %, the emission band associated with oxygen vacancies dominates over the Eu2+ emission.