Adjustable valence states of europium in CaAlBO4phosphor by means of enlarging the activator site and its luminescent properties

Eu-activated aluminoborates CaAlBO4 and its derivatives LixCa1−xAl1−xSixBO4 were synthesized by a conventional solid state reaction. The phosphors were characterized by powder X-ray diffraction (XRD). Rietveld refinement analysis was performed to reveal phase formation and changes in lattice parameters. Generation of direct white light emission was realized via incorporation of Si4+–Li+ to replace Al3+–Ca2+ for expanding the activator site in CaAlBO4. The average bond lengths of (Al, Si)–O and Ca–O were systematically shortened and elongated simultaneously, indicating that enlargement of the activator site corresponds to the doping amounts of Si4+–Li+, which play a critical role in the reduction process of Eu3+ to Eu2+. The different valence states of Europium were further confirmed by X-ray absorption near edge structure (XANES), time-resolved spectra, and photoluminescence. This work opens new vistas for designing phosphors in phosphor-converted light-emitting-diodes (pc-LEDs) by using a valence tunable activator via controlling the size of activator site.