Adjustable valence states of europium in CaAlBO4 phosphor by means of enlarging the activator site and its luminescent properties

Abstract

Eu-activated aluminoborates CaAlBO₄ and its derivatives Li_xCa_1−xAl_1−xSi_xBO₄ 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.