Tailoring super-broad photoluminescence from Eu²⁺and dual-mode Eu²⁺/Eu³⁺-doped alkaline earth aluminoborate glasses through site-similarity and ligand acidity

We report on the photoluminescence (PL) properties of Eu-doped alkaline earth aluminoborate glasses MO-Al₂O₃-B₂O₃ (M=Ca, Sr and Ba) as visually transparent warm-white phosphors for near-UV excitation. Due to the high acidity of the aluminoborate matrix, Eu²⁺ can be readily stabilized to assume six- to eight-fold oxygen coordination. The energetic gap of the active 4f⁶5d¹→4f⁷(⁸S_7/2) transition then depends on the speciation of the alkaline earth network modifier, what enables compositional tailoring of the PL colour. In addition, we find a favorably broad emission bandwidth, covering ~180 nm (> 6000 cm^-1) at half maximum, which we associate with the degree of structural disorder and the site-similarity which is imposed on the PL complex. The asymmetry ratio of Eu³⁺ emission confirms highest average site-asymmetry for the material with highest emission bandwidth. The asymmetry ratio and, eventually, the degree of site-similarity and the associated emission bandwidth decrease with either Ca²⁺ or Ba²⁺ for Sr²⁺ substitution. In consequence, finely tunable warm-white emission colour can be achieved through tailoring glass composition and – making use of dual-mode PL in the presence of multi-valent europium - through tuning the excitation wavelength.