Color tuning of Bi(3+)-doped double-perovskite Ba(2)(Gd(1−x),Lu(x))NbO(6) (0 ≤ x ≤ 0.6) solid solution compounds via crystal field modulation for white LEDs

Nowadays, rare-earth-free color-tunable solid solutions are receiving great attention. Here, we synthesize a type of double-perovskite Ba(2)(Gd(1−x),Lu(x))NbO(6):Bi(3+) (0 ≤ x ≤ 0.6) solid solution compound using high temperature solid state reaction. The structural phase purity and photoluminescence (PL) properties of the samples are characterized by powder X-ray diffraction (XRD), density functional theory (DFT) calculations, UV-visible diffuse reflectance spectroscopy and PL spectroscopy. Structural analysis shows that all the samples are crystallized in the double-perovskite structure with a cubic space group of Fm3̄m. Moreover, with the increase of Lu(3+) content, the XRD positions are gradually shifted toward higher diffraction angles, indicating the shrinkage of the crystal lattice. Our PL results show that the Ba(2)(Gd(1−x),Lu(x))NbO(6):Bi(3+) solid solutions can show Bi(3+) tunable emissions from 462 nm to 493 nm and excitation peaks from 363 nm to 390 nm with the increase of Lu(3+) content from 0 to 0.6, owing to the crystal field modulation around the Bi(3+) ion. In addition, the blue-shift of the host excitation peaks is also observed, which is ascribed to the increase of bandgap energies (i.e., from 3.01 eV to 4.14 eV based on the DFT calculations). Besides, due to the closer structural rigidity induced by the replacement of larger Gd(3+) ions with smaller Lu(3+) ions, the Ba(2)(Gd(1−x),Lu(x))NbO(6):Bi(3+) solid solutions show an increase of Bi(3+) emission intensity and QE values followed by a subsequent decrease. As a result, the highest QE value, which corresponds to Ba(2)Gd(0.8)Lu(0.2)NbO(6):Bi(3+), is 49%. Finally, by coating this optimal blue phosphor and the red CsPb(Br(0.4)I(0.6))(3) phosphor on a commercial UV LED chip, a white LED device with a color temperature (CT) of 3633 K, CIE value at (0.381, 0.379), color rendering index (CRI) of 78.4, and luminous efficiency of 48 lm W(−1) is achieved.