Color tuning of Bi3+-doped double-perovskite Ba2(Gd1−x,Lux)NbO6 (0 ≤ x ≤ 0.6) solid solution compounds via crystal field modulation for white LEDs

Abstract

Nowadays, rare-earth-free color-tunable solid solutions are receiving great attention. Here, we synthesize a type of double-perovskite Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ (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 Fmm. Moreover, with the increase of Lu³⁺ content, the XRD positions are gradually shifted toward higher diffraction angles, indicating the shrinkage of the crystal lattice. Our PL results show that the Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ solid solutions can show Bi³⁺ tunable emissions from 462 nm to 493 nm and excitation peaks from 363 nm to 390 nm with the increase of Lu³⁺ content from 0 to 0.6, owing to the crystal field modulation around the Bi³⁺ 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 Gd³⁺ ions with smaller Lu³⁺ ions, the Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ solid solutions show an increase of Bi³⁺ emission intensity and QE values followed by a subsequent decrease. As a result, the highest QE value, which corresponds to Ba₂Gd_0.8Lu_0.2NbO₆:Bi³⁺, is 49%. Finally, by coating this optimal blue phosphor and the red CsPb(Br_0.4I_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.