Tuning of Bi3+-related excitation and emission positions through crystal field modulation in the perovskite-structured La2(Znx, Mg1-x)TiO6 (0 ≤ x ≤ 1):Bi3+ solid solution for white LEDs

Abstract

In the past year, emission-tunable crystals based on the rare-earth (RE) ions as luminescent center have been frequently reported for use in UV and blue converted white LEDs, but so far tuning the non-RE Bi3+ related emissions through the crystal field modulation is still not discovered in the perovskite crystals. In this work, we design and report a type of Bi3+ doped La2(Znx,Mg1-x)TiO6 (0 ≤ x ≤ 1) perovskite solid solutions, which enable showing the tunable Bi3+ excitation and emission positions. The XRD results show that gradual substitution of smaller Mg2+ ions with larger Zn2+ ions can lead to the blue-shifting of X-ray diffraction (XRD) position, revealing the expansion of cell lattice. Together with structural analysis, our refined XRD and time-resolved spectral results reveal that there is only one type of La site available for Bi3+ substitution. With this regular crystal lattice change, the crystal field strength around Bi3+ ions is found to vary regularly, allowing to realization of the excitation and emission spectral tuning, i.e., the Bi3+ excitation and emission positions as the Mg ions are replaced by the Zn ions can tune from 348 nm to 392 nm and from 405 nm to 433 nm, respectively. This Bi3+ spectral tuning peak after calculated by the dielectric chemical bond theory features a linear relationship with the crystal field strength and, thus, is ascribed to the crystal field modulation. On basis of the La2(Zn0.4,Mg0.6)TiO6 blue, SrGa2S4:Eu2+ green and Y2O3:Eu3+ red phosphors, a UV converted warm white LED device with desirable color rendering index (CRI) of 78, correlated color temperature (CCT) of 3650 K and good luminous efficacy of 118.13 lm/W, is fabricated. This work provides new insights into using the crystal-field modulation to discover more Bi3+ emission-tunable crystals for white LEDs in the future.