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While numerous approaches have been devised to optimize the luminescence properties of Eu2+-activated phosphors, the quest for a simple design strategy to achieve color-tunable Eu2+ emission remains a significant challenge. Here, we present a straightforward, doping-free method to tailor the site occupation of Eu2+ and modify its luminescence properties. In the pristine sample, the NaBaB9O15 (NBB) host featuring two cation sites (Na+ and Ba2+), Eu2+ exclusively occupies the Ba2+ site, resulting in blue emission when the NBB:Eu2+ phosphor is synthesized in a H2 reducing atmosphere. When we use CO instead of a H2 reducing atmosphere, impressively, the formation energy required for heterovalent substitution of Na+ by Eu2+ is lowered, leading to the emergence of a distinct green emission band. Significantly, the resultant phosphor NBB:Eu2+@CO exhibits not only a high internal quantum yield of 85.1% and zero-thermal-quenching behavior within the temperature range of 25–200 °C but also wavelength-tunable emission. Benefitting from the multi-color luminescence of the NBB:Eu2+@CO phosphor, its optical applications will no longer be confined to general lighting but extend to anti-counterfeiting and encryption. This study provides a novel design methodology for optimizing the optical properties of Eu2+-activated phosphors. [ABSTRACT FROM AUTHOR]

We have synthesized and investigated a series of yellow-emitting Sr3Ce(PO4)3:Eu2+ phosphors, showing an extreme broadband at around 535 nm (with a FWHM of ca. 200 nm/6760 cm−1) and a large Stoke shift (ca. 8265 cm−1), which is attributed to the 4f–5d transition of Eu2+ without the contribution from Ce3+ emission under excitation at long wavelengths (＞370 nm). However, such broadband Eu2+ emission is rather surprising, considering that there is only one cation site for the Eu2+ luminescent center. Herein, we investigated the crystal structure by performing X-ray Rietveld refinement on the synchrotron X-ray diffraction data, and demonstrate that both cation and anion sites in Sr3Ce(PO4)3 are disordered in the host lattice. These unusual structural characters generate a variety of distinct Eu2+ sites, which is verified by decay lifetime analysis and electron paramagnetic resonance spectra, and thus result in astonishing broadband yellow-emission. Moreover, a white LED device with a color-rendering index of 86.5, a color temperature of 5996 K, and chromaticity coordinates of (0.32, 0.38) was obtained by combining a 405 nm near-UV LED chip and the phosphor blends of yellow-emitting Sr3Ce(PO4)3:Eu2+ and the commercial blue-emitting BaMgA10O17:Eu2+ phosphor. These results indicate that the as-prepared yellow-emitting Sr3Ce(PO4)3:Eu2+ phosphor has potential applications in the dual-color-phosphor-converted WLEDs. [ABSTRACT FROM AUTHOR]

Two series of K1−xNaxSrPO4:0.005Eu2+ and K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+ phosphors are synthesized via a high-temperature solid-state reaction. Their emission color can be tuned from deep blue to blue–green by modulating the crystal field strength and energy transfer. Partial substitution of K+ with Na+ results in a contraction and distortion of the unit cell of the K1−xNaxSr0.995PO4:0.005Eu2+ host, tuning the emission from 426 to 498 nm. The red-shifted emission is attributed to an increased crystal field splitting for Eu2+ in a lowered symmetry crystal field. The tunable emission is further demonstrated in the cathodoluminescence spectra, which indicates that the luminescence distribution of the K1−xNaxSr0.995PO4:0.005Eu2+ phosphor is very homogenous. Additionally, utilizing the principle of energy transfer, the emission color can be further tuned by co-doping with Tb3+. The chromaticity coordinates for the co-doped phosphor, K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+, can be adjusted from (0.202, 0.406) for y = 0 to (0.232, 0.420) for y = 0.09. The energy transfer processes from the sensitizer (Eu2+) to the activator (Tb3+) are studied and demonstrated to have a resonance-type dipole–dipole interaction mechanism, with the critical distance of the energy transfer calculated to be 12.46 Å using a concentration quenching method. [ABSTRACT FROM AUTHOR]