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Bismuth (Bi)-activated luminescence materials have attracted much attention for their tunable broad emissions ranging from a visible to near-infrared (NIR) region. However, it remains a challenge to regulate the Bi valence state and achieve NIR emission via a facile way. Here, we report the design and preparation of Ba 3 Sc 4 O 9 :Bi phosphors, which emit visible and NIR emissions simultaneously even prepared in the air condition. The self-reduction mechanism of Bi 3+ species in Ba 3 Sc 4 O 9 with a rigid crystal structure is illustrated based on the charge compensation model, and the coexistence of different Bi-active centers, Bi 3+ for visible emission, while Bi + and Bi 0 for NIR emission, is confirmed by the spectroscopic data and X-ray photoelectron spectroscopy (XPS) analysis. The enhanced NIR emission was further achieved through controlled reducing treatment and the related mechanism has also been clarified. This work paves a new way to control bismuth valence and tune the emission of Bi-based luminescence materials for emerging photonics applications.

Long persistent phosphors (LPPs) with ultraviolet (UV) luminescence have great potential for application in the fields of biomedicine, environmental, and catalysis. However, it is currently limited by the design and development of remarkable UV LPPs with a suitable spectral region and an ultralong afterglow decay time. Herein, we develop a new type of Bi 3+ -activated LiScGeO 4 LPP, which exhibits bright ultraviolet-A (UVA) persistent luminescence (PersL). Because of the existence of numerous stabilized effective traps, the as-synthesized phosphors can undergo an ultralong PersL decay time far longer than 12 h. The PersL properties, effective trap depths, distributions, and types, as well as the possible mechanism for the PersL behavior of LiScGeO 4 :Bi 3+ , are comprehensively surveyed utilizing PersL excitation spectra, PersL decay analyses, thermoluminescence experiments, and X-ray photoelectron spectroscopy. This work can cover the shortage of LPPs in the UV region and also can lay the foundation for the development of more excellent UV LPPs toward versatile novel applications.

Narrow band red-emitting Mn 4+ -doped fluoride phosphor is an essential red component of modern white-light-emitting-diode (WLED) devices. Its luminescence has sensitivity to structure and influences the performance of WLED. In this paper, we report a high-performance Mn 4+ phosphor based on a new heterodialkaline fluorogermanate, CsNaGeF 6 :Mn 4+ . As determined by the single-crystal X-ray diffraction analysis, the CsNaGeF 6 compound crystallizes in the orthorhombic crystal system with space group Pbcm (No. 57). Under excitation by 360 and 470 nm photons, CsNaGeF 6 :Mn 4+ emits intense red light near 630 nm with a high quantum yield of 95.6%. The electronic energy levels of the Mn 4+ ion in Cs 2 GeF 6 , Na 2 GeF 6 , and CsNaGeF 6 are calculated using the exchange charge model of crystal-field theory. The local Mn 4+ environment inducing different zero-phonon-line emissions in the structures is probed by electron paramagnetic resonance. The Mn 4+ -doped heterodialkaline fluorogermanate CsNaGeF 6 :Mn 4+ exhibits broader emission as a result of the lowest symmetry. It has higher quantum yield than Na 2 GeF 6 :Mn 4+ and higher spectral luminous efficacy than Cs 2 GeF 6 :Mn 4+ . Given the good thermal stability and efficient luminescence, a prototype warm-WLED device with a color rendering index of 92.5, a correlated color temperature of 3783 K, and a luminous efficacy of 176.3 lm/W has been fabricated by employing the CsNaGeF 6 :Mn 4+ phosphor as the red component. Our results not only reveal that a high-performance Mn 4+ red phosphor is achieved through cationic substitutions but also construct a relationship of performance-structure to guide the design of Mn 4+ phosphors in the future.

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu 2+ for the two distinct types of alkaline earth (AE) sites in Ba 1.9995- x Sr x Eu 0.0005 SiO 4 ( x = 0-1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu 2+ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu 2+ 4f 7 ground level position, the improved thermal stability of Eu 2+ luminescence in the intermediate composition ( x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition.

The resistance to thermal quenching is an essential factor in evaluating the performance of luminescent materials for application in white light emitting diodes (WLEDs). In this work, we studied the site occupancy preference and thermal quenching of luminescence in β-Ca 2 P 2 O 7 :Bi 2+ red phosphor at low (10-300 K) and high temperatures (303-573 K). In β-Ca 2 P 2 O 7 , the host lattice has four different calcium sites, at which Bi 2+ dopant can be located. After comparing the change of bond energy when the Bi 2+ ions are incorporated into the four calcium sites, we found out that Bi 2+ would preferentially occupy the smaller energy variation sites Ci(2) and Ci(1) in this compound, which can be assigned to Bi(2) and Bi(1), respectively. Surprisingly, we noticed that the variation of emission intensity is different under different excitations when the temperature changes from 10 to 300 K. When exciting into the typical absorption of Bi(1) sites at 419 nm, the emission intensity at 300 K remains only 38% as compared to that at 10 K, while exciting into typical Bi(2) absorption at 460 nm, the emission intensity increases to 110%. When further increasing the temperature from 303 to 573 K, we observed a similar phenomenon, and the emission at 460 nm excitation starts to quench at 453 K. The emission intensity at 573 K still remains 86.1% of that at 303 K. This might be attributed to the Bi(2) → Bi(1) energy transfer. It is also evidenced by the time-resolved emission spectra and lifetime values. This work gives new insights into better understanding luminescent behaviors of Bi 2+ -doped materials with multiple cation sites. This should be helpful in the future when designing the bismuth doped phosphor for WLEDs with better resistance to thermal quenching.