LnNbO 4 (Ln = Y, Gd and Lu) is a type of material that can be used as both outstanding self-activated phosphor and laser host has generated significant interests. However, their electronic structures and optical properties still require elucidation. To solve this intriguing problem, firstly, YNbO 4 and LuNbO 4 polycrystalline with monoclinic crystal structure were prepared by traditional high-temperature solid-state reaction method and their structure and luminescent properties were characterized in experiment. Additionally, their CIE chromaticity coordinate were presented to illustrate the luminescent color and the chromaticity coordinates for YNbO 4 and LuNbO 4 are x = 0.1581, y = 0.0725 and x = 0.1581, y = 0.0753, respectively. Secondly, systematic theoretical calculations based on density functional theory method were carried out on them and their band structures, density of states as well as optical properties were obtained in theoretical. The calculated band gap for YNbO 4 and LuNbO 4 are 4.101 and 4.214 eV, respectively. Lastly, a comparison between the experimental and calculated band gap and refractive index was conducted. The good coincident between them indicating the calculated results are reliable. Therefore, all of the obtained results could provide an essential understanding of LnNbO 4 materials.
Broadband ultraviolet (UV) excitation and narrow red-emitting phosphors are employed extensively for spectral conversion, yet finding materials with a luminous efficiency high enough for applications is still challenging. Herein, extremely efficient red emission was realized by way of the exciton-assisted energy transfer when the LuNbO4:xEu3+ phosphors were irradiated with UV light in the broadband region of 220–300 nm. When exciting the interband transition with UV light, self-trapped excitons (STEs) were generated and captured by Eu3+ ions to transfer the energy for lighting. The STE behaviors dominating the luminous properties could be optimized by tuning the Eu3+ doping. With increasing the doping levels x, more efficient energy transfer occurred, resulting in enhanced Eu3+ luminescence with the STE emission fading, and the luminescence external quantum efficiency reached an amazing value of 66.8% when x = 0.09. The STE-emission decay curves showed that the energy transfer was realized by exciton transmission from the NbO43− groups to the Eu3+ ions. The Eu3+ doping resulted in more isolated NbO4 tetrahedrons exerting a larger location force on the STEs, and then an orientational host–activator energy transfer occurred, accounting for the efficient light conversion. Since Eu3+ ions could capture more escaping STEs upon thermal stimulus and obtain extra exciting energy to resist the thermal quenching, the red emission also exhibited excellent thermal stability even at 478 K. These results suggest that the exciton-assisted energy transfer could initiate the exploration of new stably efficient red phosphors for applications in light conversion.
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