Red persistent and photostimulable phosphor SrLiAl3N4:Eu2+

J. Mater. Chem. C, 2020,8, 4956-4964.

The SrLiAl3N4:Eu2+ phosphor has attracted considerable attention owing to its highly efficient narrow-band red emission. Herein, we report for the first time its red persistent luminescence (PersL) and photostimulated luminescence (PSL). After 254 nm light pre-irradiation, the SrLiAl3N4:0.1%Eu2+ phosphor shows 395 s red PersL at a 0.32 mcd m−2 threshold value and its PSL can still be detected under 980 nm light after 15 days. The thermoluminescence spectra evidence that the shallow trap (0.47 eV) plays a major role in PersL and the deep trap (0.81 eV) is responsible for PSL. The charging process for PersL and PSL is clarified by the thermoluminescence excitation (TLE) spectrum. By the aid of density functional theory (DFT) calculations, we verify that the trap levels are due to N vacancies. The electronic structure diagram (HRBE diagram) of SrLiAl3N4:Eu2+ with traps is constructed to illustrate the mechanism of PersL and PSL. The special feature that PersL and PSL both exist makes SrLiAl3N4:Eu2+ a potential candidate for applications such as anti-counterfeiting and optical information storage.

Highly efficient near-infrared phosphor LaMgGa11O19:Cr3+

Inorg. Chem. Front., 2020,7, 1467-1473.

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are desirable for in vivo imaging and applications for nondestructive examination in the food industry. Accordingly, it is very important to exploit highly efficient and stable broad-band NIR phosphors. Herein we report a Cr3+-activated LaMgGa11O19 phosphor via a simple solid-state reaction, showing broad-band emission centered at 770 nm with internal/external quantum efficiency of 82.6%/42.5%. There are three six-coordinated octahedral crystallographic sites in the structure for Cr3+ occupancy, and changing the Cr3+ concentration can tune the NIR emission with tunable band centers from 715 to 800 nm. This spectral red-shift is mainly ascribed to energy transfer among multiple Cr3+ sites, which is further confirmed by decay lifetime analysis. The phosphor also shows excellent luminescence thermal stability, and the photoluminescence intensity at 410 K maintains 87% of that at room temperature. Our work provides a novel broadband NIR emission phosphor with high efficiency and excellent thermal quenching resistance for the field of NIR spectroscopy.

Tolerance Factor and Phase Stability of the Normal Spinel Structure

Cryst. Growth Des., 2020, 20, 3, 2014–2018

Tolerance factor for the normal-spinel structure is introduced as a structural descriptor to predict the phase stability. It is derived following similar principles as those of perovskite and garnet structures, i.e., the geometrical relationship between multitype polyhedra. The calculation of tolerance factor only requires the ionic radii of compositional components involved. A survey of the tolerance factor over 120 AB2X4-type compounds proves the reliability. The numerical values are distributed below 1, which originates from the compressed octahedra which support the framework of spinel. The tolerance factor will be helpful in machine learning and high-throughput screening methods for fast evaluation of phase stability and materials properties of spinel-type compounds.

Sunlight-activated yellow long persistent luminescence from Nb-doped Sr3SiO5:Eu2+ for warm-color mark applications

J. Mater. Chem. C, 2020,8, 1143-1150.

Warm-color persistent luminescent materials are strongly desired for signage markings and medical imaging in comparison with green or blue counterparts. Herein we report a novel yellow long-persistent phosphor, Nb-doped Sr3SiO5:Eu2+, with a peak wavelength of ∼580 nm and persistence time of more than 14 hours at the 0.32 mcd m−2 threshold value after UV radiation. A combination of thermoluminescence (TL), thermoluminescence excitation (TLE), electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations reveals that the persistent luminescence enhancement is attributed to a significant Nb-induced increase of oxygen vacancies that act as electron trapping centers with appropriate trap depths. Groups of time-dependent color-change images are realized with this material, which has potential applications as anti-counterfeit and indicator marks. This investigation also expands the application of transition metal (TM) ions to the field of persistent luminescence and would motivate further exploration of TM substitutions to design and improve silicate or aluminosilicate persistent phosphors with superior performance.