Tolerance factor, phase stability and order–disorder of the pyrochlore structure

Inorg. Chem. Front., 2020,7, 1583-1590. https://doi.org/10.1039/D0QI00016G

Graphical abstract: Tolerance factor, phase stability and order–disorder of the pyrochlore structure

The tolerance factor is a structural indicator that connects the crystal structure and chemical composition. In this work, we establish the tolerance factor for pyrochlore A2B2O7-type compounds, using ionic radii of the compositional species. It is derived following similar procedures to those of perovskite and garnet structures. More than 180 A2B2O7 type compounds are examined to test its validity in predicting the pyrochlore phase stability. It can also be used to understand the reverse and order–disorder cationic occupations. This structural descriptor could be used together with machine learning or high-throughput screening methods for new material design and discovery.

Orange super-long persistent luminescent materials: (Sr1−xBax)3SiO5:Eu2+,Nb5+

Mater. Chem. Front., 2021,5, 333-340. https://doi.org/10.1039/D0QM00488J

Persistent luminescent materials are widely used as night-vision and marking materials in various important fields. Although significant achievements have been made in blue and green persistent luminescent (PersL) materials, the research and development of PersL materials in the warm-color region (550–660 nm) are relatively lacking. Only the orange PersL phosphor Y2O2S:Eu3+,Mg2+,Ti4+ (∼610 nm) and deep red PersL phosphor (Ca1−xSrx)S:Eu2+,Tm3+ (∼650 nm) fulfill the demands of commercial applications. However, sulfide phosphors have poor chemical stability and a relatively short persistent duration time. Herein, we report a series of orange PersL materials, (Sr1−xBax)3SiO5:Eu2+,Nb5+, which exhibit a strong PersL emission band at 550–670 nm and a super-long persistent time of more than 20 h at the 0.32 mcd m−2 threshold value after UV radiation. These new orange PersL materials are compared to the commercial warm-color sulfide PersL phosphor, Y2O2S:Eu,Mg,Ti, with regards to PersL time and brightness, and it is determined that they would have great potential applications.

Red persistent and photostimulable phosphor SrLiAl3N4:Eu2+

J. Mater. Chem. C, 2020,8, 4956-4964. https://doi.org/10.1039/D0TC00277A

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. https://doi.org/10.1039/D0QI00063A

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 https://doi.org/10.1021/acs.cgd.9b01673

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.