Intense Luminescence and Good Thermal Stability in a Mn²⁺-Activated Mg-Based Phosphor with Self-Reduction

Abstract

White light-emitting diodes provide widespread applications in lighting, electronic equipment, and high-tech displays. However, thermal quenching effect severely limits their practical application. Here, we developed an orange-red phosphor β-KMg­(PO3)3:Mn2+, which emits bright orange-red light when excited by ultraviolet light without the energy transfer of sensitizer, owing to the strong crystal field provided by β-KMg­(PO3)3 for Mn2+. The self-reduction of Mn4+ → Mn2+ and good thermal stability have been realized in an ambient atmosphere. The defect types were verified by X-ray photoelectron spectroscopy, and cationic vacancy plays a significant role in the self-reduction of Mn4+ → Mn2+. Furthermore, the properties of the trap energy levels were studied by thermoluminescence. The recombination luminescence of the detrapped carriers released from the deep trap levels at high temperatures suppresses the luminescence loss of thermal quenching. Moreover, the trap energy levels play an important role in the mechanoluminescence of β-KMg­(PO3)3:Mn2+. This work emphasizes the significance of the defects in the luminescent characteristics and opens up a new approach for the development of advanced optical functional materials.

Elucidating the Multisite and Multivalence Nature of Mn Ions in Solids and Predicting Their Optical Transition Properties: A Case Study on a Series of Garnet Hosts

“…The increase in the 5 T 2 thermal population at the expense of 1 T 2 with temperature usually leads to the dominance of the deep red emission from 5 T 2 near room temperature, but the dominance of infrared emission from 1 T 2 at sufficiently low temperatures, which has a temperature-dependent emission energy and lifetime. Mn 2+ ions have abundant site occupancy; the emissions range from green to NIR, and the luminescence decay lifetimes are generally in the order of milliseconds, ,− but depend on the site symmetry, however. In the framework of the density functional theory, the excited states and optical transitions are approximated with the combination of single determinants, and the energy levels and transition energies of Mn 4+ , Mn 3+ , and Mn 2+ activators in the solids are all calculated.…”

“…Interpreting and predicting the site occupancy and valence state of dopants in multisite hosts have attracted great research interest in the field of optoelectronics and functional phosphor materials. The effect of structure-property correlations on luminescence performance has been elaborately studied in the multisite phosphors of lanthanide ion activators, and the selective partial self-reduction of Eu 3+ to Eu 2+ in a multi-cationic host has been shown as an effective strategy to design mixed-valence activated phosphors. − Recently, self-reduction of Mn 4+ to Mn 2+ has been extensively studied in the literature for intense luminescence and excellent thermal stability. − …”

Self-reduction of Mn4+ to Mn2+: NaY9Si6O26:Mn2+ red phosphors with excellent thermal stability for NUV LEDs

“…Compared to rare earth ions, Mn 4+ , a commonly used red phosphor activator with excellent optical properties and low cost, can be self-reduced in some unique crystal structures in an air environment (Mn 4+ → Mn 2+ ), e.g. , NaMg(PO 3 ) 3 :Mn 2+ , 14 Ba 3 BP 3 O 12 :Mn 2+ , 15 β-KMg(PO 3 ) 3 :Mn 2+ , 16 BaXP 2 O 7 :Mn 2+ (X = Mg/Zn), 17 NaZn(PO 3 ) 3 :Mn 2+ , 18 etc. , exhibiting high thermal stability and anti-quenching phenomena.…”

“…The ideal way to overcome these problems is to keep the low-valent ions stable in the air environment such as in Sr/Ca[B 8 O 11 (OH) 4 ]:Eu; 9,10 CaAl 2 Si 2 O 8 :Eu; 11 a-Ca 3 (PO 4 ) 2 :Eu; 12 M 2 B 5 O 9 Cl:Eu 2+ (M = Sr, Ca) 13 or to use the Eu 3+ -Eu 2+ self-reduction to achieve excellent thermal stability. Compared to rare earth ions, Mn 4+ , a commonly used red phosphor activator with excellent optical properties and low cost, can be self-reduced in some unique crystal structures in an air environment (Mn 4+ -Mn 2+ ), e.g., NaMg(PO 3 ) 3 :Mn 2+ , 14 Ba 3 BP 3 -O 12 :Mn 2+ , 15 b-KMg(PO 3 ) 3 :Mn 2+ , 16 BaXP 2 O 7 :Mn 2+ (X = Mg/Zn), 17 NaZn(PO 3 ) 3 :Mn 2+ , 18 etc., exhibiting high thermal stability and anti-quenching phenomena. Among them, the self-reduction of Mn 4+ results from a reaction between Mn 4+ ions and negatively charged defects: the non-equivalent substitution of Mn 4+ with the host ions creates defects that can provide electrons for selfreduction.…”

Anti-thermal quenching phosphors based on the new phosphate host Ca3.6In3.6(PO4)6

“…White light emitting diode (WLED), regarded as the fourth generation light source, has attracted considerable attention for its advantages of ecological friendliness, energy conservation, fast response, longer working life, etc . 1,2 The white phosphor-converted WLED (pc-WLED), which uses a combination of electrified UV or blue LED chips and excited phosphors, is one of the most frequent ways to create light sources. 3 Among them, the phosphor is a key component, which determines the performance of high-power WLED in terms of luminous efficiency, color temperature, and service life.…”