Frenkel Defect‐modulated Anti‐thermal Quenching Luminescence in Lanthanide‐doped Sc₂(WO₄)₃

Abstract

Although large amount of effort has been invested in combating thermal quenching that severely degrades the performance of luminescent materials particularly at high temperatures, not much affirmative progress has been realized. Herein, we demonstrate that the Frenkel defect formed via controlled annealing of Sc 2 (WO 4 ) 3 :Ln (Ln = Yb, Er, Eu, Tb, Sm), can work as energy reservoir and back-transfer the stored excitation energy to Ln 3 + upon heating. Therefore, except routine anti-thermal quenching, thermally enhanced 415-fold downshifting and 405-fold upconversion luminescence are even obtained in Sc 2 (WO 4 ) 3 :Yb/Er, which has set a record of both the Yb 3 + -Er 3 + energy transfer efficiency (> 85 %) and the working temperature at 500 and 1073 K, respectively. Moreover, this design strategy is extendable to other hosts possessing Frenkel defect, and modulation of which directly determines whether enhanced or decreased luminescence can be obtained. This discovery has paved new avenues to reliable generation of high-temperature luminescence.

Negative Thermal Expansion Materials as Anti-Thermal-Quenching Phosphor Matrixes: Status, Opportunities, and Challenges

“…In addition, forming extrinsic or Frenkel defects upon heating has recently been found in NTE crystal lattices as excitation energy reservoirs that can back-transfer the stored excitation energy to compensate for emission loss. 17 A similar process has also been realized for trap-mediated thermally enhanced emissions, as presented below. 19 In this Perspective, we discussed relevant anti-TQ mechanisms of reported NTE-based anti-TQ phosphors along with thermally enhanced UC and DS luminescence as well as their specific materials characterization.…”

“…Because these structural defects act as excitation energy reservoirs and populate excitation energy levels to combat TQ, monitoring temperature-dependent excitation and emission spectra has also been used to spot trap formation. 17 While detailed full characterization of NTE-based anti-TQ phosphors is still limited in such short but rapid developing stages, there are several proposed anti-TQ mechanisms to explain thermally enhanced UC and DS emissions induced by the NTE property of the NTE host matrixes (Scheme 1). First, chemical bond contraction of NTE materials in two or three dimensions can decrease the distance between the emission centers of dopants and/or defect levels, compensating for emission loss due to the increased ET rate upon heating.…”

“…Moreover, Wei et al also obtained a significant 415-fold DS increase from room temperature to 500 K from Sc 2 W 3 O 12 :Yb 3+ ,Er 3+ along with the observed anti-TQ UC emission, as discussed in the earlier section in the same study reported in 2023. 17 Similarly, they attributed the anti-TQ DS performance to Frenkel defects formed from the extrinsic WO 4 2− positions due to their local deviation upon heating. In other words, the partial overlap between the intrinsic and extrinsic energy levels of WO 4 2− formed singlet and triplet energy reservoirs and back-transferred the stored energy upon heating to combat TQ.…”

“…In 2023, Wei et al reported a 405-fold enhancement of the UC luminescence of Er 3+ at 1073 K from a NTE-based Sc 2 W 3 O 12 :Yb 3+ ,Er 3+ phosphor followed by an enriched anti-TQ UC mechanism study. 17 On the basis of their calculations on the interatomic distance of Yb 3+ −Er 3+ and the distortion index of Er 3+ , they believed that the formed Frenkel defects due to annealing, instead of the shortened distance between the activator and sensitizer or the higher coordination symmetry, contributed to the observed anti-TQ behavior because energy reservoirs to back-transfer stored excitation energy to emitting ions in the NTE-based Sc 2 W 3 O 12 :Yb 3+ ,Er 3+ phosphor. During heat treatment, highly oxygen-coordinated WO 4 tetrahedra connected with corner-shared ScO 6 octahedra form a quasi-layered structure with relatively large interstitial sites that favor the formation of Frenkel defects (Figure 3c).…”

Simultaneously tuning the luminescent color and realizing an optical temperature sensor by negative thermal expansion in Sc2(WO4)3:Tb/Eu phosphors

“…Recently, upconversion luminescent phosphors with an unusual anti-thermal quenching performance have been reported where lanthanide ions are doped into A 2 (MO 4 ) 3 -type (A = Sc 3+ , Y 3+ , Lu 3+ ; M = Mo 6+ /W 6+ ) materials with negative thermal expansion characteristics, which has attracted extensive attention from researchers. 18–24 This performance implied the potential to control the thermal quenching behavior of downshifting (DS) luminescent materials by selecting NTE materials and suitable doped-lanthanide ions. For the reported DS luminescent materials with NTE characteristics, the absolute majority of them are single lanthanide ion-doped, such as Sc 2 (MoO 4 ) 3 :Eu 3+ and Lu 2 (MoO 4 ) 3 :Eu 3+ , 25,26 and they also exhibit anti-thermal quenching behavior.…”