Eu²⁺Stabilized at Octahedrally Coordinated Ln³⁺Site Enabling Red Emission in Sr₃LnAl₂O_7.5(Ln = Y or Lu) Phosphors

Abstract

Red spectrum loss in phosphor‐converted light‐emitting diodes (pc‐LEDs) restricts high‐quality warm‐white lighting. Herein, two blue‐light excitable red‐emitting Sr3LnAl2O7.5:Eu (Ln = Y or Lu) phosphors are reported, and red emission originating from an unprecedented substitution model, with Eu2+ occupied sixfold octahedrally coordinated lanthanide (Ln3+) sites is demonstrated. Site occupancy identification reveals that three different sites are occupied by Eu2+ and one distinct site is occupied by Eu3+, and the Eu2+ stabilized at Ln3+ site accompanied by Eu3+ selectively occupies at Sr2+ site as a charge compensator. An anomalous prolonged Eu2+‐photoluminescence decay emission with increasing temperature in the low‐temperature region is demonstrated. This accounts for the trapped electrons, which are thermally released from shallow traps and eventually populate the Eu2+ 5d level to form an excited‐state Eu2+. The findings help better understand Eu2+ occupancy and luminescence. These also provide a new perspective for the exploration of novel Eu2+ activated oxide‐based red phosphor for pc‐LEDs.

Eu2+luminescence in CaYGaO4olivine: a new efficient red phosphor for warm illumination

“…1,6 To improve its light quality and construct an illumination-grade pc-WLED device, innovative red-emitting materials with sufficiently high emission efficiency are required. 7…”

“…35,36 This viewpoint is strongly supported by the recently developed Rb 3 YSi 2 O 7 :Eu 2+ , Sr 3 LnAl 2 O 7.5 :Eu 2+ (Ln = Y or Lu), and NaMgPO 4 :Eu 2+ red phosphors, and some others, whose red emission has been revealed to be linked to the octahedrally coordinated Eu 2+ ion. 7,37…”

“…1,6 To improve its light quality and construct an illumination-grade pc-WLED device, innovative red-emitting materials with sufficiently high emission efficiency are required. 7 Currently, there are several available approaches to redemitting materials: quantum dots, Mn 4+ doped fluorides and Eu 2+ doped compounds. 2,8,9 While quantum dots, on the one hand, are highly vulnerable to moisture, heat shock and light irradiation due to their low formation energy and ionic crystal nature, on the other hand, they usually contain poisonous Pb/ Cd, hindering their practical applications.…”

“…[10][11][12] Mn 4+ doped fluorides are constrained by a low absorption cross section in the blue spectrum region and long decay times, so their practical application is mostly limited to low power LEDs. [13][14][15] By contrast, Eu 2+ doped compounds, whose emission is derived from the parity allowed transitions between the 8 S 7/2 (4f 7 ) ground state and the crystal field components of the 4f 6 5d 1 excited state configuration, have a high light absorption coefficient, relatively short emission decay, and tunable emission energy and emission bandwidth, [16][17][18] making Eu 2+ doped compounds more favorable for LED applications. The prominent examples are broadband emitting CaAlSiN 3 :Eu 2+ , Sr 2 Si 5 N 8 :Eu 2+ , and narrowband emitting SrLiAl 3 N 4 :Eu 2+ red phosphors, which have dominated the commercial market since they were discovered.…”

“…35,36 This viewpoint is strongly supported by the recently developed Rb 3 YSi 2 O 7 :Eu 2+ , Sr 3 LnAl 2 O 7.5 :Eu 2+ (Ln = Y or Lu), and NaMgPO 4 :Eu 2+ red phosphors, and some others, whose red emission has been revealed to be linked to the octahedrally coordinated Eu 2+ ion. 7,37 Herein, we aim to introduce the Eu 2+ dopant into an octahedral coordination sphere to conceptually design a red phosphor. To achieve the goal, we focused on CaYGaO 4 olivine comprising two crystallographically independent cationic CaO 6 and YO 6 octahedra potentially available for Eu 2+ doping.…”