A First-Principles Explanation of the Luminescent Line Shape of SrLiAl₃N₄:Eu²⁺ Phosphor for Light-Emitting Diode Applications

Abstract

White light-emitting diodes are gaining popularity and are set to become the most common light source in the U.S. by 2025. However, their performance is still limited by the lack of an efficient red-emitting component with a narrow band emission. The red phosphor SrLiAl3N4:Eu2+ is among the first promising phosphors with a small bandwidth for next-generation lighting, but the microscopic origin of this narrow emission remains elusive. In the present work, density functional theory, the ΔSCF-constrained occupation method, and a generalized Huang–Rhys theory are used to provide an accurate description of the vibronic processes occurring at the two Sr2+ sites that the Eu2+ activator can occupy. The emission band shape of Eu­(Sr1), with a zero-phonon line at 1.906 eV and a high luminescence intensity, is shown to be controlled by the coupling between the 5d z 2 –4f electronic transition and the low-frequency phonon modes associated with the Sr and Eu displacements along the Sr channel. The good agreement between our computations and experimental results allows us to provide a structural assignment of the observed total spectrum. By computing explicitly the effect of the thermal expansion on zero-phonon line energies, the agreement is extended to the temperature-dependent spectrum. These results provide insight into the electron–phonon coupling that accompanies the 5d–4f transition in similar UCr4C4-type phosphors. Furthermore, these results highlight the importance of the Sr channel in shaping the narrow emission of SrLiAl3N4:Eu2+, and they shed new light on the structure–property relations of such phosphors.

Isomorphism of Sr[Li3AlO4] and Sr[Li3GaO4] – Syntheses, Crystal Structure, and Europium(II) Luminescence

“…We assume that Eu 2+ is distributed on two crystallographically distinct sites in Sr[Li 3 (Al/Ga)O 4 ]. A comparable situation exisits for Sr[LiAl 3 N 4 ]:Eu 2+ , where time-resolved and low-temperature luminescence spectroscopy at T = 10 K were used to decompose the virbronic structure of the emission transitions, revealing that Eu 2+ occupies two crystallographically distinct strontium sites. , The luminescence of the single crystals used for the structure determinations of the title compounds were investigated and the normalized emission spectra of the Eu 2+ -activated compounds are shown in Figure S3. Peak wavelengths were observed at λ em = 568 nm with a fwhm equals 46 nm for Sr[Li 3 AlO 4 ]:Eu 2+ and at λ em = 552 nm with a fwhm equals 48 nm for Sr[Li 3 GaO 4 ]:Eu 2+ .…”

“…The interconfigurational transitions from the excited 4f 6 5d 1 states to the 4f 7 ground state of the Eu 2+ activator ion in the above-mentioned compounds lead to narrow-band emissions in the red spectral region of the visible spectrum, making some of them promising candidates for applications in phosphor-converted light-emitting diodes (pc-LEDs) . In the cases of Ca[LiAl 3 N 4 ], Sr[LiAl 3 N 4 ] and Sr[Li 2 Al 2 O 2 N 2 ], detailed spectroscopic studies have been carried out to explore the coordination environments of Eu 2+ . − Except for Ca 18.75 Li 10.5 [Al 39 N 55 ] and Sr 4 [LiAl 11 N 14 ], the crystal structures of the described oxo(nitrido)lithoaluminates are ordered variants of the U[Cr 4 C 4 ] aristotype . The rediscovery of the U[Cr 4 C 4 ]-type structure or closely related structures was driven by the observation of (ultra-) narrow-band emissions emanating from Eu 2+ -activated alkaline-rich lithosilicates − and alkaline earth-rich lithoaluminates. ,, The nitridolithoaluminate Sr[LiAl 3 N 4 ] crystallizes in the ordered variant, which was first described for the isotypic crystal structure of Cs[Na 3 PbO 4 ] .…”

A theoretical spectroscopy study of the photoluminescence properties of narrow band Eu2+-doped phosphors containing multiple candidate doping centers. Prediction of an unprecedented narrow band red phosphor

“…Hence while at room temperature within the experimental resolution, the two sites provide similar absorption spectra and emission spectra, 1 it has been recently shown that at low temperature, they provide unique emission band signatures in the vibrationally resolved emission spectrum. 58 These phosphors define the lower and upper boundaries of a range of Eu 2+ doped phosphors in which the crystal field strength as well as the Eu/ML 8 cuboid compression is increasing. 1 It should be noted that the actual bottom boundary in the Eu 2+ -doped phosphors is defined by the narrow band red emitting Ca[LiAl 3 N 4 ]:Eu 2+ (CLA:Eu 2+ ) phosphor.…”

“…38,52–54 This approach opens new avenues for investigating and optimizing the emission properties of Eu-doped phosphors, ultimately leading to improved performance. In systems with multiple potential doping centers, aspects like the volume of the doping coordination region, 3 the probability of doping and the vibronic coupling of the rigid host structure 55–58 have been successfully employed to interpret the studied phosphor emission properties. In particular, they have been employed to provide insight over the nature of the emission band in room and low temperature spectra of RbNa[Li 3 SiO 4 ] 2 :Eu 2+ 55 and Sr[LiAl 3 N 4 ]:Eu 2+ 58 or to understand the relationship of the host environment to the heat induced blue shift (HIB) phenomenon in Eu 2+ -doped phosphors.…”