Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr,Ca)AlSiN₃:Eu Phosphor

Abstract

Nitride phosphors are suitable for white light-emitting diode applications. In this study, the structure of phosphor has been modified through cation substitution to induce charge variation and a rearrangement of neighboring nitride clusters, and consequently enhance its luminescent behavior. Substitution of Ca(2+) by Sr(2+) cations expanded the lattice volume and the bc plane, but shortened the distance between the layers along the a axis. Lattice distortion of the framework introduced high-coordination sites in the Sr/Eu centers and adequate space, thereby facilitating charge variation of activators under reduced atmosphere, as detected through X-ray absorption near-edge structure spectroscopy. As such, the photoluminescent intensity of the phosphors increased by more than 10% and a blue shift occurred. The microstructures of the samples were also analyzed using high-resolution transmission electron microscopy. Cation substitution induced a special change in the anion environment, as indicated in the solid-state Raman spectra. Moreover, typical ordering variations in the SiN4 and AlN4 clusters are generated in the lattice. Meanwhile, neighbor sequence of (Si/Al)N4 around the divalent centers were observed through solid-state nuclear magnetic resonance spectroscopy. The modified ordering distribution resulted in a rigid structure and improved the thermal quenching behavior. Thermal stability has been enhanced by 10% at 473 K when x = 0.9 in SrxCa0.993-xAlSiN3:Eu(2+)0.007 compared with that at x = 0. This study promotes the research of neighbor sequence for selective tetrahedral sites such as Li, Mg, Al, and Si coordinated by N atoms in contact with cation sites.

Thermal Quenching Mechanism of CaAlSiN3:Eu2+ Red Phosphor

“…For this thermal quenching mechanism, two possibilities have been considered. One is the thermally activated crossover from the 5d level to the 4f levels [7][8][9] , the other is the thermal ionization from the 5d excited level to the bottom of the conduction band (CB). 10,11 Zhang and Hintzen et al investigated the energy gap between the lowest 5d level of Eu 2+ and Yb 2+ and the bottom of CB (E5d-CB) and found the correlation between thermal quenching behavior and E5d-CB.…”

Critical Review—Narrow-Band Emission of Nitride Phosphors for Light-Emitting Diodes: Perspectives and Opportunities

“…Through cation or anion substitution, a bathochromic or hypsochromic shift may be achieved. [4][5][6] For example, bypassing the geometrical restriction of the activator by replacing a few elements in the lattice.…”

“…65,66 Through the gas pressure sintering with silicon (α-Si 3 N 4 ) additive, a wurtzite phase doped with Eu 2+ was prepared. The Si was particularly essential to improve the incorporation of Eu in the crystal lattice 6,7 as well as SiC which was also used to achieve the same effect (Fig. 17a).…”

Critical Review—Narrow-Band Nitride Phosphors for Wide Color-Gamut White LED Backlighting

“…The absorption band around 240-280 nm is ascribed to the host lattice absorption, and therefore the optical bandgap is estimated to be ∼4.6 eV. The luminescence spectra 62 the host lattice ions Li, Mg, and Si all occupy distinct lattice sites instead of a statistical distribution over the same lattice site contributes to the smaller Stokes shifts, limited inhomogeneous broadening of the emission bands, and therefore narrow band emission. The introduction of Sr causes an enlarged Eu-N distance, and hence reduces the crystal field splitting.…”