Site occupation and energy transfer of Ce3+-activated lithium nitridosilicate Li2SrSi2N4 with broad-yellow-light-emitting property and excellent thermal stability

Abstract

The abundant nitridosilicate-based phosphors have been found due to the various connection types of [SiN₄] tetrahedron such as corner-, edge- and even face-shared structures. The stable and tight frameworks formed by the [SiN₄] tetrahedron lead to excellent luminescence properties. In this study, the lithium nitridosilicate Li₂SrSi₂N₄ with a special crystal structure has been successfully synthesized by gas pressure sintering. The Rietveld refinement indicates that Li₂SrSi₂N₄ is made up of two kinds of [SiN₄] tetrahedron rings called dreier and siebener rings, and it is more stable than the structure formed by chains. Moreover, the channel built by the ring provides a suitable location for the cations to occupy. With the introduction of Ce³⁺ ions, the yellow light centering at 553 nm has been observed, and the broad excitation band ranging from 250 to 450 nm matches well with that of blue LED chips. Additionally, two Sr²⁺ sites occupied by Ce³⁺ ions broaden the emission band including the red region, and the energy transfer happens between the two sites, which has been proved by the time-resolved photoluminescence (TRPL) spectra. When compared with commercial YAG:Ce³⁺, Li₂SrSi₂N₄:Ce³⁺ not only has a broad emission band leading to the decrease in CCT, but also presents excellent thermal stability; at 150 °C the emission intensity still remains beyond 90% of the initial intensity at room temperature. The results indicate that this special crystal structure is worthy of further investigation for practical applications.