Governing the crystallographic sites for tuning Eu2+ emission in an apatite oxyfluoride host to be applied for superior white light emitting diodes

Abstract

Single white light-emitting phosphors for near-UV-converted white light-emitting diodes (WLEDs) are the best alternatives to tricolour phosphor blends and blue light converted yellow-emitting garnets. Nevertheless, achieving white light with elevated colour rendering (CRI) from a single-phase phosphor activated with a lone activator ion is a major challenge. This study aimed at the generation of white light from a single-phase composition activated only with europium. The study started with structural evaluations of Eu³⁺-activated Ca₄La₆Si₆O₂₄F₂ phosphors using X-ray diffraction (XRD) and Eu³⁺ photoluminescence to elucidate the local environment of rare-earth ions and the symmetric nature of the lattice sites. Ca₄La₆Si₆O₂₄F₂ crystallized in the hexagonal P6₃/m space group. The predominant ⁵D₀–⁷F₂ electric dipole transition at 614 nm, and the non-splitting as well as the zero-shifting behaviour of ⁵D₀–⁷F₀ at 578 nm, suggested that the rare-earth ionic substitutions preferably took place at the larger asymmetric sites. Introducing Sr²⁺ ions in Ca₄La₆Si₆O₂₄F₂:Eu³⁺/Eu²⁺ that is synthesized under a reducing atmosphere suppressed Eu³⁺ emission. From the optimized Ca_1.98Sr_1.98La₆Si₆O₂₄F₂:0.04Eu²⁺, a sequence of M²⁺-codoped (M = Mg/Ba) Ca_1.98Sr_1.98La₆Si₆O₂₄F₂:0.04Eu²⁺ phosphors were further developed. The substitutions of Mg²⁺ and Ba²⁺ altered the crystal field by changing the lattice parameters. The Mg²⁺ -doped samples showed a blue-shift from 520 nm (Mg²⁺ = 0) to 471 nm (Mg²⁺ = 1.0), whereas the Ba²⁺-doped compositions showed a red-shift from 520 nm (Ba²⁺ = 0) to 536 nm (Ba²⁺ = 1.2). The change of symmetry owing to the Mg²⁺/Ba²⁺ substitution could have led to the centroid shift, which was responsible for the blue- or red-shift of the emission spectra. The XRD of Ca_1.38Sr_1.38La₆Si₆O₂₄F₂:0.04Eu²⁺,1.2Ba²⁺ indicated a Ba²⁺-induced lattice site expansion. Keeping this in view, the Eu²⁺ ions concentrations were further enhanced from 0.04 to 0.3, and the resultant photoluminescence was further enhanced and red-shifted. The optimized sample showed better intensity compared with the commercial Y₃Al₅O₁₂:Ce³⁺ and exhibited decent photoluminescence above 70% at 150 °C as compared with that at room temperature. Finally, several prototype WLEDs were fabricated using the single phosphor Ca_1.365Sr_1.365La₆Si₆O₂₄F₂:0.07Eu²⁺,1.2Ba²⁺ with near-UV and violet-LED chips. The outcomes indicated the promising nature of this single-composition phosphor for indoor lighting.